In this project, I designed a nightlight that automatically dims and turns itself off. A lot of people find it difficult to fall asleep when they suddenly transition from a brightly lit room to complete darkness. A dim source of light such as a nightlight helps them to adjust to a lower brightness and fall asleep more easily. But the down side of most nightlights is that they waste electricity because they are on all night when you really only need them to be on while you are falling asleep. So I designed a nightlight that will automatically dim and turn itself off after a certain period of time that is set by the user
This is the circuit that I designed for this project. To power this project, I am using a 6V 400 mA DC power supply. Its open-circuit voltage (no load) measured by a volt meter is approximately 10V. With the load of this project the operating voltage is about 9.5V. The circuit can be broken into two main parts, a timer circuit and a dimming circuit. The timer is made from a 741 OP AMP wired as a comparator. It compares the voltage across a capacitor with a reference voltage that is set by R2 and R3. When S2 is pressed C1 is charged to the supply voltage. C1 then gradually discharges through R1. As long as the voltage across C1 is greater than the reference voltage, the output of the OP AMP is high (about 8.7V). This keeps C2 charged. When the voltage across C1 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 variable resistor is set.
When this happens, C2 begins to slowly discharge through R4. This begins the dimming cycle. The second 741 OP AMP is wired as a unity gain amplifier. The output mirrors the voltage across C2. As the voltage across C2 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. A SPST sliding switch turns the power on and off.
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).
Then if the breadboard prototype works, solder it onto a circuit board. I am using a style of printed circuit board that is laid out just like a breadboard. So I don’t have to change my layout very much. In order to conserve space I am stacking some components. So it is important to make sure that there are no shorts. To connect the variable resistor, I am using a strip of PC jumper wires with some of the wires trimmed off. For my light source I am using three LEDs in series whose combined voltages is close to the supply voltage so I am forgoing adding a resistor. I tend to use a lot of heat shrink tubing to insulate my solder connections. I find that it helps avoid unwanted shorts.
Once you have the circuit constructed, find a suitable housing. Then drill some holes for the LEDs, the switches, the dial and the power cord. Trim your circuit board so that it is only as big as it has to be. This will really help when it comes to fitting everything in the housing. Finally, load in all the components and your project is complete.
The last step is making it look nice. If you want, you can added a diffuser or something to scatter the light. You can tint the LEDs gels or just liquid highlighter. The final aesthetic details are up to you. I put mine in a decorative lantern.