# Circuit Playground Bluefruit Automatic Bike Brake Light
## Overview
https://youtu.be/5YMgtzF4oa4
This project makes use of two of the many features available on Circuit Playgrounds; the accelerometer and the NeoPixels. It uses the accelerometer to sense if the bike is braking and then uses the NeoPixels to indicate that the bike is slowing down.
For this project, you will need one Circuit Playground (Bluefruit or Express), one Circuit Playground case, one 500mAh LiPo battery, and one JST extension cable with an on/off switch. You can print a mount that goes on the rails of a bike saddle, and then attach a Circuit Playground and a battery to it.
## Two Design Options!
We included two mounting options.
The first design mounts under the seat, while the second design mounts to the seat post.
Both designs include source files to fully customize an exact fit to your bike!
## Parts
### Circuit Playground Bluefruit - Bluetooth® Low Energy
[Circuit Playground Bluefruit - Bluetooth® Low Energy](https://www.adafruit.com/product/4333)
**Circuit Playground Bluefruit** is our third board in the Circuit Playground series, another step towards a perfect introduction to electronics and programming. We've taken the popular Circuit Playground Express and made it even better! Now the main chip is an nRF52840...
Out of Stock
[Buy Now](https://www.adafruit.com/product/4333)
[Related Guides to the Product](https://learn.adafruit.com/products/4333/guides)
### Adafruit Circuit Playground Express or Bluefruit Enclosure
[Adafruit Circuit Playground Express or Bluefruit Enclosure](https://www.adafruit.com/product/3915)
We've got nice cases for many of our beloved boards, but the [Circuit Playground Express](https://www.adafruit.com/product/3333) and [Circuit Playground Bluefruit](https://www.adafruit.com/product/4333) has been left out of couture enclosure...
In Stock
[Buy Now](https://www.adafruit.com/product/3915)
[Related Guides to the Product](https://learn.adafruit.com/products/3915/guides)
### Lithium Ion Polymer Battery - 3.7v 500mAh
[Lithium Ion Polymer Battery - 3.7v 500mAh](https://www.adafruit.com/product/1578)
Lithium-ion polymer (also known as 'lipo' or 'lipoly') batteries are thin, light, and powerful. The output ranges from 4.2V when completely charged to 3.7V. This battery has a capacity of 500mAh for a total of about 1.9 Wh. If you need a larger (or smaller!) battery,
Out of Stock
[Buy Now](https://www.adafruit.com/product/1578)
[Related Guides to the Product](https://learn.adafruit.com/products/1578/guides)
### JST 2-pin Extension Cable with On/Off Switch - JST PH2
[JST 2-pin Extension Cable with On/Off Switch - JST PH2](https://www.adafruit.com/product/3064)
By popular request - we now have a way you can turn on-and-off Lithium Polymer batteries without unplugging them.
This **PH2 Female/Male JST 2-pin Extension Cable ** comes with an in-line **Switch** - click once to turn on, click again to turn off....
In Stock
[Buy Now](https://www.adafruit.com/product/3064)
[Related Guides to the Product](https://learn.adafruit.com/products/3064/guides)
## Additional Items
- One **1/4" 20 nut ** to secure the two halves of the mount together.
- One **1/4" 20 X 1/2" bolt ** to attach the Circuit Playground to the mount.
- One **1/4" 20 X 1 1/2" ** to attach to the nut, securing the two halves of the mount.
- A few **1/4" 20 washers ** as spacers for the half-inch bolt, since it is slightly too long.
- A **3D Printer** to print the mount for the light.
If you use a Circuit Playground Express, it will work fine, just not quite as well as it would on the faster Bluefruit version.
The JST on/off switch isn't necessary, but it is very helpful.
- [Next Page](https://learn.adafruit.com/circuit-playground-bluefruit-brake-light/circuitpython-on-circuit-playground-bluefruit.md)
## Featured Products
### Circuit Playground Bluefruit - Bluetooth® Low Energy
[Circuit Playground Bluefruit - Bluetooth® Low Energy](https://www.adafruit.com/product/4333)
**Circuit Playground Bluefruit** is our third board in the Circuit Playground series, another step towards a perfect introduction to electronics and programming. We've taken the popular Circuit Playground Express and made it even better! Now the main chip is an nRF52840...
Out of Stock
[Buy Now](https://www.adafruit.com/product/4333)
[Related Guides to the Product](https://learn.adafruit.com/products/4333/guides)
### Circuit Playground Express
[Circuit Playground Express](https://www.adafruit.com/product/3333)
**Circuit Playground Express** is the next step towards a perfect introduction to electronics and programming. We've taken the original Circuit Playground Classic and made it even better! Not only did we pack even more sensors in, we also made it even easier to...
In Stock
[Buy Now](https://www.adafruit.com/product/3333)
[Related Guides to the Product](https://learn.adafruit.com/products/3333/guides)
### Adafruit Circuit Playground Express or Bluefruit Enclosure
[Adafruit Circuit Playground Express or Bluefruit Enclosure](https://www.adafruit.com/product/3915)
We've got nice cases for many of our beloved boards, but the [Circuit Playground Express](https://www.adafruit.com/product/3333) and [Circuit Playground Bluefruit](https://www.adafruit.com/product/4333) has been left out of couture enclosure...
In Stock
[Buy Now](https://www.adafruit.com/product/3915)
[Related Guides to the Product](https://learn.adafruit.com/products/3915/guides)
### Lithium Ion Polymer Battery - 3.7v 500mAh
[Lithium Ion Polymer Battery - 3.7v 500mAh](https://www.adafruit.com/product/1578)
Lithium-ion polymer (also known as 'lipo' or 'lipoly') batteries are thin, light, and powerful. The output ranges from 4.2V when completely charged to 3.7V. This battery has a capacity of 500mAh for a total of about 1.9 Wh. If you need a larger (or smaller!) battery,
Out of Stock
[Buy Now](https://www.adafruit.com/product/1578)
[Related Guides to the Product](https://learn.adafruit.com/products/1578/guides)
### JST 2-pin Extension Cable with On/Off Switch - JST PH2
[JST 2-pin Extension Cable with On/Off Switch - JST PH2](https://www.adafruit.com/product/3064)
By popular request - we now have a way you can turn on-and-off Lithium Polymer batteries without unplugging them.
This **PH2 Female/Male JST 2-pin Extension Cable ** comes with an in-line **Switch** - click once to turn on, click again to turn off....
In Stock
[Buy Now](https://www.adafruit.com/product/3064)
[Related Guides to the Product](https://learn.adafruit.com/products/3064/guides)
## Related Guides
- [Adafruit Circuit Playground Express](https://learn.adafruit.com/adafruit-circuit-playground-express.md)
- [Adafruit Circuit Playground Bluefruit](https://learn.adafruit.com/adafruit-circuit-playground-bluefruit.md)
- [Combo Dial Safe with Circuit Playground Express](https://learn.adafruit.com/combo-dial-safe-with-circuit-playground-express.md)
- [UFO Flying Saucer with Circuit Playground Express](https://learn.adafruit.com/ufo-circuit-playground-express.md)
- [Remote Control Candy Dispenser Ghost](https://learn.adafruit.com/ble-claw.md)
- [Case for Circuit Playground](https://learn.adafruit.com/case-for-circuit-playground.md)
- [How to Choose a Microcontroller](https://learn.adafruit.com/how-to-choose-a-microcontroller.md)
- [Wirelessly Code your Bluetooth Device with CircuitPython](https://learn.adafruit.com/wirelessly-code-your-bluetooth-device-with-circuitpython.md)
- [Meeting Time Keeper Stick](https://learn.adafruit.com/meeting-time-keeper-stick-with-cpx.md)
- [Adafruit Circuit Playground TFT Gizmo](https://learn.adafruit.com/adafruit-tft-gizmo.md)
- [3D Printed Heel Clips](https://learn.adafruit.com/3d-printed-heel-clips.md)
- [CircuitPython Hardware: ILI9341 TFT & FeatherWing](https://learn.adafruit.com/micropython-hardware-ili9341-tft-and-featherwing.md)
- [Make It Shake, Rattle, and Roll: Accelerometer Use](https://learn.adafruit.com/make-it-shake-rattle-and-roll.md)
- [Mount for CRICKIT](https://learn.adafruit.com/mount-for-crickit.md)
- [Unicorn Christmas Stocking with Rainbow Lights & Sound](https://learn.adafruit.com/unicorn-christmas-stocking-with-lights-sound.md)
- [CircuitPython BLE Libraries on Any Computer](https://learn.adafruit.com/circuitpython-ble-libraries-on-any-computer.md)
- [Drama Piñata](https://learn.adafruit.com/customizable-reusable-pinata.md)