Watson’s Buzz Beeper
I built this Buzz Beeper to find out if an audible tone could be used for sending simple commands, like turn on. Recently I built a microphone preamp with a bandpass filter that can detect my whistle. But it can be triggered by loud noise. So how about I generate a tone modulated with an on-off square wave that would have to be detected to trigger the command. The unmodulated tone itself won’t trigger the command, giving some noise immunity.
I connected a piezo beeper to 9 volts DC and I could hear the 4 kHz tone out the room’s door and down the hall. This tone could be used for sensing objects like ultrasonic transducers – I can hear the standing waves as I walk around. So I added an astable multivibrator to drive the piezo with a square wave (see schematic). This gives a “buzzing beep”, a 4 kHz tone that’s buzzing or humming on and off. I think one way of describing it is if you had a 4 kHz signal generator connected to an amplifier without a ground connected.
This tone “carrier” can be received, filtered, detected and the square wave recovered. If it’s present for so many milliseconds it can be considered as a positive command to switch or toggle a circuit.
Without the .01 uF across the piezo beeper, the astable multivibrator wouldn’t oscillate at its proper frequency; it was being triggered by the pulses from the piezo. The 0.01 uF capacitor helped a lot. But the piezo’s on and off is slower.
