This circuit was found in a 1992 publication. What it does is to light an external lamp when it gets dark. The trigger point adjustment is made in a potentiometer or trimpot and the sensor is a common LDR.
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....