Continuity tester (CB7386E)

This circuit was found in a Hands-On Electronics from the 90's. The FET can be any general purpose.

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Heterodyne Frequency Meter (CB22320E)
This circuit determines the frequency of a signal by beating with a known signal. The circuit was...
Variable Timer 555 (CB15097E)
With the values of the components indicated, this timer controls intervals from a few seconds to a...
Water Pump Automatic Control (CB18101E)
The purpose of this circuit is to automatically turn a water pump on and off. Sensor 1 is the...