Today we have more compact solutions. This is from a Radio Electronics magazine from October 1990. It uses the technology of the time.
This circuit was obtained from an old American documentation having a limited adjustment range....
This 555 timed touch switch was found in a 90s Hands-On Electronics. It triggers a buzzer, but a...
The transistors of this old circuit from the 60s can be the BC548, in a current version. The...