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I have a small circuit from a toy sword that has an LED and a small speaker. It was powered by a couple of small coin cells.

I am trying to figure out how to connect it to my Arduino so I can trigger it. If I attached the circuit directly to the GND and +3.3V and press the switch in it I get the sound of a sword and flashing LED as desired.

Now I want to somehow get the Arduino to trigger the circuit but am very new to Arduino and quite unsure of how to approach this.

Can anyone advise how best to work with an existing low power circuit that can use the Arduino's power rails but needs the switch integrated?

Sorry for the noob question.

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You'll probably want to use a transistor to switch it on/off. You could just use the Arduino's digital pin but they can only supply a limited amount of current. Transistors can work with larger currents. The Arduino switches the transistor and it powers the external circuit.

There are a couple of existing questions that may help you.

jippie's answer in the first link shows an example schematic for hooking up a transistor with a bunch of LEDs but you would replace them with your own circuit:

enter image description here

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  • Thanks for this. I had been looking at this diagram earlier when trying to figure this out. In this diagram do you think I could just use the GND and +3.3V pins as the external circuits power supply? – dibs May 17 '14 at 5:51
  • It looks like the DC current for 3.3 pin is 50mA. If your circuit requires less than that, it should be okay. – sachleen May 17 '14 at 5:59

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