555 PWM Motor Controller
Manual-knob PWM speed controller for a small DC motor, driven by a 555 timer and an N-channel MOSFET.
Difficulty: Breadboard. Estimated build time: about 40 minutes. Estimated parts cost: about US$4.50. 11-line bill of materials. Compare supplier offers when available. A pulse-width-modulated motor controller built without any microcontroller.
Power and bypass
Wire 6 V (4×AA pack, or a bench supply) to the upper rail and ground to the lower rail. Drop a 100 nF ceramic across the supply close to the 555 to keep its rail clean.
555 in astable
Place the NE555P. Tie pin 4 (reset) and pin 8 (Vcc) together to +6 V; pin 1 (GND) to ground; 10 nF from pin 5 to ground. Bridge pin 2 to pin 6. Connect a 100 nF timing capacitor from pin 6 to ground.
PWM steering network
Connect a 10 kΩ potentiometer with one end at pin 7 (discharge) and the other at pin 6 (threshold). Wipers go nowhere on their own — instead we add two 1N4148 diodes back-to-back across the pot ends so charge current flows one way through one half of the pot, and discharge current flows the other way through the other half. Sliding the wiper changes the ratio of charge time to discharge time without changing the total period — that is the duty-cycle knob.
MOSFET driver
Connect pin 3 (output) through a 220 Ω gate resistor to the gate of the IRLZ44N. Tie a 100 kΩ from gate to ground so the MOSFET is held off if the 555 is removed. The source goes to ground; the drain goes to the motor's negative terminal. The motor's positive terminal goes straight to +6 V.
Add the flyback diode
Place a 1N4007 across the motor with the cathode (banded end) facing +6 V. When the MOSFET turns off, the motor's coil tries to keep current flowing — without this diode that energy would spike the drain voltage and slowly degrade the MOSFET. Apply power, turn the pot, and watch the motor spool up and down smoothly. Listen carefully — at 1 kHz you may hear a faint whistle; raise the timing cap to 220 nF if it bothers you (you'll lose a little top-end speed).