# Single-Pole, Double-Throw Switch

Part of the Electricity Merit Badge Digital Resource Guide: [Introduction & Overview](https://merit-badge.university/merit-badges/electricity/guide/index.md)

*10. Hands-On Projects*

> **10d.** Build a single-pole, double-throw switch. Show that it works.

A **single-pole, double-throw (SPDT)** switch has one input connection and two possible output connections. Instead of simply opening and closing a circuit, it chooses which of two paths the current will take.

## What the Name Means

- **Single-pole:** one common input path.
- **Double-throw:** two possible output paths.

You can think of it like a railroad switch. One track comes in, but the switch sends the train onto one of two routes.

## How to Demonstrate It

An easy way to show an SPDT switch working is to connect one battery source to the common terminal and two different loads to the two output terminals. When you flip the switch one way, load A works. Flip it the other way, and load B works.

That makes the switching action visible right away.

![SPDT switch routing current from one common terminal to one of two different light loads](https://ik.imagekit.io/doulacoop/tr:w-800/mbu-assets/merit-badges/electricity/guide/spdt-switch-two-load-paths?v=a7d52ce5)

**SPDT demonstration idea** — Keep the routing obvious

- [ ] **Connect the battery** to the common side.
- [ ] **Connect one load** to throw A.
- [ ] **Connect another load** to throw B.
- [ ] **Flip the switch** to show current reaching only one load at a time.
- [ ] **Explain which connection is common** and which two are the alternate paths.

SPDT logic helps you understand more advanced switching ideas, including the 3-way lighting concept in [Req 10e — How a 3-Way Switch Works](https://merit-badge.university/merit-badges/electricity/guide/req10e/).

> **Tip**
>
> If you use two small lights of the same type, the result is easy to see: one switch position lights bulb A, and the other lights bulb B.

Video: [SPDT switch | Home-made robots | Electrical engineering | Khan Academy — Khan Academy - Projects](https://www.youtube.com/watch?v=1_OieLuWGyw)

> **Safety First!**
>
> Use only low-voltage project parts. Make sure no bare conductors can easily touch and create an accidental short between the two output paths.

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- Previous: [Build a Rheostat](https://merit-badge.university/merit-badges/electricity/guide/req10c/index.md)
- Next: [How a 3-Way Switch Works](https://merit-badge.university/merit-badges/electricity/guide/req10e/index.md)