Step #1: CircuitPrevNext
The circuit can be broken into two main parts, a timer circuit and a dimming circuit. The timer (left) is made from a 741 OP AMP wired as a comparator. The dimmer (right) is made from 741 OP AMP wired as a voltage follower (or a unity gain amplifier)
Step #2: Timer circuitPrevNext
- The timer is made from a 741 op amp (operational amplifier) wired as a comparator. It compares the voltage across a capacitor with a reference voltage that is set by the 2.2k resistor and the 5k potentiometer.
- When S2 is pressed the capacitor is charged to the supply voltage. It then gradually discharges through the 1M resistor. As long as the voltage across the capacitor is greater than the reference voltage, the output of the op amp is high (about 8.7V). When the voltage across the capacitor drops below the reference, the output of the op amp goes low (about 1.9V). This can take 0-45 minutes depending on how the potentiometer is set.
Step #3: Dimmer CircuitPrevNext
While the output of the timer is HIGH, it keeps the second capacitor charged. When the timer output goes LOW, this capacitor begins to slowly discharge through the 3.3M resistor. This begins the dimming cycle. The output of the second 741 op amp mirrors the voltage across the capacitor. As the voltage across the second capacitor drops, so does the output voltage and the LEDs dim. It should take about 45 minutes for the LEDs to go from full brightness to full darkness. Pressing the button at any point will reset the whole cycle.
Step #4: Breadboard PrototypePrevNext
Testing your circuit on a breadboard before soldering can help work out bugs.
Step #9: Modify the designPrevNext
The duration of time that the lights are on at full brightness and the time that they dim can be modified by changing the values of R1, C1, R4 and C2. By changing the ratios of the resistors and capacitors you change how quickly the capacitors will discharge. For a decent estimate of how the capacitors will discharge you can use the formula Vc=Vo×e(-t/RC). Feel free to change the values to meet your needs.