Introduction: "Candle Flicker" LED As Quasi-Square Wave Generator (Geiger Counter Simulator)

About: I've been taking things apart since I was 10. My mother wasn't impressed, even though I told her I knew how to put it back together... I've been making things since I picked up my first soldering iron (By The …

Those ubiquitous "Candle Flicker" LEDs are fascinating in more ways than one. There is plenty of information on the internet about getting an audio signal from a flicker LED. They seem to either play a tune or produce a weird sort of "music" reminiscent of a cheap sci-fi movie. The one I first experimented with did the latter. (I'm working with the type that has the "flicker" chip built into the LED.) Like a lot of other consumer junk, these devices can also be hacked into something more interesting.

This all started because I already had a 555 timer breadboarded and I started thinking about making a fake Geiger counter sound effect.

Step 1: What Can You Do With a "Flicker" LED?

The flicker LED is a special form of flashing LED. Unlike the flasher, it appears to use a form of pulse modulation rather than simple dimming. This makes sense from a digital circuit standpoint. It's also much more efficient from a power usage standpoint, just as a switching power regulator doesn't waste a lot of power as heat.

The above photo shows the waveform my test LED generated. I wanted an "On-Off" effect rather than dimming, chiefly because I wanted to simulate the clicks from a Geiger counter without having to go to the expense of using an Arduino, and I don't much like dealing with programming, at least not for something this simple.

The circuit I came up with does that, but even if you don't need a fake Geiger counter, it's also useful for producing a quasi-random "flashing" effect instead of a "dimming" effect.

Step 2: Theory of Operation

This circuit, except for the flicker LED, is based on the old reliable 555 timer IC, available from everybody that's anybody in the electronics world (Even Radio Shack!) the 555 is an extremely versatile and easy to work with IC, available in many variations including CMOS, and operates with a wide variety of supply voltages from 3V up.

This 555 is wired as a monostable multivibrator (That's a mouthful!), also called a one-shot. This means it will output one pulse whenever it's triggered by an external signal that arrives at pin 2. The pulse duration is controlled by R and C in this diagram.

A pull-up resistor (shown on the next page) keeps the 555 un-triggered until the LED flickers.

Every time the flicker LED flickers, it triggers the 555, producing a square output wave.

All you need to know about the 555 timer.

Step 3: Design

Here's the actual circuit. It's identical to the previous diagram except for the means of triggering and the output device.

R3 serves 2 purposes: it's a series resistor to lower the flicker LED voltage to around 3 volts - the working voltage for these devices. It also, with R2, forms a voltage divider across the 555 trigger pin. As the current through R3 varies, so does the voltage across it. R2 serves as a pull-up resistor, keeping the trigger pin high until the LED flickers, lowering the voltage across R3 and triggering the 555. The ratio of R2 to R3 may need to be tweaked for different flicker LEDs, but R3 should be around 1K for a supply voltage of 9 volts. The second flicker LED I experimented with caused some circuit instability. Placing a .01 uF capacitor (not shown) across R3 fixed this.

R1 and C1 control the "on" time of the circuit. these can be played around with to get the desired effect. R1 should be no smaller than 1K. C1 can be any size, I've had the most pleasing effect with 4.7-10 uF.

I really don't know why C2 is there, but it's recommended, so I use it. The circuit seems to work fine without it, though.

C3 keeps DC out of the speaker and modifies the resulting sound. The value of C3 can be changed, and will change the tone of the 'Clicks.' The output can be fed into a small amplifier for more volume. A piezo speaker can also be used, and does not require the capacitor.

An optional LED (a normal one, this time) can be connected to the output, and will flash in time with the clicks.

Step 4: Parts and Tools Required

Parts list for the circuit I built:

"Flickering" LED (From "flickering" LED Candle, any color)

U1 LM555C

C1 4.7-10 uF

C2 .01 uF

C3 .22-.47 uF (See Notes)

C4 .01 uF (Optional, see step 5)

R1 10K

R2 3K

R3 1K

R4 470 Ohms

SP1 Small speaker or piezo element (See Notes)

Optional output LED (Any Color)

Perfboard, solderless breadboard or PC board, wire, solder, etc

9 Volt battery or power supply

Notes: Can substitute Piezo element for speaker and C3.
R1 and C1 control timing. Values shown give 110 Milliseconds. These can be changed, within reason. Experiment!

Pull-up resistor R2 may need changing for different flicker LEDs.

Tools required:

None if you use a solderless breadboard... Otherwise:

Soldering iron, solder, and soldering ability

wire cutters

small needle-nose pliers

This is a minimum list. This instructable assumes you know something about assembling electronics. If you've built even one simple kit successfully, you can build this. If you need help on the basics, try:

And by the way, "solder" is pronounced "saw-der," not "soul-der!" There, I've gotten that off my chest!

Edit: One of our friends from "across the Pond" informs me that it is, in fact pronounced "soul-der" over there. I guess we really are separated by a common language!

Step 5: Assembly and Testing

This is an experimenter's project and, as they say, your mileage may vary, so put down the soldering iron and back away slowly!

I strongly suggest you breadboard your circuit before you finalize anything. Solderless breadboards are cheap compared to the heartbreak of putting it together with solder and finding out you then have to unsolder a bunch of stuff to change something.

You can see my breadboarding is anything but neat. That's because I save the neatness for the finished product. There is a danger here, though. If your breadboard is too sloppy, you might cause a short and fry something. If you have to put your project away for a while, you'll probably knock a component loose and then go crazy troubleshooting something that already worked once. So if you're going to be using the circuit for a while on it's breadboard, take a little time to make it neater.

As you can see from the video, it works! (After I fried one 555 by hooking up power backwards. Buy several 555s. They're ridiculously cheap, you'll find lots of uses for them, and you'll eventually ruin one or two.) The flashing light is a blue LED I hooked up for testing.

The project will be put into it's final form after I get the rest of my parts, which are on a slow boat from China... They say you can have it good, fast, or cheap: Pick any two. Well, I guess I'd rather have it cheap than fast!

The rest of my flicker LEDs finally arrived, and these caused an instability where they would stop flickering after a while. I placed a .01 uF capacitor across R3, and that fixed it. As I said earlier, breadboard first!

Step 6: The Result

We get a nice clean quasi-random square wave output from the 555. The pulse width (and the resulting sound and light) can be modified by changing either R1 or C1. As you can see from the video, the red (standard) LED flashes on and off every time the blue (flicker) LED flickers. This project is ready for solder!

If you need a primer on soldering, read the "soldering comic." It's the best simplest basic instruction I've found so far.

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