This circuit was found in 1980's Radio Electronics. The circuit produces a 1500 Hz modulate 35 kHz. The components used are still common.
This 10 kHz filter is based on a 741 operational amplifier. The circuit is from Electronics Now...
This circuit can be used with common drills connected to the 110 V network. With change of R1 and...
Found in an Electronics World of 1966 this circuit is intended for use with a microphone as an...