# Tutorial – LM3915 Logarithmic Dot/Bar Display Driver IC

Introduction

This is the second of three articles that will examine the LM391x series of LED driver ICs. The first covered the LM3914, this will cover the LM3915 and the LM3916 will follow. The goal of these is to have you using the parts in a small amount of time and experiment with your driver ICs, from which point you can research further into their theory and application.

Although these parts have been around for many years, the LM3915 isn’t used that much however for the sake of completeness we’re writing the tutorial. The LM3915 offers a simple way to display a logarithmic voltage level using one or more groups of ten LEDs with a minimum of fuss. If you’re wanting to make a VU meter, you should use the LM3916 which we will cover in the final instalment of this trilogy.

Instead of having each LED represent a voltage level as with the LM3914, each LED connected to the LM3915 represents a 3 dB (decibel) change in the power level of the signal. For more on decibels, check out Wikipedia.

To display these power level changes we’ll run through a couple of examples that you can use in your own projects and hopefully give you some ideas for the future. Originally by National Semiconductor, the LM391X series is now handled by Texas Instruments.

Getting Started

You will need the LM3915 data sheet, so please download that and keep it as a reference. First – back to basics. The LM3915 controls ten LEDs. It controls the current through the LEDs with the use of only one resistor, and the LEDs can appear in a bar graph or single ‘dot’ when in use. The LM3915 contains a ten-stage voltage divider, each stage when reached will illuminate the matching LED (and those below it in level meter mode).

Let’s consider the most basic of examples (from page two of the data sheet) – a simple logarithmic display of voltage between 0 and 10V:

After building the circuit you can connect a signal to measure via pin 5, and the GND to pin 2. We’ve built the circuit exactly as above on some stripboard for demonstration purposes, with the only difference being the use of an 8.2kΩ resistor for R2:

To show this in action we use a signal of varying AC voltage – a sine wave at around 2 kHz. In the following video, you can see the comparison of the signal’s voltage against the LEDs being illuminated, and you will see the logarithmic voltage increase represented by the LEDs:

We used the bar display mode for the voltage increase, and the dot display mode for the voltage decrease. Did you notice that during the voltage decrease, the LEDs below the maximum level being displayed were dim? As the signal’s voltage was varying very quickly, the change in the LED’s location is a blur due to the speed of change. In the video below, we’ve slowed the frequency right down but kept the same maximum voltage.

Well that was a lot of fun, and gives you an idea of what is possible with the LM3915.

Displaying weaker signals

In non-theoretical situations your input signal won’t conveniently be between 0 and 10 V. For example the line level on audio equipment can vary between 1 and 3V peak to peak. For example, here’s a random DSO image from measuring the headphone output on my computer whilst playing some typical music:

Although it’s an AC signal we’ll treat it as DC for simplicity. So to display this random low DC voltage signal we’ll reduce the range of the display to 0~3V DC. This is done using  the same method as with the LM3914 – with maths and different resistors.

Consider the following formulae:

As you can see the LED current (Iled) is simple, however we’ll need to solve for R1 and R2 with the first formula to get our required Vref of 3V. For our example circuit I use 2.2kΩ for R2 which gives a value of 1.8kΩ for R1. However putting those values in the ILED formula gives a pretty low current for the LEDs, about 8.3 mA. Live and learn – so spend time experimenting with values so you can match the required Vref and ILED.

Nevertheless in this video below we have the Vref of 3V and some music in from the computer as a sample source of low-voltage DC. This is not a VU meter! Wait for the LM3916 article to do that.

Again due to the rapid rate of change of the voltage, there is the blue between the maximum level at the time and 0V.

Chaining multiple LM3915s

This is covered well in the data sheet, so read it for more on using two LM3915s. Plus there are some great example circuits in the data sheet, for example the 100W audio power meter on page 26 and the vibration meter (using a piezo) on page 18.

Conclusion

As always I hope you found this useful. Don’t forget to stay tuned for the final instalment about the LM3916. And if you made it this far – check out my new book “Arduino Workshop” from No Starch Press.

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.

# Tutorial – LM3914 Dot/Bar Display Driver IC

Introduction

This is the first of three tutorials that will examine the LM391x series of LED driver ICs. In this first tutorial we cover the LM3914, then the LM3915 and LM3916 will follow. The goal of these tutorials is to have you using the parts in a small amount of time and experiment with your driver ICs, from which point you can research further into their theory and application.

Although these parts have been around for many years, the LM3914 in particular is still quite popular. It offers a simple way to display a linear voltage level using one or more groups of ten LEDs with a minimum of fuss.

With a variety of external parts or circuitry these LEDs can then represent all sorts of data, or just blink for your amusement. We’ll run through a few example circuits that you can use in your own projects and hopefully give you some ideas for the future. Originally by National Semiconductor, the LM391X series is now handled by Texas Instruments.

Getting Started

You will need the LM3914 data sheet, so please download that and keep it as a reference. So – back to basics. The LM3914 controls ten LEDs. It controls the current through the LEDs with the use of only one resistor, and the LEDs can appear in a bar graph or single ‘dot’ when in use. The LM3914 contains a ten-stage voltage divider, each stage when reached will illuminate the matching LED (and those below it in level meter mode).

Let’s consider the most basic of examples (from page two of the data sheet) – a voltmeter with a range of 0~5V:

The Vled rail is also connected to the supply voltage in our example. Pin 9 controls the bar/dot display mode – with it connected to pin 3 the LEDs will operate in bar graph mode, leave it open for dot mode. The 2.2uF capacitor is required only when “leads to the LED supply are 6″ or longer”. We’ve hooked up the circuit above, and created a 0~5V DC source via a 10kΩ potentiometer with a multimeter to show the voltage – in the following video you can see the results of this circuit in action, in both dot and bar graph mode:

Customising the upper range and LED current

Well that was exciting, however what if you want a different reference voltage? That is you want your display to have a range of 0~3 V DC? And how do you control the current flow through each LED? With maths and resistors. Consider the following formulae:

As you can see the LED current (Iled) is simple, our example is 12.5/1210 which returned 10.3 mA – and in real life 12.7 mA (resistor tolerance is going to affect the value of the calculations).

Now to calculate a new Ref Out voltage – for example  we’ll shoot for a 3 V meter, and keep the same current for the LEDs. This requires solving for R2 in the equation above, which results with R2 = -R1 + 0.8R1V. Substituting the values – R2 = -1210 + 0.8 x 1210 x 3 gives a value of 1694Ω for R2. Not everyone will have the E48 resistor range, so try and get something as close as possible. We found a 1.8 kΩ for R2 and show the results in the following video:

You can of course have larger display range values, but a supply voltage of no more than 25 V will need to be equal to or greater than that value. E.g. if you want a 0~10 V display, the supply voltage must be >= 10V DC.

Creating custom ranges

Now we’ll look at how to create  a lower range limit, so you can have displays that (for example) can range from a non-zero positive value. For example, you want to display levels between 3 and 5V DC. From the previous section, you know how to set the upper limit, and setting the lower limit is simple – just apply the lower voltage to pin 4 (Rlo).

You can derive this using a resistor divider or other form of supply with a common GND. When creating such circuits, remember that the tolerance of the resistors used in the voltage dividers will have an affect on the accuracy. Some may wish to fit trimpots, which after alignment can be set permanently with a blob of glue.

Finally, for more reading on this topic – download and review the TI application note.

Chaining multiple LM3914s

Two or more LM3914s can be chained together to increase the number of LEDs used to display the levels over an expanded range. The circuitry is similar to using two independent units, except the REFout (pin 7) from the first LM3914 is fed to the REFlo (pin 4) of the second LM3914 – whose REFout is set as required for the upper range limit. Consider the following example schematic which gave a real-world range of 0~3.8V DC:

The 20~22kΩ resistor is required if you’re using dot mode (see “Dot mode carry” in page ten of the data sheet). Moving on, the circuit above results with the following:

Where to from here?

Now you can visually represent all sorts of low voltages for many purposes. There’s more example circuits and notes in the LM3914 data sheet, so have a read through and delve deeper into the operation of the LM3914. Furthermore Dave Jones from eevblog.com has made a great video whcih describes a practical application of the LM3914:

Conclusion

As always I hope you found this useful. Don’t forget to stay tuned for the second and third instalments using the LM3915 and LM3916. Full-sized images are on flickr. And if you made it this far – check out my new book “Arduino Workshop” from No Starch Press.

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.

# Tutorial – Arduino and PCF8563 real time clock IC

Use the NXP PCF8563 real-time clock IC with Arduino in chapter fifty-four of our Arduino Tutorials. The first chapter is here, the complete series is detailed here.

Updated 20/08/2013

Introduction

Recently a few people have been asking about the PCF8563 real-time clock IC from NXP – so this is a tutorial on how to use it for time, date, alarm clock and square-wave generation purposes.

The PCF8563 is another inexpensive RTC that can be used with an Arduino or other platforms due to the wide operating voltage (1 to 5.5V DC), I2C interface, and very low power consumption (when powered by a backup battery it only draws 0.25 μA). If you aren’t up to speed on the I2C interface, please review the I2C tutorials before moving forward. And please download the data sheet (.pdf).

The PCF8563 is available in various chip packages, for the curious we’re using the TSSOP8 version mounted on a breakout board:

Don’t panic – you can also get it in a breadboard-friendly DIP (through-hole) package as well, and also on a pre-built module from the usual suspects.

Demonstration Circuit

If you have a pre-made module, you can skip to the next section. However if you’re making up the circuit yourself, you will need:

• One 32.768 kHz crystal
• Two 1N4148 diodes*
• One 3V coin cell (with holder)*
• Two 10kΩ resistors
• One 0.1 uF capacitor

And here’s the schematic:

* You can skip the diodes and battery if you don’t want a backup power supply when the main power is turned off or removed. Pin 3 is for the interrupt output (we’ll consider that later) and pin 7 is for the square-wave oscillator output.

Communicating with the PCF8563

Now to get down into the land of I2C once more. When looking through the data sheet NXP mentions two bus addresses, which have the same 7-bits finished with either a 1 for read or 0 for write. However you can just bitshift it over one bit as we don’t need the R/W bit – which gives you a bus address of 0x51.

Next you need to know which registers store the time and date – check the register map (table 4) on page 7 of the data sheet:

There will be a few other registers of interest, but we’ll return to those later. For now, note that the time and date start from 0x02. And one more thing – data is stored in the BCD (binary-coded- decimal) format. But don’t panic, we have a couple of functions to convert numbers between BCD and decimal.

Writing the time and date is a simple matter of collating the seconds, minutes, hours, day of week, day of month, month and year into bytes, converting to BCD then sending them to the PCF8563 with seven Wire.write() functions. Reading the data is also easy, just set the pointer to 0x02 and request seven bytes of data – then run them through a BCD to decimal conversion. With a catch.

And that catch is the need to sort out unwanted bits. Revisit table 4 in the data sheet – if you see an x that’s an unused bit. If any of them are a 1 they will mess up the BCD-decimal conversion when reading the register, so they need to be eliminated just like a whack-a-mole. To do this, we perform an & (bitwise AND) operation on the returned byte and mask out the unwanted bits with a zero. How does that work?

Example – the byte for dayOfMonth is returned – we only need bits 5 to 0. So 6 and 7 are superfluous. If you use (dayOfMonth & B00111111) the & function will set bits 6 and 7 to zero, and leave the other bits as they were.

Now to put all that together in a demonstration sketch. It puts everything mentioned to work and simply sets the time to the PCF8563, and then returns it to the serial monitor. The data is kept in global variables declared at the start of the sketch, and the conversions between BCD and decimal are done “on the fly” in the functions used to send or retrieve data from the PCF8563. Read through the following sketch and see how it works for yourself:

And a quick video of this in operation:

If all you need to do is write and read the time with the PCF8563, you’re ready to go. However there’s a few more features of this unassuming little part which you might find useful, so at least keep reading…

Square-wave output

As with any clock or RTC IC, an oscillator is involved, and as mentioned earlier you can take this from pin 7 of the PCF8563. However – it’s an open-drain output – which means current flows from the supply voltage into pin 7. For example if you want to blink an LED, connect a 560Ω resistor between 5V and the anode of the LED, then connect the cathode to pin 7 of the PCF8563.

The frequency is controlled from the register at 0x0D. Simply write one of the following values for the respective frequencies:

• 10000000 for 32.768 kHz;
• 10000001 for 1.024 kHz;
• 10000010 for 32 kHz;
• 10000011 for 1 Hz;
• 0 turns the output off and sets it to high impedance.

The following is a quick demonstration sketch which runs through the options:

And the resulting waveforms from slowest to highest frequency. Note the sample was measured from a point between the LED and resistor, so the oscillations don’t vary between the supply voltage and zero:

Self-awareness of clock accuracy

The PCF8563 monitors the oscillator and supply voltage, and if the oscillator stops or the voltage drops below a certain point – the first bit of the seconds register (called the VL bit) is set to 1. Thus your sketch can tell you if there’s a chance of the time not being accurate by reading this bit. The default value is 1 on power-up, so you need to set it back to zero after setting the time in your sketch – which is done when you write seconds using the code in our example sketches. Then from that point it can be monitored by reading the seconds register, isolating the bit and returning the value.

Examine the function checkVLerror() in the following example sketch. It reads the seconds byte, isolates the VL bit, then turns on D13 (the onboard LED) if there’s a problem. The only way to restore the error bit to “OK” is to re-set the time:

And now for a demonstration of the error-checking at work. We have the PCF8563 happily returning the data to the serial monitor. Then the power is removed and restored. You see D13 on the Arduino-compatible board turn on and then the error is displayed in the serial monitor:

This function may sound frivolous, however if you’re building a real product or serious project using the PCF8563, you can use this feature to add a level of professionalism and instil confidence in the end user.

Alarm Clock

You can use the PCF8563 as an alarm clock, that is be notified of a certain time, day and/or day of the week – at which point an action can take place. For example, trigger an interrupt or turn on a digital output pin for an external siren. Etcetera. Using the alarm in the sketch is quite similar to reading and writing the time, the data is stored in certain registers – as shown in the following table from page seven of the data sheet:

However there is a catch – the MSB (most significant bit, 7) in the registers above is used to determine whether that particular register plays a part in the alarm. For example, if you want your alarm to include hours and minutes, bit 7 needs to be set to 1 for the hour and minute alarm register. Don’t panic – you can easily set that bit by using a bitwise OR (“|”) and B10000000 to set the bit on with the matching data before writing it to the register.

Checking if the alarm has occurred can be done with two methods – software and hardware. Using software you check bit 3 of the register at 0x01 (the “AF” alarm flag bit). If it’s 1 – it’s alarm time! Then you can turn the alarm off by setting that bit to zero. Using hardware, first set bit 1 of register 0x01 to 1 – then whenever an alarm occurs, current can flow into pin 3 of the PCF8563. Yes – it’s an open-drain output – which means current flows from the supply voltage into pin 3. For example if you want to turn on an LED, connect a 560Ω resistor between 5V and the anode of the LED, then connect the cathode to pin 3 of the PCF8563. To turn off this current, you need to turn off the alarm flag bit as mentioned earlier.

Now let’s put all that into a demonstration sketch. It’s documented and if you’ve been following along it shouldn’t be difficult at all:

This is the same as the example 54.1, however we’ve added the required functions to use the alarm. The required alarm data is stored in the global bytes:

and is written to the PCF8563 using the function:

Note the use of bitwise OR (“|”) to add the enable bit 7 to the data before writing to the register. The interrupt pin is also set to activate at the end of this function, however you can remove that part of the code if unnecessary. We also demonstrate checking the alarm status via software using the function:

which simply reads the AF bit in the register at 0x01 and let’s us know if the alarm has occurred via the Serial Monitor. In this function you can add code to take action for your required needs. It also calls the function:

which retrieves the contents of the register at 0x01, sets the AF bit to zero and writes it back. We do this to preserve the status of the other bits in that register. For the curious and non-believers you can see this sketch in action through the following video, first the software and then the hardware interrupt pin method (an LED comes on at the alarm time and is then turned off:

Conclusion

Hopefully you found this tutorial useful and now have the confidence to use the PCF8563 in your own projects. Furthermore I hope you learned something about the I2C bus and can have satisfaction in that you didn’t take the lazy option of using the library. People often say to me “Oh, there’s a library for that”, however if you used every library – you’d never learn how to interface things for yourself. One day there might not be a library! And then where would you be? So learning the hard way is better for you in the long run. If you have any questions leave a comment below, or ask privately via the contact page.

And if you enjoy my tutorials, or want to introduce someone else to the interesting world of Arduino – check out my book (now in a second printing) “Arduino Workshop” from No Starch Press.

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.

# Tutorial – Arduino and MC14489 LED Display Driver

Learn how to use MC14489 LED display driver ICs with Arduino in chapter fifty-one of a series originally titled “Getting Started/Moving Forward with Arduino!” by John Boxall – A tutorial on the Arduino universe. The first chapter is here, the complete series is detailed here.

Updated 12/05/2013

Introduction

Recently we’ve been looking at alternatives to the MAX7219 LED display driver IC due to pricing and availability issues (stay tuned for that one) – and came across an old but still quite useful IC – the MC14489 from Motorola (now Freescale Semiconductor). The MC14489 can drive five seven-segment LED numbers with decimal point, or a combination of numbers and separate LEDs. You can also daisy-chain more than one to drive more digits, and it’s controlled with a simple serial data-clock method in the same way as a 74HC595 shift register. Sourcing the MC14489 isn’t too difficult – it’s available from element14, Newark, Digikey, and so on – or if you’re not in a hurry, try the usual suspects like Futurlec.

For the purpose of the tutorial we’ll show you how to send commands easily from your Arduino or compatible board to control a five-digit 7-segment LED display module – and the instructions are quite simple so they should translate easily to other platforms. Once you have mastered the single module, using more than one MC14489 will be just as easy. So let’s get started.

Hardware

Before moving forward, download the data sheet (pdf). You will need to refer to this as you build the circuit(s). And here’s our subject in real life:

For our demonstration display we’ll be using a vintage HP 5082-7415 LED display module. However you can use almost any 7-segment modules as long as they’re common-cathode – for example, Sparkfun part number COM-11405. If you’re using a four-digit module and want an extra digit, you can add another single digit display. If you want a ruler, the design files are here.

Connecting the MC14489 to an LED display isn’t complex at all. From the data sheet consider Figure 9:

Each of the anode control pins from the MC14489 connect to the matching anodes on your display module, and the BANK1~5 pins connect to the matching digit cathode pins on the display module. You can find the MC14489 pin assignments on page 1 of the data sheet. Seeing as this is chapter fifty-one  – by now you should be confident with finding such information on the data sheets, so I will be encouraging you to do a little more of the work.

Interesting point – you don’t need current-limiting resistors. However you do need the resistor Rx – this controls the current flow to each LED segment. But which value to use? You need to find out the forward current of your LED display (for example 20 mA) then check Figure 7 on page 7 of the data sheet:

To be conservative I’m using a value of 2k0 for Rx, however you can choose your own based on the data sheet for your display and the graph above.  Next – connect the data, clock and enable pins of the MC14489 to three Arduino digital pints – for our example we’re using 5, 6 and 7 for data, clock and enable respectively. Then it’s just 5V and GND to Arduino 5V and GND – and put a 0.1uF capacitor between 5V and GND. Before moving on double-check the connections – especially between the MC14489 and the LED display.

Controlling the MC14489

To control the display we need to send data to two registers in the MC14489 – the configuration register  (one byte) and the display register (three bytes). See page 9 of the data sheet for the overview. The MC14489 will understand that if we send out one byte of data it is to send it the configuration register, and if it receives three bytes of data to send it to the display register. To keep things simple we’ll only worry about the first bit (C0) in the configuration register – this turns the display outputs on or off. To do this, use the following:

and to turn it off, send bit C0 as zero. The small delay is necessary after each command.

Once you have turned the display on – the next step is to send three bytes of data which represent the numbers to display and decimal points if necessary. Review the table on page 8 of the data sheet. See how they have the binary nibble values for the digits in the third column. Thankfully the nibble for each digit is the binary value for that digit. Furthermore you might want to set the decimal point – that is set using three bits in the first nibble of the three bytes (go back to page 9 and see the display register). Finally you can halve the brightness by setting the very first bit to zero (or one for full brightness).

As an example for that – if you want to display 5.4321 the three bytes of data to send in binary will be:

Let’s break that down. The first bit is 1 for full brightness, then the next three bits (101) turn on the decimal point for BANK5 (the left-most digit). Then you have five nibbles of data, one for each of the digits from left to right. So there’s binary for 5, then four, then three, then two, then one.

To demonstrate everything described so far, it’s been neatly packaged into our first example sketch:

… with the results in the following video:

Now that we can display numbers and a few letters with binary, life would be easier if there was a way to take a number and just send it to the display.

So consider the following function that takes an integer between 0 and 99999, does the work and sends it to the display:

So how does that work? First it splits the 5-digit number into separate digits and stores them in the array numbers[]. It then places the fourth digit into a byte, then moves the data four bits to the left – then we bitwise OR the fifth digit into the same byte. This leaves us with a byte of data containing the nibbles for the fourth and fifth digit. The process is repeated for digits 2 and 3. Finally the brightness bit and decimal point bits are assigned to another byte which then has the first digit’s nibble OR’d into it. Which leaves us with bytes a, b and c ready to send to the MC14489. Note that there isn’t any error-checking – however you could add a test to check that the number to be displayed was within the parameter, and if not either switch off the display (see example 51.1) or throw up all the decimal points or … whatever you want.

You can download the demonstration sketch for the function – Example 51.2, and view the results in the following video:

You can also display the letters A to F by sending the values 10 to 15 respectivel to each digit’s nibble. However that would be part of a larger application, which you can (hopefully) by now work out for yourself. Furthermore there’s some other characters that can be displayed – however trying to display the alphabet using 7-segment displays is somewhat passé. Instead, get some 16-segment LED modules or an LCD.

Finally, you can cascade more than one MC14489 to control more digits. Just run a connection from the data out pin on the first MC14889 to the data pin of the second one, and all the clock and enable lines together. Then send out more data – see page 11 of the data sheet. If you’re going to do that in volume other ICs may be a cheaper option and thus lead you back to the MAX7219.

Conclusion

For a chance find the MC14489 is a fun an inexpensive way to drive those LED digit displays. We haven’t covered every single possible option or feature of the part – however you will now have the core knowledge to go further with the MC14489 if you need to move further with it. And if you enjoy my tutorials, or want to introduce someone else to the interesting world of Arduino – check out my new book “Arduino Workshop” from No Starch Press.

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.

# The 555 Precision Timer IC

Learn about the useful and inexpensive 555 timer IC in this detailed tutorial!

Hello readers

Today we revisit one of the most popular integrated circuits ever conceived – the 555 timer IC. “Triple-five”, “five-five-five”, “triple-nickel” … call it what you will, it has been around for thirty-eight years. Considering the pace of change in the electronics industry, the 555 could be the constant in an ever-changing universe. But what is the 555? How does it work? How can we use it? And … why do we still use it? In this introductory article we will try to answer these questions. If you would like to see some examples, visit here.

What is the 555?

The 555 timer is the solution to a problem found by the inventor – Hans Camenzind.  He saw the need through his radio work for a part that could act as an oscillator or a timer [1]; and working as a contractor for Signetics developed the 555. (Signetics was purchased by Philips in 1975, and their semiconductor division was spun off as NXP in 2006). The 555 has to be one of the most used ICs ever invented. It is used for timing, from microseconds to hours; and creating oscillations (which is another form of timing for the pedants out there). It is very flexible with operation voltage, you can throw from 4.5 to 18V at it; you can sink or source 200mA of current through the output; and it is very cheap – down to around nine cents if you order several thousand units. Finally, the 555 can achieve all of this with a minimum of basic components – some resistors and capacitors.

Here are some examples in the common DIP casing:

Furthermore a quick scan of suppliers’ websites show that the 555 is also available in surface-mount packages such as SOIC, MSOP and TSSOP. You can also source a 556 timer IC, which contains two 555 ICs. (What’s 555 + 555? 556…) Furthermore, a 558 was available in the past, but seems rather tricky to source these days.

How does the 555 work?

The 555 contains two major items:

• A comparator – a device which compares two voltages, and switches its output to indicate which is larger, and
• A flip-flop – a circuit that has two stable states, and those states can be changed by applying a voltage to one of the flip-flop’s inputs.

Here is the 555 functional diagram from the TI 555 data sheet.pdf:

… and the matching pin-out diagram:

Don’t let the diagrams above put you off. It is easier to explain how the 555 operates within the context of some applications, so we will now explore the three major uses of the 555 timer IC in detail – these being astable,  monostable, and bistable operations, in theory and in practice.

Astable operation

Astable is an on-off-on… type of oscillation – and generates what is known as a square wave, for example:

There are three values to take note of:

• time (s) – the time for a complete cycle. The number of cycles per second is known as the frequency, which is the reciprocal of time (s);
• tm (s) – the duration of time for which the voltage (or logic state) is high;
• ts (s) – the duration of time for which the voltage (or logic state) is low.

With the use of two resistors and one capacitor, you can determine the period durations. Consider the following schematic:

Calculating values for R1, R2 and C1 was quite simple. You can either determine the length of time you need (t) in seconds, or the frequency (Hz) – the number of pulses per second.

t (time) = 0.7 x (R1 + [2 x R2]) x C1

f (frequency) = 1.4 / {(R1 + [2 x R2]) x C1}

Where R1 and R2 are measured in ohms, and C1 is measured in farads. Remember that 1 microfarad = 1.0 × 10-6 farads, so be careful to convert your capacitor values to farads carefully. It is preferable to keep the value of C1 as low as possible for two reasons – one, as capacitor tolerances can be quite large, the larger the capacitor, the greater your margin of error; and two, capacitor values can be affected by temperature.

How the circuit works is relatively simple. At the time power is applied, the voltage at pin 2 (trigger) is less than 1/3Vcc. So the flip-flop is switched to set the 555 output to high. C1 will charge via R1 and R2. After a period of time (Tm from the diagram above) the voltage at pin 6 (threshold) goes above 2/3Vcc. At this point, the flip-flop is switched to set the 555 output to low. Furthermore, this enables the discharge function – so C1 will discharge via R2. After a period of time (Ts from the diagram above) the voltage at pin 2 (trigger) is less than 1/3Vcc. So the flip-flop is switched to set the 555 output to high… and the cycle repeats.

Now, for an example, I want to create a pulse of 1Hz (that is, one cycle per second). It would be good to use a small value capacitor, a 0.1uF. In farads this is 0.0000001 farads. Phew. So our equation is 1=1.4/{(R1 + [2 x R2]) x C1}. Which twists out leaving us R1=8.2Mohm, R2=2.9MOhm and C1 is 0.1uF. I don’t have a 2.9MOhm resistor, so will try a 2.7MOhm value, which will give a time value of around 0.9s. C2 in astable mode is optional, and used if there is a lot of electrical noise in the circuit. Personally, I use one every time, a 0.01uF ceramic capacitor does nicely. Here is our example in operation:

Notice how the LED is on for longer than it is off, that is due to the ‘on’ time being determined by R1+R2, however the ‘off’ time is determined by R2 only. The ‘on’ time can be expressed as a percentage of the total pulse time, and this is called the duty cycle. If you have a 50% duty cycle, the LED would be on and off for equal periods of time. To alter the duty cycle, place a small diode (e.g. a 1N4148) over pins 7 (anode) and 2 (cathode). Then you can calculate the duty cycle as:

Tm = 0.7 x R1 x C1 (the ‘on’ time)

Ts = 0.7 x R2 x C1 (the ‘off’ time)

Furthermore, the 555 can only control around 200mA of current from the output to earth, so if you need to oscillate something with more current, use a switching transistor or a relay between the output on pin 3 and earth. If you are to use a relay, put a 1N4001 diode between pin 3 (anode) and the relay coil (cathode); and a 1N418 in parallel with the relay coil, but with the anode on the earth side. This stops any reverse current from the relay coil when it switches contacts.

Monostable operation

Mono for one – one pulse that is. Monostable use is also known as a “one-shot” timer.  So the output pin (3) stays low until the 555 receives a trigger pulse (drop to low) on pin 2. The length of the resulting pulse is easy to calculate:

T = 1.1 x R1 x C1;

where T is time in seconds, R1 is resistance in ohms, and C1 is capacitance in farads. Once again, due to the tolerances of capacitors, the longest time you should aim for is around ten minutes. Even though your theoretical result for T might be 9 minutes, you could end up with 8 minutes 11 seconds. You might really need those extra 49 seconds to run away…  Though you could always have one 555 trigger another 555… but if you were to do that, you might as well use a circuit built around an ATmega328 with Arduino bootloader.

Now time for an example. Let’s have a pulse output length of (as close as possible to) five seconds. So, using the equation, 5 = 1.1 x R1 x C1… I have a 10 uF capacitor, so C1 will be 0.00001 farads. Therefore R1 will be 454,545 ohms (in theory)… the closest I have is a 470k, so will try that and see what happens. Note that it you don’t want a reset button (to cancel your pulse mid-way), just connect pin 4 to Vs. Here is the schematic for our example:

How the monostable works is quite simple. Nothing happens when power is applied, as R2 is holding the trigger voltage above 1/3Vcc. When button S1 is pushed, the trigger voltage falls below 1/3Vcc, which causes the flip-flop to set the 555’s output to high. Then C1 is charged via R1 until the threshold voltage 2/3Vcc is reached, at which point the flip-flip sets the output low and C1 discharges. Nothing further happens until S1 is pressed again. The presence of the second button S2 is to function as a reset switch. That is, while the output is high the reset button, if pressed, will set the output low and set C1 to discharge.

Below is a video of my example at work. First I let it run the whole way through, then the second and subsequent times I reset it shortly after the trigger. No audio in clip:

Once again, we now have a useful form of a one-shot timer with our 555.

Bistable operation

Bistable operation is where the 555′s output is either high, or low – but not oscillating. If you pulse the trigger, the output becomes and stays high, until you pulse reset. With a bistable 555 you can make a nice soft-touch electronic switch for a project… let’s do that now, it is so simple you don’t need one of my quality schematics. But here you are anyway:

In this example. pressing S1 sets the voltage at pin 2 (trigger) to below 1/3Vcc, thereby setting the output to high – therefore we call S1 our ‘on’ switch. As pin 6 (threshold) is permanently connected to GND, it cannot be used to set the output to low. The only way to set the output back to low is by pressing S2 – the reset button, which we can call the ‘off’ switch. Couldn’t be easier, could it? And that output pin could switch a transistor or a relay on or off, who knows? Your only limit is your imagination. And here’s one more video clip:

And there you have it – three ways in which we can use our 555 timer ICs. But in the year 2011, why do we still use a 555? Price, simplicity, an old habit, or the fact that there are so many existing designs out there ready to use. There will be many arguments for and against continued use of the 555 – but as long as people keep learning about electronics, the 555 may still have a long and varied future ahead of it.

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References

[1] “The 555 Timer IC – An interview with Hans Camenzind” (Jack Ward – semiconductormuseum.com)

Various diagrams and images from the Texas Instruments NE555 data sheet.