This circuit makes use of an integrated LM2907. The circuit is from an old documentation being applied to cars that still used platinum, common at the time.
This 1967 Raytheon circuit uses an optical coupler of the era and has a range of 800 to 200 Hz....
This interesting digital counter with relays with complementary outputs was found in the Radio...
This circuit uses a diode as a radiation sensor. The operational ones are old because the...