Detecting low voltage AC current.

Other Hardware General Electronics
1

I'm using an UNO for a project in a pinball machine modification. I want the UNO to be triggered to respond when a particular light on the playfield turns on. The lighting circuit is 6.3V AC. I simply need it to detect the lights circuit as on or off.

With it being an AC current is there a simple way to do this without purchasing any sensors or auxiliary boards? Would simply placing, say, a 1N4004 diode with a resistor to drop the voltage in the circuit work to essentially create a pulsing DC circuit that could be sensed by the UNO?

Any simple circuit suggestions or advice would be appreciated. Thanks!

2

Sharing grounds with Arduino and machine could be a problem (groundloops).
To avoid that, you could use an opto coupler.

Opto transistor: emitter to Arduino ground, collector to a digital pin with internal pull up resistor enabled in pinMode.
Opto LED: current limiting resistor (~470ohm), and diode anti-parallel for reverse voltage protection.
Leo..

3

Hi moses1592,

I agree with Wawa. Assuming the pinball machine has its own power supply and circuitry, separate from the Uno. Gounding the two can lead to problems.

I know you said no additional sensors, however you might look into a photo resistor (aka cadmium sulfide cell). They are super cheap and if it could be placed near the light it would provide a very simple isolated sensor. Also because the brightness of an incandescent bulb does not follow the 60Hz power none of the speed issues mentioned below will come into play.

If you do decide to go with the diode capacitor route I would suggest not tying the Uno ground directly to the pinball machine. A 1 k (maybe as low as 200) resistor between the two would protect the Uno from ground loops.

Things to consider:

  • diode - capacitor will charge to 1.414 * 6.3V
  • you will have to limit the current to the Uno or create a voltage divider so the ~9v doesn't fry your Uno
  • The diode capacitor will tend to stay charged when the light goes off causing the signal to the Uno will have a significant delay
  • If you put a resistor load on the diode capacitor to make it go low faster (when the light goes off) you run the risk of sending a pulsing signal to the Uno (could lead to apparent inconsistency of the sensing).

Hope this helps

Good luck

John

4

Thank you for the help!! I will definitely try the opto transistor route.