Automatic Light Dimmer

This lab consists in Automatically Dimming an LED according with ambient light. Besides involving an interesting analog transducer, the Photoresistor, this exercise also approaches Pulse Width Modulation (PWM), a technique widely used to control the power supplied to electrical devices, especially to inertial loads such as motors.

In summary, PWM is a modulation technique that consists in dynamically varying the duration (width) of an electrical pulse based on a digital signal. The switching of the output signal between on and off must be fast enough so the circuit being controlled perceives it as a waveform (and not as consecutive stop-and-go signals). Respected the switching rate, PWM acts on the controlled circuit varying the voltage (and current) proportionally to the time the signal is kept on (called duty cycle).

For this lab, a switching frequency a bit above 100 Hz will do. The photoresistor (or LDR) will be connected to the an ADC channel, so the microcontroller can asses the ambient light.

Pseudo code

Let's begin with a pseudo code (actual coding is up to you):

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14. 		int main() {
    set_point = 400;
    duty_cycle = 0;
    while(1) {
        light = adc->read();
        if (light > set_point && duty_cycle > 0)
            duty_cycle--;
        else if (light < set_point && duty_cycle < 255)
            duty_cycle++;
        pwm->write(duty_cycle);
        delay();
    }
    return 0;
}

The pseudo-code is self-explaining: an infinite loop reading the ADC channel to which the photoresitor has been connected, a scaling from the read value to yield the PWM duty cycle and a delay to avoid overshooting.

Schematic

Schematic

Part List

For this lab, you'll need the following components:

Assembly

Fritzing

Solutions

Tools