Found in an American documentation from the 90s, this circuit uses small low voltage lamps (6 to 12 V0. The circuit can be adapted to use LED indicators.
We found this sensitive high input impedance voltage amplifier in the November 1970 Radio...
In the figure we show how to add a 100k ohms potentiometer to implement an oscillator controlled by this...
In the figure we have a low-pass filter, with a cutoff frequency of 10 kHz, also suggested by...