Re: 1970s disco light displays
There was a circuit design in a 1970s Practical Electronics magazine which I built - it used 741 op-amps as active filters to separate the audio into 3 channels [bass, mid, treble] which then fed Triacs to activate the bulbs.
The entire circuit sat at mains-voltage - the 741 bit was powered by a capacitive dropper. 'Safety' was obtained by feeding the audio signal through a 1:1 600-Ohm line-transformer.
My version worked reasonably well but would occasionally suffer a sort of latch-up where all 3 bulbs came on at full brightness and wouldn't go out even when the audio input was disconnected, and needed the mains removed to reset it. I suspect something somewhere was oscillating but I wasn;t going to go poking around with a 'scope bearing in mind the live-chassis.
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