Seventy years ago this month, the November 1956 issue of Popular Electronics showed this keying monitor circuit, which had been sent in to the magazine by reader L. B. Wyckoff. The circuit is nothing more than an audio oscillator, with two power sources. There is one battery, but the rectifier takes power from a coil placed near the transmitter’s tank coil and rectifies it. So when the key is down, the circuit is fully powered up and generates audio. The magazine notes that the circuit can be used as a code oscillator by replacing the loop with another battery. Of course, the heart of the circuit is the venerable CK722.