Light Switch Circuit

Do you want to take your electronics knowledge to the next level? This project is perfect for beginners with some theoretical knowledge and experience in electronics and will allow you to develop your understanding further through project base learning. It is a Light Switch Circuit, meaning that the light level in the room acts as a switch.

When this circuit is built successfully you will be able to turn an LED on and off by simply changing the brightness.

This process involves

• gathering the supplies
• understanding and planning the circuit
• assembling the breadboard
• soldering the the perf/stripboard

Citations - schematic picture

Zahid, F. (2020). Temperature Sensor Circuit using Thermistor. [online] Circuits DIY. Available at: https://www.circuits-diy.com/temperature-sensor-circuit-using-thermistor/.

The tools you will need are a soldering kit, wire strippers, wire cutters and helping hands.

For the circuit you will need

2 jumper cables

solid core wires

1 green 5mm LED

1 450Ω resistor

1 NPN transistor

1 250k variable resistor

1 LDR

1 9v battery

1 long breadboard

1 perf board

If you don't have a green 5mm LED or a 450Ω resistor, you need to do some calculations to figure out what parts to use. You will need to find the current requirements of your led. To do this you need to know the size (5mm, 3mm etc) and the colour (red, green etc). You can then search into google "how much current does a (insert size) (insert colour) led take. In this website, input the 9 volts from the battery, and the current requirements you get from google and click calculate. you will then need to find a resistor of the value stated on the website.

The breadboard circuit in the picture has been 'translated' from the schematic on the right. The only change that has been made is adding a resistor to the part of the circuit connected to the LED. They don't have this resistor in the original schematic as it is designed for a lamp, not an LED.

If you just want to build the circuit, copy the circuit into your tinkerCAD file. If you're interested in why the circuit works, read the rest of the section.

As the current flows through the circuit, it is first split between the LED and the variable resistor. Once the current has gone through the variable resistor, it has two paths connecting to the LDR or the transistor.

In a perfectly dark room, the resistance of an LDR can be up to 1M ohm meaning that almost all of the current will go down the path of the transistor. In an npn transistor, current is only sent out through the emitter if enough has gone through the base, and in a dark room, because of the resistance of the LDR, enough current will go through the base and come out of the emitter thus connecting the LED to the circuit. As the light level increases, less and less current will be supplied to the transistor until it doesn't switch on anymore and the LED is effectively not connected to the circuit. You can see this displayed in the schematic.

My original tinkerCAD file is also linked here..

Once you have the TinkerCAD file, it should be easy to make it onto a breadboard in real life. Breadboards are used to mock up designs and are not permanent, so it is okay to make mistakes. If your design works in tinkerCAD, it should work on the breadboard, PROVIDED you copy it exactly. This step is to make sure your parts, Solid Core wire and everything else is working before we get soldering.

In this step you need to put the components into place on the board. There are a couple types of board, one being perfboard and one being stripboard. If you have a strip board like me, you can copy my board. Otherwise you will need to translate the breadboard you designed into perfboard. This is a relatively simply process, and you just need to make sure that every component that was connected in the breadboard is connected in the same way in the perf board.

After you've soldered the board, congratulations, you've completed the LDR light switch circuit.