Component guide
How to Use an LED
- Grades 3-8
- 10 minute activity
- NGSS 4-PS3-2
Quick answer
To light an LED in a play dough circuit, push its long leg into the dough connected to the red (positive) wire and its short leg into the dough connected to the black (negative) wire, with a gap between the two pieces of dough. LEDs only work one way, so if it does not light, turn it around. With a wired LED, match the colors instead: red wire to the red side, black wire to the black side.
What is an LED?
LED stands for light-emitting diode. A diode is a part that only lets electricity flow in one direction, so an LED has a positive side and a negative side. When electricity flows through it the right way, it gives off light.
LEDs use much less energy than old-fashioned light bulbs and stay cool to the touch. You will find them in flashlights, traffic lights, TV screens, phones and holiday lights.
Standard LEDs and wired LEDs
Squishy Circuits uses two kinds of LEDs.
- Standard LEDs are the classic kind, with two thin metal legs that push straight into the dough. They are inexpensive, but the legs bend and wear out with use, so they need to be replaced over time. Because they are standard LEDs, replacements are easy to find: get them on our site as an LED assortment or by color, or from electronics suppliers online.
- Wired LEDs have the LED on a red wire and a black wire, with our metal terminals on the ends. The terminals push into the dough and do not break with use, so wired LEDs last longer and are easier for younger builders and classrooms. Match the colors: red wire to the red side, black wire to the black side. They come in our Wired LED Assortment.
Try it with dough
Light an LED with play dough
You need
- Battery holder with 4 AA batteries
- Conductive (salty) play dough
- 1 LED (standard or wired)
Steps
- Make two pieces of conductive dough and set them a finger-width apart.
- Push the red battery wire into one piece and the black wire into the other.
- Gently spread the LED legs apart.
- Push the long leg into the dough with the red wire and the short leg into the dough with the black wire. Using a wired LED? Push its red terminal into the dough with the red wire and its black terminal into the dough with the black wire.
- Switch the battery holder on.
What happens
The LED glows. Electricity flows from the battery holder through one piece of dough, through the LED and back through the other piece.
Not working?
- Turn the LED around. The long leg must be on the red side.
- Check that the two pieces of dough are not touching.
- Push the LED legs and battery wires deeper into the dough.
- Try fresh batteries or a different LED.
Why the dough matters
Never connect an LED straight to the battery holder wires. The batteries would push too much electricity through it and burn it out in an instant.
Conductive dough has a little resistance, so it acts like a speed bump that limits how much electricity reaches the LED. That is what keeps the LED safe and glowing. Electronics engineers do the same job with a part called a resistor.
Light more than one LED
Ready for more light? You can connect LEDs in two ways.
- In series: set three pieces of dough in a ring with a gap between each. Put the red and black wires in two pieces that sit next to each other, then put an LED across each of the other two gaps. The LEDs share one path and the energy, so they glow dimmer.
- In parallel: roll two long cylinders of dough and set them side by side with a gap. Put the red wire in one and the black wire in the other, then bridge the gap with several LEDs. Each LED gets its own path, so they all glow brightly.
Mixing colors? Red and yellow LEDs need less push than blue, green and white ones. In parallel, a red LED can take most of the electricity and leave a blue one dark. Give each color its own gap if that happens.
LED do's and don'ts
Do
- Put the long leg on the red-wire side
- Keep each leg in a different piece of dough
- Always have dough between the batteries and the LED
- Spread the legs gently, just once
Don't
- Connect an LED straight to the battery holder wires
- Let the two pieces of dough touch
- Bend the legs back and forth, which makes them snap
- Pull LEDs out of hard, dried dough by the legs
Key words
- LED
- Light-emitting diode, a part that gives off light when electricity flows through it.
- Diode
- A part that lets electricity flow in only one direction.
- Anode
- The positive side of an LED: the long leg.
- Cathode
- The negative side of an LED: the short leg.
- Wired LED
- An LED on a red wire and a black wire, with metal terminals that push into the dough.
- Resistor
- A part that limits how much electricity flows. In a dough circuit, the dough does this job.
- Series and parallel
- Two ways to connect more than one part: on one shared path, or on separate paths.
Watch: how to use an LED
LED questions
What does LED stand for?
LED stands for light-emitting diode. A diode only lets electricity flow one way, which is why an LED has to face the right direction to light.
Which leg of an LED is positive?
The long leg is positive (the anode) and goes to the red wire side. The short leg is negative (the cathode) and goes to the black wire side.
Why won't my LED light up?
Most of the time, it is in backwards: turn it around. Also check that the two pieces of dough are not touching, that the legs and wires are pushed in deeply, and that the batteries are fresh.
Can an LED burn out?
Yes. Connecting an LED straight to a battery holder without dough in between sends too much electricity through it and burns it out. LED legs can also break if they are bent back and forth, so expect to replace standard LEDs now and then. Wired LEDs have sturdy metal terminals instead of legs, so they do not break with use.
What is the difference between standard and wired LEDs?
A standard LED has two bare metal legs: the long leg goes to the red side and the short leg to the black side. A wired LED has a red wire and a black wire with metal terminals on the ends, so you match the colors instead. Standard LEDs are inexpensive but wear out over time. Wired LEDs are sturdier and easier for younger builders.
Where can I buy replacement LEDs?
Squishy Circuits uses standard LEDs, so replacements are easy to find. You can get them on our site as an LED assortment or by color, or from electronics suppliers online. Wired LEDs come in our Wired LED Assortment.
How many LEDs can I light at once?
Several. Put them in parallel across the same gap, or across different gaps, and they will all light. The more you add, the more power they draw, so fresh batteries help.
Why does one LED light but another one doesn't?
Different colors need different amounts of push. In parallel, a red or yellow LED can take most of the electricity and leave a blue, green or white LED dark. Give the dark LED its own gap.
Printable guide: Components Explained (PDF)
Next, try a project
Build the Fireflies Project A free step-by-step project with lots of LEDs.