Using a 555, this configuration appears elsewhere on this site. This is from American documentation from the 90s. The indicator can be an LED or some device controlled by a relay.
This circuit produces the ticking of a clock on a common piezoelectric transducer. The circuit has an...
The basis of this circuit is a double T in the feedback, which has the components calculated to...
This configuration was found in the Texas Instruments optoelectronics manual. PNP transistors...