This low pass filter has attenuation of 5.5 dB and the frequencies depend on the values of the components in the case 1 kHz.
Approaching the hand or an object to the sensor of this circuit, the relay closes its contacts. The...
For those who want to recover an old multimeter of this type, the circuit presented is worth...
The circuit shown is only for analog telephone lines. I found it in Popular Electronics (1991). It...