This circuit can be used to set the compass for walks or exercises, being small enough to be carried in your pocket. The base is a single junction transistor, and the power is made from a 9 V battery. As the current consumption is very low, the battery life will be very long. The handset must have a low impedance headset and the rhythm is adjusted in the potentiometer. A crystal phone with a 4.7k-ohm resistor can be used in parallel.
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