Getting Started on a Breadboard

Here's a simple circuit for producing a sound from a piezo disc. Normally, beginners get introduced to the astable multivibrator as their first oscillator (I think that was the first true electronic circuit I ever built), but I've gone in a different direction here. This is a current-mode astable. I've simplified it a bit - resistors rather than current sources, and the bias arrangement holding the base of the first transistor is a bit useless but seems to work anyway [on the evidence of this, don't give me a job as an engineer anyone].

Here is the circuit

Here it is on a breadboard - the red and black leads go to a power supply set to 9V. The current consumption is a few milliamps, so it would be fine on a small 9V battery.

And here is a waveform, because waveforms always look cool when you're blogging about electronics (this is the voltage across the piezo disc). With the 100n capacitor, the frequency is just over 2kHz.

As it stands it's a bit boring - just a single, monotonous (literally, a mono tone) sound - but it could be extended to do other things. With a latch it could be an alarm. With a selection of different resistor values it could be an electronic organ (try experimenting with different value resistors from the emitter of the lefthand transistor to ground - from 1k to 10k gives about an octave range in sound). Anyway, just an idea - it's certainly cheap and all the bits can be reused in other circuits.

Here's a simple circuit for producing a sound from a piezo disc. Normally, beginners get introduced to the astable multivibrator as their first oscillator (I think that was the first true electronic circuit I ever built), but I've gone in a different direction here. This is a current-mode astable. I've simplified it a bit - resistors rather than current sources, and the bias arrangement holding the base of the first transistor is a bit useless but seems to work anyway [on the evidence of this, don't give me a job as an engineer anyone].

Here is the circuit

Here it is on a breadboard - the red and black leads go to a power supply set to 9V. The current consumption is a few milliamps, so it would be fine on a small 9V battery.

And here is a waveform, because waveforms always look cool when you're blogging about electronics (this is the voltage across the piezo disc). With the 100n capacitor, the frequency is just over 2kHz.

As it stands it's a bit boring - just a single, monotonous (literally, a mono tone) sound - but it could be extended to do other things. With a latch it could be an alarm. With a selection of different resistor values it could be an electronic organ (try experimenting with different value resistors from the emitter of the lefthand transistor to ground - from 1k to 10k gives about an octave range in sound). Anyway, just an idea - it's certainly cheap and all the bits can be reused in other circuits.

Was going to suggest that. My first circuit was an astable multivibrator and I had lots of fun with it. Particularly, I was using it to mess up the TV reception when my grandparents were watching the news and I couldn't follow my favorite cartoons. Fun times.

I'd also suggest a couple of off the shelf voltage regulators to show how you can get different voltages and also explain power and power dissipation.

I was being a bit optimistic when I suggested that the circuit above could be used as an electronic organ. I've just tried it with a potentiometer to vary the value of the emitter resistor of the leftmost transistor and, instead of the tone varying smoothly, it jumped in steps - the circuit was evidently being pulled to the different physical resonances of the piezo disc. It's interesting to play with - I've just spent a while getting all manner of weird sounds out of it - but for a beginner the astable multivibrator or a 555 timer would probably be a safer bet.

Other problems, if anyone feels inclined to experiment with it, are that the right transistor will sometimes oscillate at a few megahertz (superimposed on the wanted  waveform; emitter followers are prone to that kind of thing; adding a small-value resistor to the base will stop it) and it doesn't always start up, depending on the components used and the frequency you're trying to achieve.