Also a good starting point for novices wanting to get their hands dirty with the 555 timer IC. Some would argue that this is not the most efficient approach, but hey (read the title), it's simple, and it works.
Check the video out.
More images and description here.
Pulse Width Modulation (PWM)
Put simply PWM is the process of switching power ON and OFF to a device in pulses at a specific frequency. Same approach used in commercial light dimmers, DC motor speed controller, CPU fan speed controllers and etc.
That's what we seek to achieve here.
Step 1: Part List
1) 555 timer IC - 1
2) 100K variable resistor - 1
3) 1N4148 Diode - 2
4) 100nF capacitor - 2
The 555 Timer IC
The 555 timer is arguably one of the most popular IC ever made. There are thousands of resources online if you're interested to delve deeper into the subject. I'm just going to give the simple description directly relevant to the build
PIN 1 - Ground
PIN 2 - Trigger
When LOW, it causes the Output pin to go HIGH. Activated when voltage fall
below 1/3 of +V.
PIN 3 - Output
Output is HIGH when Trigger pin is LOW. Output is LOW when Threshold pin is
HIGH. Output is LOW when reset pin is LOW.Output pin is able to source or sink
PIN 4 - Reset
Short to +V when not in use.
PIN 5 - Control Voltage
Grounded through a capacitor when not in use.
PIN 6 - Threshold
When voltage reaches 2/3 of +V, this pin will cause Output to be driven LOW.
PIN 7 - Discharge
Grounded when Output pin goes HIGH.
PIN 8 - +V
Step 2: How It Works.
When the circuit is powered up, the C1 capacitor will initially be in a discharged state. Thus, the Trigger (pin 2) will be LOW, driving the Output (pin 3) to go HIGH. Discharge (pin 7) goes HIGH and goes ground. The cycle begins.
The HIGH Output will cause C1 capacitor to be charged through the R1 and D1 path. Upon C1 voltage reaching 2/3 of +V, the Threshold (pin 6) will be activated and drive the Output (pin 3) LOW. Discharge (pin 7) goes LOW. The time it takes for C1 to charge depends on the position of R1.
Since Output (pin 3) is now LOW, capacitor C1 will start to discharge through the D2 and R1 path. When the voltage of C1 drops below 1/3 of +V, Trigger (pin 2) will be LOW, driving Output (pin 3) to go HIGH, and Discharge (pin 7) to go HIGH and shorts to ground. The cycle repeats itself.
You've probably noticed by now that the circuit is using Discharge (pin 7) to drive the motor, simply by going ground in each cycle. You can add some amount of protection if you're concerned about back EMF from the motor.
Pin 4 and 5 are not used, and pin 1 is simply tied to ground. The circuit can take between +3v to +18v. The Frequency is around 144Hz. Do note that, doubling the value of C1 will reduce frequency to half, tripling will will reduce frequency to 1/3, and so on.