We found this circuit in a 1996 publication. The circuit uses components that are still common. The timing depends on C1, which can be changed, and the triac can be of any type.
We found this project in a 1982 publication. What we have here is an audio signal relay trigger, which...
We found this circuit in an 80's Radio Shack publication. The circuit uses a company buzzer, but...
This circuit was found in a 90s Popular Electronics. It uses a varicap that admits equivalents. The...