Knock Block

Step 4 Wire the circuit (solenoid)

You'll need to get a high-current transistor (we used a TIP31C) to act as a switch for the solenoid - if it's connected directly to the Arduino it may reset the whole thing every time it activates, since it draws too much current for the Arduino to supply. Then the Arduino is only turning the base on and off, and the collector/emitter circuit is sourced by the 9V directly. The solenoid we used was designed for 12V but it has some tolerance for lower voltage at the cost of knocking volume.

Schematic and board overview attached. Notes:
- We wired the battery right into the Vin and GND pins on the Arduino (right under the resistor).
- The middle pin of the transistor is bent out so it's not in the Arduino, and is only connected to the solenoid via the blue wire.

Comments

[buteomont]

This is a great idea! The only suggestion I can make is that you may want to put a reverse-biased diode across the solenoid, to keep the back EMF from destroying the transistor. Apparently the particular transistor that you are using can handle it, but a less robust one might not.

[jkestner]

Good point! I didn't put one in while prototyping, and forgot it after that.