Practice code oscillator (CB21090E)

Found in Popular Electronics from the 90's, this circuit consists of an audio oscillator for Morse code practice.

Temperature Control (CB6003E)
This circuit functions as a thermostat triggering a relay when the boil control temperature...
Latching relay (CB25722E)
The figure shows how to connect a relay to obtain the latch effect. The circuit is from an...
Extension Bell (CB15685E)
With this circuit it is possible to use a DC buzzer (not a transducer) as an extension of the...