## Various 555 Timer circuits

The purpose of this article is to follow on from our explanation of the 555 timer IC by demonstrating some simple yet interesting, noisy and plain annoying uses of the 555. They are by no means that complex, and intended to help move theory into practice.

Button de-bouncer

De-bouncer? How does one bounce a button in the first place? Many years ago I bounced a button on the arcade Sonic the Hedgehog – hit it so hard it popped out and bounced over the table… But seriously, when working with digital logic circuits, you may need to use  a momentary button to accept user input. For example, to pulse a trigger or so on. However with some buttons, they are not all that they seem to be. You press them once, but they can register multiple contacts – i.e. register two or more ‘presses’ for what seems like only one press. This could possibly cause trouble, so we can use a 555 timer monostable circuit to solver the problem. In our de-bounce example, when the button is pressed, the output is kept at high for around half a second. Here is the schematic:

What we have is a basic monostable timer circuit. For my example the output delay (t) is to be half a second. The formula for t is: t=1.1xR1xC1. The closest resistor I had at hand was 2k ohms, so to find the required value for C1, the formula is rearranged into: C1=t/(1.1xR1). Substituting the values for t and R1 gives a value of C1 as 227.274 uF. So for C1 we have used a 220 uF capacitor.

Now for a visual demonstration of the de-bouncer at work. In the following video clip, the oscilloscope is displaying the button level on the lower channel, and the output level on the upper channel. The button level when open is high, as the 555 requires a low pulse to activate. The output level is normally low. You can see when the button is pressed that the button level momentarily drops to low, and then the output level goes high for around half a second:

Make some noise

As we know the 555 can oscillate at frequencies from less than 1Hz to around 500 kHz. The human ear can theoretically hear sounds between (approximately) 20 and 20 kHz. So if we create an astable timing circuit with an output frequency that falls within the range of the human ear, and connect that output to a small speaker – a range of tones can be emitted.

The circuit required is a standard 555 astable, with the output signal heading through a small 8 ohm 0.25 watt speaker and a 4.7 uF electrolytic capacitor to ground. The capacitor stops any DC current flowing to ground, without this we will overload the current-handling ability of the 555. (I couldn’t help myself by trying it without the capacitor – pulled 550 mA from the 555 before it stopped working…). To choose the values of R1 and C1 to emit out required frequency, the following formula is used: f (frequency) = 1.4 / {(R1 + [2 x R2]) x C1}. To cover the range required, a 100k ohm trimpot was used for R1. Here is the resulting schematic:

The input voltage can fall within the specification of the 555, however for optimum results a supply of between 5 and 9 volts DC should be used. In the following demonstration, we used a 9V supply. The purpose of the video is to learn the relationship between the tones and their frequencies. You can see the frequency on my old counter and hopefully hear the result:

Our next example is to create a  siren effect, using two 555 circuits – one for a low frequency and one for a high frequency. To determine the value for R1 for the low and high frequency, I used the previous circuit and chose two tones that were quite different, and measured the resistance of the trimpot (R1) at those frequencies. My R1 value for the ‘low’ tone is 82k ohm and 36k ohm for the ‘high’ frequency.

The switching between low and high frequency will be handled by a 4047 multivibrator – the Q and Q outputs will control NPN transistors. The transistors are used as switches to allow current to flow from the supply to the 555 high or low tone circuit. We use this method as the 4047 is not able to source enough current to drive the 555 circuits. Here is the schematic:

Don’t forget to connect pin 14 of the 4047 to supply voltage. This circuit has been tested with a supply voltage between 5 and 12 volts. As the supply voltage increases, so does the amplitude of the square wave emanating from the 555 output pins, which in turn in creases the volume of the siren. At 5 volts, the entire circuit drew only 20 milliamps. Speaking of which, you can listen to a recording of the output here. If you wish to alter the time for each tone, adjust the value of what is the 47k ohm resistor on pins 2 and 3 of the 4047.

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.

## Kit Review – adafruit industries Ice Tube clock v1.1

Today we examine a kit that perhaps transcends from general electronic fun and games into the world of modern art – the adafruitIce Tube” clock.

What is an Ice Tube clock? Before LCDs (liquid-crystal displays) were prevalent another form of display technology was popular – the vacuum-fluorescent display (or VFD). This clock uses a VFD originally manufactured in the former Soviet Union (link for the kids) or Russia (I think mine is date-stamped January 1993). This particular VFD contains a series of seven-segment digits and a dot, which allow the display of time in a bright and retro fashion.

Since this kit was released I had always desired one, however my general parsimonious traits and the wavering exchange rate against the US dollar kept my spending in check. But lately my wallet was hit by a perfect storm: the Australian dollar hit parity with the greenback, adafruit had a discount code and I felt like spending some money – so before the strange feelings passed I ordered a kit post-haste.

Sixteen slow, hot days later the box arrived. I must admit to enjoying a good parcel-opening:

As always, the packaging was excellent and everything arrived as it should have. But what was everything?

Included is the anti-static bag containing the PCB and general components, a bag with the laser-cut acrylic pieces to assemble the housing, another bag with the housing fasteners and the back-up coin cell for the clock, a mains adaptor, and finally another solid cardboard box containing the classic display unit – albeit with the following sensible warning:

And finally the Russian IV-18 display tube:

The tube is a fascinating piece of work, certainly a piece of perfect retro-technology and a welcome addition to my household. Assembling the clock will not be a fast process, and in doing so I recommend reviewing the detailed instructions several times over at the adafruit website. Furthermore, it is a good idea to identify, measure and line up the components ready for use, to save time and confusion along the way. Your experience may vary, however this kit took around three hours for me to construct.

Normally with most kits you can just solder the components in any order, however it is recommended you follow the instructions, as they are well written and allow for testing along the way. For example, after installing the power regulator, you can check the output:

At this stage, you can test your progress with the piezo beeping at power-on:

These mid-construction tests are a good idea as you can hopefully locate any problems before things get out of hand. Another item to be careful with is the PLCC socket for the Maxim MAX6921 VFD driver IC (second from the left):

However with time and patience there is no reason why you would have any problems. Once the main PCB is completed, the next item is the end PCB which connects to the VFD:

At this point it is a good time to have a break and a bit of a stretch, as you need all your patience for soldering in the VFD. Before attempting to do so, try and carefully straighten all the wires from the VFD so they are parallel with each other. Then using the adafruit instructions, make sure you have the tube wires lined up with the correct hole on the PCB:

It is also a good idea to check the gap between the VFD and the PCB is correct, by checking the fit within the housing:

And after much patience, wire pulling with pliers, and light soldering –  the VFD was married to the PCB:

So now the difficult soldering work has been completed and now it was time for another test – the big one… does it all work?

Yes, yes it does. *phew* The low brightness is normal, as that is the default level set by the software. Please note: if you run your VFD without an enclosure that you must be careful of the high voltages on the right-hand side of the PCB and also the VFD PCB. If you test your VFD in this manner, don’t forget to allow ten minutes for the voltage to return to a safe level after removing the power supply. If you have been following the instructions (I hope so!) there is some more soldering to do, after which you can put away your soldering iron.

Now to remove the liner from the acrylic housing pieces and put it all together. Be very careful not to over-tighten the bolts otherwise you will shatter the housing pieces and be cranky. If all is well, you’re finished clock will appear as such:

The clock in use:

And finally, our ubiquitous video demonstration:

VFDs can lose their brightness over the years, and can be difficult to replace – so if you want many, many years of retro-time it would be smart to buy an extra tube from adafruit with your kit, or a modified DeLorean.

Overall, this was an interesting and satisfying kit to assemble. Not for the beginner, but if you have built a few easier kits such as  the “TV-B-Gone” with success, the Ice Tube clock will be within your reach. Furthermore, due to the clear housing, this kit is a good demonstration of your soldering and assembly skills. High resolution images are available on flickr.

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

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