Using the CA3140 integrated circuit, this switch was found in the June 1986 Radio Electronics magazine. Sensitivity adjustment is made at the trimpot in series with the sensor.
This circuit was found in a 1967 Raytheon manual. The transistors are from the time. Equivalent modern...
This circuit was published in a 1979 magazine, but it can still be easily assembled as the...
The circuit presented generates short pulses. This configuration was found in a publication from...