The Chirpie

By Corey Menscher on Day 4

You’ve heard of the Throwie. Well, today I attempted to create the Chirpie! The Chirpie is an audio throwie…a small electronic device that emits a continuous series of chirps and can be attached to metallic surfaces via a magnet. Multiple throwies could be thrown and attached to a surface to provide a cacophony of chirps! I have no idea if Chirpies have been made before, but I decided to attempt it to learn something about tone generation with analog circuits.

My first attempt to create the Chirpie involved using two 555 timers…one in an astable configuration, and one in a monostable configuration. The astable 555 would have a period of ~1hz, and would trigger the monostable 555. The monostable 555 would in turn emit a tone…ideally around 440hz. I don’t know if you’ve every dealt with 555 timers, but they are a pain in the ass. They’re difficult to configure, and doing so requires a number of components (multiple resistors and capacitors, with leads running between voltage dividers and from one chip pin to another). Since I’d be using TWO 555 chips, plus the components in a “freeform” circuit (i.e. no circuitboard), I nixed this idea due to the fact that this tangled mess would be extremely difficult to solder.

Chris Cerrito recommended I use hex inverters (specifically, 74HC14’s) to generate my audio. These are very simple to use, and since the 74HC14’s contain four inverters per chip, this could be a compact one chip solution. I managed to get a tone, but then I realized that there was no way to use one hex inverter on a chip to trigger another one to play at a regular cycle.

It turned out that I needed to use a 4093 NAND gate/hex inverter chip. Each inverter has TWO inputs, plus one output (the 4HC14 only had one input). This enabled me to generate a slow cycle with one inverter, and use the output pin to trigger another inverter with a higher frequency…providing an audible tone. The beauty of this solution is that it only required one chip and four components (two resistors and two capacitors…which determine each inverter’s period). Here are some shots of my circuit:

The problem with this circuit is that the two 3v coin cells I was going to use simply couldn’t handle the current draw. They made an awesome series of descending-frequency chirps…but died after about 5 chirps. I figured I could use either a 9V or a AA battery with a step-up circuit…but I never got far enough to have to make this decision.

My troubles began when I soldered the freeform circuit. Creating a such a circuit is difficult…you have to solder the components directly to chip pins, as well as to each other. You also have to solder wire to the components and chip to act as power and ground bus. And you have to do all this without shorting anything. Unfortunately, that must be exactly what I did. I melted two small speakers, and apparently fried the chip itself with my 9V test. On the off chance that I had simply soldered the circuit incorrectly, I attempted a second try using the other two inverters on the chip…but go nothing. I suppose I fried the one and only 4093 chip I had. (Chris gave it to me.) So, unfortunately, my 5in5 project for today will not be complete. Well not entirely. Here’s what I unded up with:

But the day wasn’t a TOTAL wash. Since I was inspired by LED Throwies, I decided to make one with a superbright LED and the coin cells I was going to use for the Chirpie:

movie of the Throwie in action

So yeah I made SOMETHING…but it ain’t all that. Still, I learned far more than I ever expected to about creating oscillators with analog circuits. I’m actually very happy that I can come away with that useful knowledge…and I’m committed to actually COMPLETING a Chirpie when I can get my hands on another 4093. Only next time, I’ll probably do the whole thing on a perf board and cut it down as small as possible. Freeform circuits are just too unpredictable.

August 1st, 2008