This circuit represents a very simple and inexpensive alternative to an electrical switch used to turn ON and OFF the battery-operated dc motor typically employed in handheld fans. It eliminates the mechanical component that could suffer from humidity, condensation or bad contacts.
The circuit works by toggling between two stability points (see the figure). When the motor is OFF, it shows a low impedance to its terminals by forcing Q1 into its OFF state. Thus, transistor Q2 has no bias current to turn on the motor. The fan stays off indefinitely and the circuit power consumption depends solely on the junction leakage of the two transistors.
You can switch ON the motor by simply giving it a small rotation with your hand. This movement produces a voltage drop between the motor terminals, which is sufficient to turn on transistor Q1, in turn switching ON Q2 and the motor circuit. When the motor is rotating, Q2 is ON, forcing node A to low voltage. This allows Q1 to remain ON, providing a collector current that maintains the circuit in this stability point. The fan rotates continuously until you switch it off by simply stopping it (obviously, this circuit should only be used in very specific applications). The value of resistor R modifies the two stability conditions of the circuit.
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