A friend showed me this sound effect circuit from Wilf Rigter. It produces surprisingly nice bird sounds with good variations, while using very few components.
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| Original schematic |
I wanted to understand the circuit better, so I started by drawing it in Eagle.
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| Redraw |
I then rearranged the circuit for clarity. Starting at the top left, we have an LFO (IC1B) modulating the pitch of another oscillator (IC1C). IC1C also has a Light Dependent Resistor that affects its pitch.
IC1C modulates the pitch of the main oscillator formed by IC1E and IC1D. It appears to be a take on the textbook transistor astable-multivibrator. Its output drives a speaker via IC1F.
At the bottom left we have one more LFO that serves to turn the main oscillator on and off. This keeps the oscillator from droning on. You instead get bursts of sound.
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| Rearranged |
Finally I made a rough board layout. It was easy to route with a single layer, and seems like a good candidate for stripboard adaptation.
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| Board layout |
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