Hoping you can help a newbie. I'm putting together a board for a Christmas present this year. It's replacing the factory board that comes on a beer fridge/vending machine, with minimal interactions.

Basically, when the beer can falls into an outlet chute, it trips a switch that triggers some LEDs and audio. Pretty simple stuff. The fridge supplied +5v to the previous board, so I thought I could remove it and simply swap in the Arduino.

The code runs fine when the Arduino is powered by USB/laptop. I spliced a USB header onto the +5v wire supplied by the fridge and plugged it all in. Now, it will boot up, play some weird fuzz out of the speaker, and reset after about 10 seconds.

How can I use the +5v to power my nutty creation? The previous board had a capacitor on it which I removed (I know next to nothing about some of this stuff but it seems like maybe that would help?)

Here's a photo of what I'm working with. If I need to provide any additional information or photos I can do that.

My board

Fridge Schematic


enter image description here If you have any advice, please ELI5 with regards to what I might be able to place in my circuit to get this working (links or definitions are incredibly helpful!)

Any help is appreciated!

  • 1
    Without knowing the specs of the power supply it's impossible to tell. It may only have a very low current capacity, or it may be unsmoothed, or similar.
    – Majenko
    Commented Dec 11, 2017 at 13:45
  • @Majenko thanks for the reply. what's the best way to determine more information about the specs? Multimeter?
    – Dave Kiss
    Commented Dec 11, 2017 at 14:28
  • 1
    First step is to examine the power supply and see if there are any clues. Second step is probably to monitor the power with an oscilloscope looking for brownouts.
    – Majenko
    Commented Dec 11, 2017 at 14:29
  • Holy jumper wires, Batman! It might help if you used ribbon jumpers, so you can make sure you don't have an accidental short somewhere. Commented Dec 11, 2017 at 19:28
  • 1
    @DaveKiss well, a (recent) laptop USB usually can provide at least 500mA (or even more, if it has some USB3 ports). If it is a current issue, I don't think you can solve it with just a capacitor (BTW, use a large electrolytic capacitor rated at 10V or more; connect the pin labelled "-" to the ground and the other to +5V - but don't invert the pins!!! Explosion risk). But maybe it is worth trying. In any case, another supply rated at 1A or more in my opinion will solve your problem
    – frarugi87
    Commented Dec 12, 2017 at 15:26

1 Answer 1


How much current takes your board with all components? Maybe the 5V power supply can not give you the wanted current.

  • You can use a oscilloscope to see what is happening with the 5V when you use your board (maybe not enough current) for example interruptions on your connection or the voltage is not stable.
  • Maybe you can find a datasheed for the power supply for more information
  • try your board with another power supply to see if this is a problem of the board or the power supply

I just recognized in the third picture that the transformers output have 5V and 200mA. Your speaker will maybe have 8 Ohm (please write us the resistance). This means it will take 5V/8 Ohm this equals a current of 0,625 or 625mA. This current is 3 times higher than the output current of the transformer.

My theory 1 is:

Your circuit worked on your computers USB because you have got usb 3.0 and the max current for it is 900mA wich is much greater than 625mA and enough for leds, speaker and arduino. Your speaker is not connected directly to a pin and gnd. Some seperate electronics manages the pulses and takes directly the 5V of the USB.

My theory 2 is: You used USB 2.0 (500mA) or USB 3.0 (900mA) and the speaker is connected directly to a pin (the resistance is ok or you added some resistance). The sound was probably not loud because the power was low. The current for both USBs were enough for leds, speaker and arduino.

Now to your problem with the resetting.

The solution:

Switch off the speaker and test your circuit without it. (I think it will work fine).

If it works: You took too much current and this current heated the pin and influenced the charecteristics of the pin and finally the voltage at your pin (weird fuzz). Add a variable resistor in series to your speaker an try it.

If it not works: Measure the output voltage of the transformer and wirte us the value.

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