Engineer an LED Night-Light *

Side by side comparison of LED night light circuit, with LED off in daylight and LED on at night
Areas of Science Electricity & Electronics
Time Required Short (2-5 days)
*Note: This is an abbreviated Project Idea, without notes to start your background research, a specific list of materials, or a procedure for how to do the experiment. You can identify abbreviated Project Ideas by the asterisk at the end of the title. If you want a Project Idea with full instructions, please pick one without an asterisk.


Night-lights can be found in many homes; you might even have one in your bedroom! Using a night-light is a great way to avoid stubbing your toe on furniture during a midnight trip to the bathroom. In this fun engineering project, you will design and build your own night-light. You will need to build the circuit, but you will also need to think about the physical design of the product. Most people probably do not want an ugly, bare circuit board sitting around their house. Can you make a functional, aesthetically pleasing case or enclosure for your circuit? You could make a case using craft materials, wood, or even a 3D printer if you have access to one.

If you need some help getting started and learning how to build an LED night-light circuit, first watch this video for an overview and explanation of how the circuit works:

As you saw in the video, you will need to make some design decisions for your circuit. The major decision is whether to make it battery-powered or wall-powered. The voltage of your power supply (3 V for 2xAA/2xAAA/coin cell batteries, 5 V for USB wall power) will determine the required resistance of the current-limiting resistor in series with the LED. If you are not familiar with Ohm's Law or how to calculate the current through an LED, see the references in the Bibliography.

The circuit required to build an LED night-light is very similar to the one in our Light Tracking Bristlebot project. If you have already purchased the kit for that project, you can reuse many of the parts. If not, here is the main list of parts you will need. Remember that the exact parts you use may vary depending on some of your design decisions.

  • Mini breadboard or protoboard
  • Power supply, such as a 2xAA or 2xAAA battery holder. For wall power you will need a USB wall adapter and either a USB cable or male USB plug. You will need to connect to the proper pins in the USB adapter—see the Bibliography.
  • Super bright LED, available in different colors.
  • Current-limiting resistor to put in series with the LED. You can calculate an exact value for the resistor as described in the Bibliography references. However, if you already have some resistors laying around, something in the 30–50 Ω range should work with a 3 V power supply.
  • Potentiometer. The potentiometer used in the example circuit has a resistance of 10 kΩ. Other values should work, as long as the resistance of the photoresistor is not dramatically larger or smaller (see next point).
  • Photoresistor. The photoresistor used in the example circuit has a quoted resistance range of 16 kΩ (light) to 500 kΩ (dark). Other values will work as long as they are roughly in the same range as the potentiometer (i.e. both in the tens of kilo-ohms). Do not use a 100 Ω potentiometer with a 20 MΩ photoresistor, or the circuit will not work properly (see the Bibliography reference about voltage dividers to learn more).
  • Transistor. The transistor in the example circuit is an N-channel MOSFET. There are many different types of transistors, and you could build this circuit using other types with some small modifications. See the Bibliography to learn more.
  • Jumper wires, which come in a variety of assorted kits.

To wire the circuit, see the diagrams in Figures 1 and 2. Figure 1 shows the connections directly between the leads of the different circuit components. You can use this diagram if you plan to solder the circuit yourself on a protoboard (you will need to decide how to arrange the components). Figure 2 shows the circuit on a breadboard. You can follow this diagram exactly. If you do not know how to use a breadboard, see the breadboard tutorial in the Bibliography.

Wiring diagram for an LED night light circuit.
Figure 1. Wiring diagram for the LED night-light circuit. Image made with Fritzing.

Breadboard diagram for an LED night light circuit
Figure 2. Breadboard diagram for LED night-light circuit. Image made with Fritzing.

How does the circuit work? Remember to watch the video (starting at about 3:25) for an explanation. Figure 3 shows a copy of the schematic. See the Bibliography if you do not know how to read a circuit schematic. The potentiometer and photoresistor are combined to form a voltage divider. The output of the voltage divider turns the MOSFET on and off (therefore turning the LED on and off) depending on the amount of light hitting the photoresistor. The potentiometer allows you to adjust the threshold at which the LED will turn on.

Circuit diagram for LED night light.
Figure 3. Circuit schematic for the LED night-light.

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Ben Finio, PhD, Science Buddies

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Finio, Ben. "Engineer an LED Night-Light." Science Buddies, 4 Oct. 2019, Accessed 13 Nov. 2019.

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Finio, B. (2019, October 4). Engineer an LED Night-Light. Retrieved from

Last edit date: 2019-10-04


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