Tag Archive | "frequency"

Australian Electronics Nostalgia – Talking Electronics Kits

Introduction

From 1981, Australian electrical engineer Colin Mitchell started publishing his home-grown electronics magazine “Talking Electronics”. His goal was to get people interested and learning about electronics, and more so with a focus on digital electronics. It was (and still is) a lofty goal – in which he succeeded. From a couple of rooms in his home the magazine flourished, and many projects described within were sold as kits. At one stage there were over 150 Talking Electronics kits on the market. You could find the books and kits in retail outlets such as Dick Smith Electronics, and for a short while there was a TE store in Moorabbin (Victoria). Colin and the team’s style of writing was easy to read and very understandable – but don’t take my word for it, you can download the magazines from his website (they’re near the bottom of the left column). Dave Jones recently interviewed Colin, and you can watch those for much more background information.

Over fifteen issues you could learn about blinking LEDs all the way to making your own expandable Z80 board computer, and some of the kits may still be available. Colin also published a series of tutorial books on electronics, and also single-magazine projects. And thus the subjects of our review … we came across the first of these single-issue projects from 1981 – the Mini Frequency Counter (then afterwards we have another kit):

cover

How great is that? The PCB comes with the magazine. This is what set TE apart from the rest, and helped people learn by actually making it easy to build what was described in the magazine instead of just reading about it. For 1981 the PCB was quite good – they were silk-screened which was quite rare at the time:

pcb

pcbrear

And if you weren’t quite ready, the magazine also included details of a square-wave oscillator to make and a 52-page short course in digital electronics. However back to the kit…

Assembly

The kit uses common parts and I hoard CMOS ICs so building wasn’t a problem. This (original) version of the kit used LEDs instead of 7-segment displays (which were expensive at the time) so there was plenty of  careful soldering to do:

LEDsin

And after a while the counter started to come together. I used IC sockets just in case:

almostthere

The rest was straight-forward, and before long 9 V was supplied, and we found success:

powerup

To be honest progress floundered for about an hour at this point – the display wouldn’t budge off zero. After checking the multi-vibrator output, calibrating the RC circuits and finally tracing out the circuit with a continuity tester, it turned out one of the links just wasn’t soldered in far enough – and the IC socket for the 4047 was broken So a new link and directly fitting the 4047 fixed it. You live and learn.

Operation

So – we now have a frequency counter that’s good for 100 Hz to the megahertz range, with a minimum of parts. Younger, non-microcontroller people may wonder how that is possible – so here’s the schematic:

schematic

The counter works by using a multi-vibrator using a CD4047 to generate a square-wave at 50, 500 and 5 kHz, and the three trimpots are adjusted to calibrate the output. The incoming pulses to measure are fed to the 4026 decade counter/divider ICs. Three of these operate in tandem and each divide the incoming count by ten – and display or reset by the alternating signal from the 4047. However for larger frequencies (above 900 Hz) you need to change the frequency fed to the display circuit in order to display the higher (left-most) digits of the result. A jumper wire is used to select the required level (however if you mounted the kit in a case, a knob or switch could be used).

For example, if you’re measuring 3.456 MHz you start with the jumper on H and the display reads 345 – then you switch to M to read 456 – then you switch to the L jumper and read 560, giving you 3456000 Hz. If desired, you can extend the kit with another PCB to create a 5-digit display. The counter won’t be winning any precision contests – however it has two purposes, which are fulfilled very well. It gives the reader an inexpensive piece of test equipment that works reasonably well, and a fully-documented project so the reader can understand how it works (and more).

And for the curious –  here it is in action:

[Update 20/07/2013] Siren Kit

Found another kit last week, the Talking Electronics “DIY Kit #31 – 9V siren”. It’s an effective and loud siren with true rise and fall, unlike other kits of the era that alternated between two fixed tones. The packaging was quite strong and idea for mail-order at the time:

kitbox

The label sells the product (and shows the age):

kitlabel

The kit included every part required to work, apart from a PP3 battery, and a single instruction sheet with a good explanation of how the circuit works, and some data about the LM358:

kitparts

… and as usual the PCB was ahead of its’ time with full silk-screen and solder mask:

pcbtop

sirenpcbbottom

Assembly was quite straight-forward. The design is quite compact, so a lot of vertical resistor mounting was necessary due to the lack of space. However it was refreshing to not have any links to fit. After around twenty minutes of relaxed construction, it was ready to test:

PCBfinished

finished

It’s a 1/2 watt speaker, however much louder than originally anticipated:

Once again, another complete and well-produced kit.

Conclusion

That was a lot of fun, and I’m off to make the matching square-wave oscillator for the frequency counter. Kudos to Colin for all those years of publication and helping people learn. Lots of companies bang on about offering tutorials and information on the Internet for free, but Colin has been doing it for over ten years. Check out his Talking Electronics website for a huge variety of knowledge, an excellent electronics course you can get on CD – and go easy on him if you have any questions.

Full-sized images available on flickr. This kit was purchased without notifying the supplier.

And if you made it this far – check out my new book “Arduino Workshop” from No Starch Press.

LEDborder

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Posted in australia, counter, digital, electronics, frequency, history, kit review, learning electronics, magazine, talking, talking electronics, test equipment, tronixstuff, vintageComments (8)

Kit Review – JYE Tech FG085 DDS Function Generator

Introduction

There has been a lot of talk lately about inexpensive DDS (direct digital synthesis) function generators, and I always enjoy a kit – so it was time to check out the subject of this review. It’s the “FG085 miniDDS function generator” from JYE Tech. JYE is a small company in China that makes inexpensive test equipment kits, for example their capacitance meter (my first kit review!) and DSO. The capacitance meter was good, the DSO not so good – so let’s hope this is better than their last efforts.

Assembly

The instructions (AssemblyGuide_085G) are much better than previous efforts, and if you have bought the kit – read them. The kit arrives in a large zip-lock bag, with the following bundle of parts:

The AC adaptor is 100~240V in, 15V DC out. Everything is included with the kit including a short BNC to alligator clips lead for output. The PCBs are very good, with a nice solder mask and silk screen:

and back:

At this point we realise that most of the work is already done. There’s two microcontrollers ATmega48 and ATmega168- one for display and user-interface control, and the other for function generation. It takes only a few minutes to solder in the through-hole parts, headers and sockets:

… then you flip over the PCB and add the LCD:

… followed by the buttons and rotary encoder. From previous research this is the part that causes people a lot of trouble – so read carefully. There’s a lot of buttons – and if they aren’t inserted into the PCB correctly your life will become very difficult. The buttons must be inserted a certain way – they’re “polarised” – for example:

As you can see above, one side has a double-vertical line and the other side has a single. When you fit the buttons to the PCB – the side with the double-vertical must face the left-hand side of the PCB – the side with the DC socket. For example:

Furthermore, don’t be in a rush and put all the buttons in then try to solder them all at once.  Do them one at a time, and hold them tight to the PCB with some blu-tac or similar. If they don’t sit flush with the PCB the front panel won’t fit properly and the buttons will stick when in use. So exercise some patience, and you’ll be rewarded with an easy to use function generator. Rush them in and you’ll be very unhappy. I warned you! After fitting each button, test fit the front panel to check the alignment, for example:

Then you end up with nicely-aligned buttons:

… which all operate smoothly when the panel is fitted:

After the buttons comes the rotary encoder. Be very careful when fitting it to the PCB – the data legs are really weak, and bend without much effort. If you push in the encoder, be mindful of the legs not going through the holes and bending upwards. Furthermore, when soldering in the encoder note that you’re really close to an electrolytic – you don’t want to stab it with a hot iron:

The CP2012 chip in the image above is for the USB interface. More on that later. Now the next stage is the power-test. Connect DC power and turn it on – you should be greeted by a short copyright message followed by the operation display:

If you didn’t – remove the power and check your soldering –  including the capacitor polarities and look for bridges, especially around the USB socket. Now it’s time to fit the output BNC socket. For some reason only known to the designers, they have this poking out the front of the panel for the kit – however previous revisions have used a simple side-entry socket. Thus you need to do some modifications to the supplied socket. First, chop the tag from the sprocket washer:

… then remove the paper from the front panel:

Now solder a link to the washer in a vertical position:

… then fit the BNC socket to the panel, with the washer aligned as such:

Finally, align the top panel with the PCB so the BNC socket pin and washer link drop into the PCB and solder them in:

If you want to use the servo mode, solder three short wires that can attach to a servo form the three “output” pads between the BNC and USB socket.

Finally, screw in the panels and you’re finished!

Using the function generator

Operation is quite simple, and your first reference should be the manual (manual.pdf). The display defaults to normal function generator mode at power-up – where you can adjust the frequency, offset, amplitude and type of output – sine, square, triangle, ramp up, ramp down, staircase up and down:

The ranges for all functions is 0~10 khz, except for sine which can hit 200 kHz. You can enter higher frequencies, such as up to 250 kHz for sine – but the results aren’t so good.

Instead of filling this review with lots of screen dumps from an oscilloscope to demonstrate the output – I’ve made the following video where you can see various functions being displayed on a DSO:

You can also create signals to test servos, with adjustable pulse-width, amplitude and cycle times. However you’ll need to solder three wires onto the PCB (next to the BNC socket area) to attach to the servo.

According to the user manual and various retailers’ websites – the FG085 can generate frequency sweeping signals. These are signals that sweep from a start to as finish frequency over a period of time. However the firmware on the supplied unit is old and needs updating to enable this function. You can download the firmware in .hex file format from here. Then go and dig up an AVR programmer and avrdudeAt the time of writing we had some issues with the signature not being recognised when updating the firmware, and solidly bricked the FG085. Our fault – so when that’s sorted out we’ll update the review – stay tuned.

There is also a USB port on the side – after installing CP2102 drivers in Windows we could connect at 115200 bps with terminal, however all the FG085 returned was the firmware version number. Perhaps later on the designers will update the firmware to allow for PC control. Somehow I wouldn’t bank on it.

Oh – if you’re wondering what DDS is – click here!

Conclusion

It’s an interesting piece of equipment. Putting the firmware upgrade issues to one side, the FG085 does what it sets out to do. During testing it worked well, and we didn’t come across any obvious inaccuracies during use.  The price varies between US$43 and $50 – so for that money it’s  a good kit. Just take care during construction and you’ll be fine.

The function generator is available in kit form or assembled, with or without panels from China. The kit version with panels is also available from Sparkfun (KIT-11394) and their resellers. Full-sized images available on flickr. This kit was purchased and reviewed without notifying the supplier.

In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Posted in dds, ds2102, FG085, function generator, jyetech, kit review, KIT-11394, learning electronics, rigol, test equipmentComments (7)

Tektronix CFC250 Teardown

Introduction

Time for something different – and perhaps the start of several new articles containing teardowns. In our first instalment we examine the Tektronix CFC250 100 MHz frequency counter circa 1994: Not the most spectacular of designs, but it has worked well right until the present day. The update speed of the display wasn’t lightning fast, however for the time it would have been quite reasonable. Here is a short video I shot last year comparing it against a small frequency counter kit:

However after staring at this thing every day on my desk for a couple of years it has now become impossible to overcome the temptation to have a look inside. Therefore the reason for this article. You can click on the images to see the full-size version. So let’s go back to 1988 and check out the CFC250…

External tour

A quick look around the outside. The casing is reminiscent of the Escort brand of test equipment from the era, and (I suspect that) they OEM’d the CFC250 for Tektronix. (Interestingly enough Agilent bought the assets of Escort in 2008). Moving forward, the external images of the CFC250 starting with the front:

… and the rear. The AC transformer is tapped out to accept four different mains voltages, which you can select with the slide switches:

Opening up the unit involves removing screws from the base. The first ones were only for the feet, so they could stay put:

It was the screw on the right of the foot that was the key to entry. After removing them from each side and the other pair on the rear-bottom, the top casing pulls off easily…

Internal tour

… leaving us with the internals for all to see:

Although the LED display is a fair giveaway to the age of the CFC250,  a quick look around the PCB confirms it… and the display is ultimately controlled by an LSI Systems LS7031 “Six decade MOS up counter” (data sheet.pdf). It is matched to some DS75492N MOS-to-LED hex digit driver ICs (data sheet.pdf) and some other logic ICs. It is interesting to compare the number of parts required to drive the LEDs compared to a contemporary microcontroller and something like the TM1640 used in this module.

Now for the LED display board:

Nothing too out of the ordinary. A closer look at the rear panel shows some very neat AC mains wiring:

Now for some more close-ups. Here we can see the use of the MM5369 17-stage oscillator/divider (data sheet.pdf). I haven’t seen one of these for a while, the last time we used them was for a 60Hz timebase. However in this case it would be used to create an accurate timebase within which the CFC250 would count the number of incoming pulses:

 The removal of two more screws allows removal of the main PCB from the base of the cabinet, which reveals as such:

There is also an opaque plastic sheet cut to fit, helping insulate the PCB from the rest of the world:

 The PCB is single-sided and very easy to follow. I wonder if it was laid out by hand?

It reminds me of some old kits from the past decade.  Moving forward, there is a metal shield around the PCB area of signal input and low-pass filter:

A quick desolder of three points allows removal of the shield, and reveals the following:

At the top-left of the above image reveals a resistor in a somewhat elevated position, as shown below:

If anyone can explain this one, please leave a comment below.

 Conclusion

What impressed me the most during this teardown was the simple way in that the unit was designed – all through-hole parts, mechanical connections either soldered or nuts and bolts, and all components labelled. I can imagine that during the lifespan of the CFC250 it would have been relatively simple to repair. Such is the price of progress. And yes, it worked after putting it all back together again.

In the meanwhile, full-sized original images are available on flickr. I hope you found this article of interest. Coming soon we will have some more older-technology items to examine and some new tutorials as well.

In the meanwhile, have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column, or join our Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Posted in cfc250, teardown, tektronix, test equipment, vintageComments (9)

Kit review – Sparkfun Frequency Counter kit

Hello everyone

Today we examine a kit that is simple to construct and an interesting educational tool – the Sparkfun Frequency Counter kit. This is a revised design from a kit originally released by nuxie1 (the same people who brought us the original function generator kit). As a frequency counter, it can effectively measure within the range of 1 to a claimed 6.5 MHz. Unfortunately the update speed and perhaps accuracy is limited by the speed of the microcontroller the kit is based upon – the Atmel ATmega328. Arduino fans will recognise this as the heart of many of their projects.

Interestingly enough the kit itself is a cut-down version of an Arduino Duemilanove-standard board, without the USB and power regulation hardware. The ATmega328 has the Arduino bootloader and the software (“sketch”) is open source (as is the whole kit) and easily modifiable. This means you can tinker away with your frequency counter and also use your kit as a barebones Arduino board with LCD display. More about this later.

This becomes more obvious when looking at the PCB:

pcbss

It was a little disappointing to not find any power regulator or DC socket – you need to provide your own 5V supply. However Sparkfun have been “clever” enough to include a cable with JST plug and socket to allow you to feed the frequency counter from their function generator kit. In other words, buy both. Frankly they might as well just have produced a function generator with frequency counter kit all on one PCB. Anyhow, let’s get building.

The kit comes in a nice reusable stiff red cardboard box. One could probably mount the kit in this box if they felt like it. The components included are just enough to get by. The LCD is a standard 16 x 2 character HD44780-compatible display. (More on these here). It has a black on green colour scheme. You could always substitute your own if you wanted a different colour scheme:

partsss

An IC socket is not included. You will need to install one if you intend to reprogram the microcontroller with another Arduino board.

Assembly was quick and painless. I couldn’t find any actual step-by-step instructions on the internet (Sparkfun could learn a lot from adafruit in this regard) however the component values are printed on the PCB silk-screen; furthermore no mention of LCD connection, but the main PCB can serve as a ‘backpack’ and therefore the pins line up.

To make experimenting with this kit easier I soldered in some header pins to the LCD and matching socket to the main PCB; as well as adding pins for an FTDI cable (5V) to allow reprogramming direct from the Arduino IDE:

lcdsocketss

So there are in fact two ways to reprogram the microcontroller – either pull it out and insert into another Arduino board, or do it in-place with a 5V FTDI cable. Either way should be accessible for most enthusiasts. At this point one can put the screen and LCD together and have a test run. Find a nice smooth 5V DC power source (from an existing Arduino is fine), or perhaps plug it into USB via a 5V FTDI cable – and fire it up:

itworksss

Well, that’s a start. The backlight is on and someone is home. The next step is to get some sort of idea of the measurement range, and compare the accuracy of the completed kit against that of a more professional frequency counter. For this exercise you can observer the kit and my Tek CFC-250 frequency counter measuring the same function generator output:

As you can see the update speed isn’t that lively, and there are some discrepancies as the frequencies move upward into the kHz range. Perhaps this would be an example of the limitations caused by the CPU speed. Next on the to-do list was to make the suggested connection between the function generator kit and the frequency counter. This is quite simple, you can solder the included JST socket into the function generator board, and solder the wires of the lead included with the frequency counter as such:

boardsss

When doing so, be sure to take notice about which PCB hole is connected to which hole, the colours of the wire don’t match the assumed description on the function generator PCB. Furthermore, the voltage applied via the WAVE pin (the frequency source) should not fall outside of 0~+5V.

As mentioned earlier, this kit is basically a minimalist Arduino board, and this gives the user some scope with regards to modification of the software/sketch. Furthermore, the kit has been released under a Creative Commons by-sa  license. So you can download the schematic, Arduino sketch and EAGLE files and create your own versions or updates. If doing so, don’t forget to attribute when necessary.

Overall, this was anther interesting and easy kit to assemble. It is ideal for beginners as there isn’t that much soldering, they end up with something relatively useful, and if you have a standard Arduino Uno or similar board you can upgrade the firmware yourself.

However as a standalone frequency counter, perhaps not the best choice. Think of this kit as an educational tool – involving soldering, Arduino programming and learning how frequency counters work. In this regard, the kit is well suited.

As always, thank you for reading and I look forward to your comments and so on. Furthermore, don’t be shy in pointing out errors or places that could use improvement. Please subscribe using one of the methods at the top-right of this web page to receive updates on new posts. Or join our Google Group.

High resolution images are available on flickr.

[Note – The kit was purchased by myself personally and reviewed without notifying the manufacturer or retailer]

Posted in arduino, kit review, KIT-10140Comments (2)

Part review – Linear Technology LTC6991 “Timerblox” low frequency oscillator

Hello Readers

Time for a new component review – the Linear Technology LTC6991 low frequency oscillator. This is part of Linear‘s Timerblox series of tiny timing devices. The full range is described on their web site. It is available in DFN or SOT-23 (below)  packaging. Our example for today:

The graph paper in the image is 5mm square, so the IC itself is tiny yet worthwhile challenge. Although reading the data sheet may convince you it is a difficult part to use, it is actually quite simple. This article will give you the “simple way”. Once again I have lashed out and will hand-solder an SMD onto a SOT-23 board:

Messy, but it works. Moving along…

My reason for examining the LTC6991 was as a lower-power substitute to using a 555 timer to create a square wave at various frequencies. Normally I wouldn’t give two hoots about the current draw, as everything on my bench is powered from a lab supply.

However when designing things for external use, they are usually powered by a battery of some sort or solar – so the less current drawn the better. The bog-standard TI NE555 has a current draw (with output high) of between two and five milliamps (at 5V). Which doesn’t sound like much – but our 6991 is around 100 to 170 microamps at 5V. These figures are for the respective timers without an output load. You can source up to 20mA from the output of the 6991, and when doing so will naturally increase the current load – but still it will be less than our triple-nickel.

The LTC6991 offers a period range of 1 millisecond to 9.5 hours; which translates to a frequency range of 29.1 microhertz to 977 Hz, with a maximum frequency error or <1.5%. Only one to three external resistors are required to setup your timing requirements. For a more detailed explanation, please see the data sheet.pdf. The duty cycle defaults to 50% however this can be altered by using the IC in voltage-controlled period mode.

Linear have made using the IC very easy by providing an Excel spreadsheet you can use to make your required calculations, available from this page. For example, to create a 1 Hz oscillator, we enter our figures in as such:

and the macro returns the following details:

xls2

Very convenient – a schematic, the required resistors, and example timing diagram. I recreated this example, however not having the exact values in stock caused a slight increase in frequency – with Rset at 750k,  Rdiv1 at 910k and Rdiv2 at 180k my frequency was 3.1 Hz. Therefore to match the accuracy of the LTC6991 you need to ensure a your external components are close to spec and a very low tolerance. It produces a good square-wave:

sqw1hzss

If you cannot use the exact resistor values recommended, use resistors in series or parallel to achieve the desired values. Don’t forget to measure them in real life if possible to ensure your accuracy does not suffer.

Pin one (RST) can be left floating for nomal oscillation, when high it resets the IC and forces output (pin six) low. As you can see, it is very simple to use especially with the provided spreadsheet. The required formulae are also provided in the data sheet if you wish to do your own calculations. Pulse width can be controlled with a fourth resistor Rpw, and is explained on page sixteen of the data sheet.

Although physically it may be difficult to use as it is SMD, the power requirements and the ability to generate such a wide range of oscillations with so few external parts makes the LTC6991 an attractive proposition.

The LTC6991 and the Timerblox series are new to market and should be available from the usual suppliers in the very near future such as RS and element-14.

As always, thank you for reading and I look forward to your comments and so on. Furthermore, don’t be shy in pointing out errors or places that could use improvement. Please subscribe using one of the methods at the top-right of this web page to receive updates on new posts. Or join our Google Group.

[Note – The LTC6991was a personally-ordered sample unit from Linear and reviewed without notification]

Posted in education, LTC6991, part review, tutorialComments (4)

Education – Introduction to the Optocoupler

Hello everyone!

Today we are going to start examining Optocouplers. These are an interesting and quite convenient component, and relatively easy to implement.

First of all, what is an optocoupler?

It is a small device that allows the transmission of a signal between parts of a circuit while keeping those two parts electrically isolated. How is this so? Inside our typical optocoupler are two things – an LED and a phototransistor. When a current runs through the LED, it switches on  – at which point the phototransitor detects the light and allows another current to flow through it. And then when the LED is off, current cannot flow through the phototransistor. All the while the two currents are completely electrically isolated (when operated within their stated parameters!)

Let’s have a look at some typical optocouplers. Here are the schematic symbols for some more common units:

schematicsss

Switching DC current will flow from A to B, causing current to flow from C to D. The schematic for figure one is a simple optocoupler, consisting of the LED and the photo-transistor. However, this is not suitable for AC current, as the diode will only conduct current in one direction. For AC currents, we have an example in figure two – it has diodes positioned to allow current to flow in either polarity. Figure three is an optocoupler with a photodarlington output type. These have a much higher output gain, however can only handle lesser frequencies (that is, they need more time to switch on and off).

Physically, optocouplers can be found in the usual range of packaging, such as:

4n25

Notice the DIP casing doesn’t have the semi-circle moulded into one end like ICs do, so the white dot indicates pin one.

TO-78 (“Sputnik!”)

surface-mount SOIC-8

Some of you may be thinking “why use an optocoupler, I have a relay?” Good question. There are many reasons, including:

  • Size and weight. Relays are much larger, and heavier;
  • Solid state – no moving parts, so no metal fatigue;
  • Optocouplers are more suited to digital electronics – as they don’t have moving parts they can switch on and off much quicker than a relay;
  • Much less current required to activate than a relay coil
  • The input signal’s impedance may change, which could affect the circuit – using an optocoupler to split the signal removes this issue;

Furthermore, the optocoupler has many more interesting uses. Their property of electrical isolation between the two signals allows many things to be done. For example:

  • you might wish to detect when a telephone is ringing, in order to switch on a beacon. However you cannot just tap into the telephone line. As the ring is an AC current, this can be used with an AC-input optocoupler. Then when the line current starts (ring signal) the optocoupler can turn on the rest of your beacon circuit. Please note that you most likely need to be licensed to do such things. Have a look at the example circuits in this guide from Vishay: Vishay Optocouplers.pdf.
  • You need to send digital signals from an external device into a computer input – an optocoupler allows the signals to pass while keeping the external device electrically isolated from the computer
  • You need to switch a very large current or voltage, but with a very small input current;
  • and so on…

But as expected, the optocoupler has several parameters to be aware of. Let’s look at a data sheet for a very common optocoupler, the 4N25 – 4N25 data sheet.pdf – and turn to page two. The parameters for the input and output stages are quite simple, as they resemble those of the LED and transistor. Then there is the input to output isolation voltage – which is critical. This is the highest voltage that can usually be applied for one second that will not breach the isolation inside the optocoupler.

Side note: You may hear about optoisolators. These are generally known as optocouplers that have output isolation voltages of greater than 5000 volts; however some people regularly interchange optocouplers and optoisolators.

The next parameter of interest is the current-transfer ratio, or CTR. This is the ratio between the output current flow and the input current that caused it. Normally this is around ten to fifty percent – our 4N25 example is twenty percent at optimum input current. CTR will be at a maximum when the LED is the brightest – and not necessarily at the maximum current the LED can handle. Once the CTR is known, you can configure your circuit for an analogue response, in that the input current (due to the CTR) controls the output current.

needabench

Finally, the frequency, or bandwidth the optocoupler can accept.  Although this can be measured in microseconds, these parameters can be altered by other factors. For example, the higher the frequency of the current through the input stage, the less accurate the output stage can render the signal. The phototransistors can also be a function of the maximum bandwidth; furthermore if the optocoupler has a darlington output stage, the bandwidth can be reduced by a factor of ten. Here is an example shown on the old cathode-ray oscilloscope. I have set up a digital pulse, at varying frequencies. The upper channel on the display is the input stage, and the lower channel is the output stage:

Notice as the frequency increases, the ability of the output stage to accurately represent the input signal decreases, for example the jitter and the generally slow fall time. Therefore, especially working with high speed digital electronics, the bandwidth of your optocoupler choice does need to be taken into account.

Thus ends the introduction to optocouplers. I hope you understood and can apply what we have discussed today. In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other –  and we can all learn something.

Some information from various Isocom and Vishay data sheets and publications; various optocoupler images from element14.

Posted in 4N25, education, learning electronics, lesson, optocoupler, tutorialComments (14)

Education – Introduction to Alternating Current

Hello everyone!

Today we are going to introduce the basics of AC – alternating current. This is necessary in order to understand future articles, and also to explain in layperson’s terms what AC is all about. So let’s go!

AC – Alternating Current. We see those two letters all around us. But what is alternating current? How does current alternate? We know that DC (direct current) is the result of a chemical reaction of some sort – for example in a battery, or from a solar cell. We know that it can travel in either direction, and we have made use of it in our experimenting. DC voltage does not alter (unless we want it to).

Therein lies the basic difference – and why alternating current is what is is – it alternates! 🙂 This is due to the way AC current is created, usually by a generator of some sort. In simple terms a generator can be thought of as containing a rotating coil of wire between two magnets. When a coil passes a magnet, a current is induced by the magnetic field. So when the coil rotates, a current is induced, and the resulting voltage is relative to the coil’s positioning with the magnets.

For example, consider the diagram below (exploded view, it is normally more compact):

generator

This is a very basic generator. A rotating coil of wire is between two magnets. The spacing of the magnets in real life is much closer. So as the coil rotates, the magnetic fields induce a current through the coil, which is our alternating current. But as the coil rotates around and around, the level of voltage is relative to the distance between the coil and the magnet. The voltage increases from zero, then decreases, then increases… as the coil constantly rotates. If you were to graph the voltage level (y-axis) against time (x-axis), it would look something like below:

sin1

That graph is a sine wave… and is a representation of perfect AC current. If you were to graph DC voltage against time, it would be a straight horizontal line. For example, compare the two images below, 2 volts DC and AC, shown on an oscilloscope:

2v-dc-cro-small

2 volts DC

The following clip is 2 volts AC, as shown on the oscilloscope:

So as you can see, AC is not a negative and positive current like DC, it swings between negative and positive very quickly. So how do you take the voltage measurement? Consider the following:

sin2

The zero-axis is the point of reference with regards to voltage. That is, it is the point of zero volts. In the oscilloscope video above, the maximum and minimum was 2 volts. Therefore we would say it was 2 volts peak, or 2Vp. It could also be referred to as 4 volts peak to peak, or 4Vpp – as there is a four volt spread between the maximum and minimum values of the sine wave.

There is another measurement in the diagram above – Vrms, or volts root mean squared. The Vrms value is the amount of AC that can do the same amount of work as the equivalent DC voltage. Vrms = 0.707 x Vp; and Vp = 1.41 * Vrms. Voltages of power outlets are rated at Vrms instead of peak as this is relative to calculations. For example, in Australia we have 240 volts:

241vacs

Well, close enough. In fact, our electricity distributor says we can have a tolerance of +/- 10%… some rural households can have around 260 volts. Moving on…

The final parameter of AC is the frequency, or how many times per second the voltage changes from zero to each peak then back to zero. That is the time for one complete cycle. The number of times this happens per second is the frequency, and is measured in Hertz (Hz). The most common frequency you will hear about is your domestic supply frequency. Australia is 50 Hz:

50-hzss

… the US is 60 Hz, etc. In areas that have a frequency of 60 Hz, accurate mains-powered time pieces can be used, as the seconds hand or counter can be driven from the frequency of the AC current.

The higher the frequency, the shorter the period of time taken by one cycle. The frequency and time are inversely proportional, so frequency = 1/time; and time – 1/frequency. For example, if your domestic supply is 50 Hz, the time for each cycle is 1/50 = 0.02 seconds. This change can be demonstrated quite well on an oscilloscope, for example:

In the video above there is 2 volts AC, and the frequency starts from 100 Hz, then moves around the range of 10 to 200 Hz. As you can see, the amplitude of the sine wave does not change (the height, which indicates the voltage) but the time period does alter, indicating the frequency is changing. And here is the opposite:

This video is a demonstration of changing the voltage, whilst maintaining a fixed frequency. Thus ends the introduction to alternating current. In the next instalment about AC we will look at how AC works in electronic circuits, and how it is handled by various components.

I hope you understood and can apply what we have discussed today. As always, thank you for reading and I look forward to your comments and so on. Furthermore, don’t be shy in pointing out errors or places that could use improvement.

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