This variable filter that lets frequencies from 500 Hz to 1500 Hz pass through was found in a Popular Electronics from the 90's. The operational amplifiers are the 741.
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....