Saturday, 30 October, 2004

Code Practice Oscillator

I know the picture's not all that great, but it's about the best I can do with this little Creative PC Cam.  What you're looking at is a code practice oscillator that I build from a schematic I found on the Web.  The thing is incredibly simple, but I managed to have quite a bit of trouble putting it together.  But I was fairly impressed, considering that it was my first electronics project.  I managed to get it working without smoking any parts or burning anything important with the soldering iron.

The heart of the circuit is a 555 timer chip, that performs some kind of magic that normally would take a transistor and a few other parts.  I'm still trying to understand exactly how the circuit works.  My knowledge of resistors, capacitors, and the like is limited to their theory of operation.  I can kind of explain what one of these little do-dads does, but I can't really say what it's good for in a circuit.  I keep learning.

I like the circuit except for a couple of things.  First, the sound is very high pitched.  I lowered the value of the 150 K ohm resister in the circuit to lower the tone, but it didn't seem to help much.  That's a minor problem, and I might try an even lower value to get the squeak out.  The more annoying problem is that the thing "chirps" after it's been resting a while.  That is, I'll press the key and the speaker will output a tone and then quickly transition to a higher tone.  It sounds almost like the "boo-BEEP" of a video game.  I think the problem is that the battery isn't a very well regulated supply.  That's the theory.  One of my ham radio friends suggested I install a voltage regulator chip to see if that solves the problem.

People seem to like putting these little circuits in Altoids tins.  Just to be different, I gutted an old two-button mouse and mounted the thing in there.  The tail consists of the two wires going out to the code key.

A code practice oscillator, by the way, is basically a box that beeps when you complete the circuit.  It's used for practicing Morse Code.