Found in a 1948 MIT documentation, this circuit uses a 0 to 100 uA microammeter as an indicator. The power supply is 150 V.
This circuit for recognizing a frequency with a narrow range was found in Radio Electronics...
Any small sound amplifier from 200mW to 1W can be used in this simple circuit. Or circuit of two...
This Linear Technology circuit was found in a 1980s documentation. The circuit can swing up to a...