This circuit reproduces the dial noise in a small speaker. The transistor can be a general-purpose type. We found it in an 80's Electronic Design.
This bank of switches can be programmed in a variety of ways for logic experiments. The circuit is from...
Found in a Popular Electronics from the 90s, this circuit can act as a transmitter for an audio...
This circuit was found in a 90s Hands-on Electronics. The transistor can be a BC548. What it does...