A PIC12F675 provides all of the necessary functions. It reads the (10k) linear potentiometer and generates appropriately timed pulses to control the DC motor running the fan. The extreme low position cuts the power entirely.The circuit is relatively simple. The wire to the blower motor (either high or low side) is cut and connection is made to the circuit with the more positive side at the top connection.
The diode and 100uf capacitor always provide full 12 volts because the Pulse Width Modulation scheme always provides a minimum ‘off’ time of the power cycle.
The LM78L05 (note unusual pinouts) provides 5 volts for the PIC. Depending on the setting of the 10k pot, GP5 sends out positive pulses with duty cycles between 0 and 98 percent.
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