So sanity check for a potential project.

We have an old dishwasher at work, a GE QuietPower 3, and being a technology company, we want to connect it to our chat room to notify that the cycle is done, so someone can unload it.

The first iteration of the project we were just going to cover the "finished" indicator light with a photoresistor, so when it lights up, the microcontroller will know it is done.

However, that is not very nice looking, the second approach, after quickly looking at the technical maintenance manual. Is to literally solder the ground and + of the "finished" indicator light to a digital or analog pin, and the ground pin on the micro controller, so the results are more precise.

I suspect the LED is probably less than 12V, (will confirm with a multimeter). I will step it down to 5V.

Then it is just a matter of checking if the pin is high or not on the microcontroller, is this simple enough to solve the problem, or am I overlooking something obvious?

  • This is enough to solve half the problem - detection. The other half, communication isn't addressed yet (here, anyway) but since you haven't mentioned it I suspect you already have a handle on that part.
    – JRobert
    Commented Jun 17, 2015 at 1:13
  • Communication was solved by using CC3000 wifi shield.
    – Bill
    Commented Jun 17, 2015 at 1:14
  • 1
    Use a resistor (10k or something) between the arduino and the diswasher led, in case the voltage is higher that the arduino. The arduino avr chip has internal clamping diode which will protect the chip from possible overvoltages. If the voltage on the led lead is too low (< 0.7*Vcc), you can search for the current limiting resistor that is connected to the led, and tap of the signal before this resistor.
    – Gerben
    Commented Jun 17, 2015 at 12:10
  • 2
    If you want to see your Arduino alive again send $10....... I mean, if you want your Arduino to live, and the people using it to not die semi randomly, then read my answer carefully and consider carefully why you wouldn't do it. Optos cost maybe $1, maybe about free. Keep them away from burning PCBs (ask me how I know) and they will keep the AC mains and similar where it belongs. Non isolated connection to a "looks like it should be OK" device is an invitation to disaster. Commented Jun 17, 2015 at 13:33

2 Answers 2


If connecting to the wiring, using an opto-coupler will ensure that your Arduino's days will be long on the face of the land. If it is an LED indicator you may be able to connect the opto-coupler LED input directly across the existing LED, but use of an appropriate series resistor would be wise. Optocoupler LEDs may have about 2 V forward operating voltage which is in the same range as a red LED. Some use lower forwards voltage LEDS (probably IR).

If your opto coupler has low input current requirements then connecting it via a series resistor across the existing LED will allow the indicator to still operate.

R series = (Vsource - V_opto_LED) / I_opto.
eg 12V source, 2V opto_LED (typical), 1 mA (should be enough for logiclevel signal.)
R = (12-2) / .001 A = 10/.001 = 10 k Ohm.

For direct connection across an existing red LED a 100 Ohm series resistor should avert disaster.
V drop in resistor at 1 mA:
V = IR = 0.001 x 100 = 0.1 V
= liable to be OK.

Ask if more information wanted.


More or less. LEDs are often around the 2.something volt range, which won't be enough to power an Arduino. In addition, if you set it to run in parallel with the LED, then the path of least resistance will get most of the current.

I would probably use the led power to turn on a transistor, then use the transistor to signal the Arduino. Be aware that if you want to power it off an external source, you may find that the ground on the washing machine is not the same as the ground on your power supply. If you're powering it off a battery, this won't matter, but otherwise power can flow from one "ground" to another. If you stick a resistor (maybe start with 1kOhm, if that's too much drop it down) between the washing machine led + and the transistor, and the same between the arduino and the transistor.

  • The Arduino will be powered via the USB port. I am planning to use a digital or analog pin plus ground on the Arduino in parallel with the indicator LEDs.
    – Bill
    Commented Jun 17, 2015 at 1:28
  • This is a perfect example of a situation where ground may be different. If you connect the two grounds together (USB ground and washing machine ground), and the grounds are at different levels, you will likely destroy the washing machine or the laptop. Even different USB ports on the same computer may have different ground levels. Commented Jun 17, 2015 at 1:31
  • Sorry, I should have been more clear, by ground, I meant the ground pin on the Arduino. The Arduino will be powered via a wallwart AC -> USB 1Amp charger.
    – Bill
    Commented Jun 17, 2015 at 1:34
  • @Bill seeing that it's an old dishwasher, I would add software denouncing as well.
    – xyz
    Commented Jun 17, 2015 at 2:52
  • 3
    @RussellMcMahon's answer of using an opto-coupler would get around all these problems. In this kind of situation opto-coupling is the normal (and recommended) method.
    – Majenko
    Commented Jun 17, 2015 at 12:03

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