Found in a PE Hobbyst Handbook of 1991, this circuit was originally proposed to time photographic baths in developing rooms but can be used for other purposes. The circuit beeps at 30-second intervals.
This VCO makes use of an RC4558 which is an operational amplifier with characteristics close to...
Found in an EDN from the 90s, this circuit uses operational amplifiers from that time, which are...
Found in a Popular Electronics of the 90s this configuration can be seen elsewhere on our site....