We found this circuit in an 80s Intersil documentation. Modern equivalent operating systems can be used. The potentiometer adjusts the detection level.
This fast trigger circuit allows the counting of up to 300 pulses per minute. The circuit is from...
This intercom uses transistors from the 60s. It was found in a Japanese publication at the time....
This circuit is from an old publication from the 90s, but it can be adapted to operate with LEDs....