Heartbeat Detector (CB25472S)

This heartbeat sensor circuit was found in the July 1992 issue of Radio Electronics magazine. The circuit uses a 741 with a single power supply.

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Optical control notch filter (CB6717E)
This 1967 Raytheon circuit uses an optical coupler of the era and has a range of 800 to 200 Hz....
Even and odd counter (CB23780E)
This interesting digital counter with relays with complementary outputs was found in the Radio...
Dosimeter (CB5922E)
This circuit uses a diode as a radiation sensor. The operational ones are old because the...