This variable filter circuit was found in July 1975 Radio Electronics magazine. The circuit uses 3 general purpose op-amps like the 741 and must use a symmetrical source.
The automatic modulated siren circuit shown in the figure really produces an interesting sound...
This circuit was found in the August 1973 issue of Radio Electronics magazine. The Darlington...
This circuit has three settings, central sound frequency, modulation speed and modulation depth. The...