The ISD1820 voice recording IC is a simple, relatively low quality voice recorder. There are lots of details on how to tweak this chip here.
After some experimentation I found that the chip can also be used as a fun voltage controlled filter.
The ISD1820 has a built in low pass filter. This is intended to remove high frequency noise that emerges as the clock speed/pitch is lowered. Under “normal” settings the filter effect is relatively soft. By adding an extended pitch control (as shown in the schematics above and below) the filter range is also drastically increased giving a fairly wide filter cut range.
MAKING A FILTER
By activating the FEEDTHROUGH switch, the sound at the input will be fed to the output through the built in filter. A feedback potentiometer can also be added to give a crude resonance control. Its noisy and a bit unpredictable but effective.
In the schematic below I’ve removed all of the extra hardware thats not necessary to make a filter.
The only extra feature here is the FEEDBACK/RESONANCE setting. The resonance behaviour will change depending on which speaker output is fed back into the mic in. Try both and see which you prefer.