Light Sensing Circuit (without Arduino)

For this project, I made a simple light-sensing circuit that turns on an LED when the surroundings are dark. It can be used as an automatic lamp and it is very intuitive. It is quite easy to make and is also a great project to start off with if you are a beginner. It also comes with its own casing to hide the electronics and look appealing, just like a real lamp would in your own house, just on a smaller scale.

You can find the supplies I used to make the circuit above. Do note that the perfboard is only required for making the circuit permanent. I started off by prototyping and making sure my circuit works on a breadboard first, which allows you to easily reconfigure your components. If you have never done this before, then I would recommend this option. Additionally, I used a few wires to connect some of the components, like the LED to the potentiometer, and two wires for the positive and negative terminals of the circuits.

Parts:

5mm White LED

Transistor

Photoresistor/LDR

100K Potentiometer

1K Resistor

470Ω Resistor

Wires

The first step I took was prototyping and testing my circuit out on temporary options. Firstly, I used Tinkercad, which is an online platform for modelling circuits and has various electronic components digitally. I used it to model my circuit online and test for various types of resistors to ensure that the voltage at the LED would not fry it. I found the best range was between 400-500 ohms for this circuit. In my case, I used 460, which allowed the white LED to appear bright at 6-9 Volts. After modelling my circuit on Tinkercad, I used a breadboard to make a physical but temporary version of the LDR circuit. I used a lot of jumper wires so that I could understand and recognise all of the connections in my circuit; however, on the permanent version, there are not that many. I followed the schematic accounting for the extra resistor needed for the LED and modelled the circuit on the breadboard based on that.

Step 1: Arrange all the components on the perf board as shown. (Ensure to orient the transistor and LED in the right way, otherwise, the circuit will not work)

⚠️Please ensure you are being safe when soldering, wear safety goggles as the temperature of the solder can go up to 450 °C, and when cutting wiring, pieces can fly unpredictably as well.

Step 2: Solder all of the components in place. You can also solder one of the terminals of the potentiometer to the wiper, because that terminal is unused; this allows for the connection of each of the components to be easier. Do not solder the terminal on the positive side, as it has to connect to the LED.

Step 3: Cut excess wiring and solder the components together. Notice that the LDR and the 460 ohm resistor are soldered together with one straight "solder line". This is used to establish the negative side.

Step 4: Add additional wiring to the circuit. You'll need to add one connecting to the anode of the LED and the other terminal of the potentiometer if you followed my circuit. This is because they are too far apart, and a "solder line" cannot be used otherwise; it will interfere with the other components.

Step 5: Add the positive and negative wires to power the circuit! They will simply be sticking out on each end to connect to a power source. Add the positive wire to the potentiometer, and add the negative one to the 460 ohm resistor.

Step 6: Light it up! Connect the wires to a power source. You can solder it to a 9V battery if you'd like. When the power turns on, your LED will be off, cover your hand over the LDR, and the LED lights up! You might have to adjust the potentiometer setting if it does not light up at first. If so, cover your hand over the LDR and increase the dial until the LED lights up.

(Other): If your circuit doesn't work, try using a multimeter to pinpoint the location where current is not flowing, as it is very easy for something to appear soldered, but not actually connected. If you can't find a loose connection, ensure that some components like the LED or transistor, are alternated correctly.

To make the circuit look more appealing, I 3D printed a casing for it to hide all the messy electronics. I used Shapr3D; however, you can also use Autodesk Fusion. You can either print both parts separately or all at once, but if you'd like to modify the design, I recommend printing them separately in case you need to make adjustments to the other half. If you have Shapr, open up the STL file, and it will copy the design.

Enjoy your automated lamp!

Now that you have built the circuit, take the time to understand the functions of each of the components, this can help you manipulate various other circuits and maybe build some of your own. Every circuit is compromised of the 3 main steps, the input, process, and output.

In the Light Sensing Circuit, the input and output are simple; the LDR detects the surrounding light, and the bulb turns on/off, respectively. The process is a bit more complicated.

Process:

1. The LDR's resistance changes based on how much light is detected
2. This affects the voltage at the base of the NPN transistor
3. The LDR and the variable resistor form a potential divider (the output voltage at the LED is a fraction of what is inputted) - LDR is at the bottom
4. If the voltage exceeds 0.6 volts at the base of the transistor it turns on allowing current to flow from the collector (c) to the emitter (e)
5. When the LDR detects dark there is a higher resistance
6. A higher resistance = higher drop at the LDR = More voltage at transistor's base
7. When there is enough voltage at the base - the LED turns on