# Filtering Reverse voltage from a coil

I am working on a interface between an Arduino microprocessor and a old electromechanical pinball machine to track the scoring and light up some LED's based upon that.

I made a circuit diagram... or I think it might be described as that. I have never done that before and may have more than one tragic mistake.

The pinball machine is the 26V AC coil on the top left, that coil when energized turns a score reel... I am going to tap into the input voltage for that coil and hope to convert to DC, trigger a relay and send a signal to the microprocessor to enable me to "watch" the scoring on the pinball machine with the arduino processor and control some LEDs (shown bottom left). All the circuit stuff (but for the 26 V AC coil) are in a project box.

Question I have, is do I have to worry about filtering (and how would I) the reverse voltage (shut down voltage?) coming off that 26 V AC coil inside the machine. Basically, I am going to run wires from that to a rectifier/smoothing capacitor to trigger a relay (24 V DC relay) and have that switch a 5V DC signal to an Arduino input pin.

I am not sure I need to filter it... will the relay chatter from the reverse voltage? Will it possibly destroy the relay? Kinda new territory. Lets hope my diagram is functional and try to be nice to the new guy... happy to make corrections and listen. Thats what I am here for.

Any other stupid mistakes you can point out would be great. I can also list the exact components I am using if required.

• I upvoted you - now that you have 10 rep, could you make that an image instead? ;) – user17592 May 28 '13 at 17:58

you just need to check whether the relay coil is energized or not. you can go for a simpler solution rather using mechanical relay (relays require higher power and it has limited life time). let me give you three solutions.

Solution 1: Opto coupler based: MCT2E(DC input)

solution 2: Opto coupler based: MOC3021(Direct AC input)

Solution 3: Transistor Switch based:

solution 1 and 2 gives isolation between the 26V AC input and your arduino

solution 3 doesnt give any isolation

you can select any solution based on your requirement

• I am aware of the opto coupler, but I wanted to work with things I could understand and use- the opto is cool but I allready purchased everything and have most of the stuff soldered down, just running connections now so its a good time to add the pull up and zener as no undoing of work need be done... conversion to opto is too different from what I am doing. Thanks for your help... but I am too far down the road here to start over! – rufessor May 28 '13 at 18:43
• but i am unable to understand why you want use relays.you can use relay to power some device,but here its used to know the logical condition of a circuit(ON/OFF).is this project? or product? – yogece May 28 '13 at 19:02
• Well... after looking at this... I will see how much they are. I could just put it in place of the relay since it uses a rectified DC input. If its cheap and fits on the board I will at least consider it. Thanks. – rufessor May 28 '13 at 19:13

Two things: -

• The 24V relay expects maybe 28Vmax and 24VAC will peak at possible 37V. Subtract a couple of volt drops for the diodes in the bridge and you are still at 35V so, I would put a 10V zener in series with the relay. Smoothing cap value needs to be 10uF plus but plan on it being as high as 100uF i.e. make room for a bigger component should it be needed. You might also have a catch diode across the relay coil to prevent back-emfs whn it switches off.
• You'll need a pull-down resistor on the digital input so that when the contact is open circuit a definite state is forced onto the IO pin. (EDITED from pull-up)

Here's the mods and where the zener is circled you'll need a regular diode across the relay, cathode to positive feed after the zener (BZX84C10V for the zener) and possibly a 1N400x for the diode : -

Good luck

• I changed the diagram to include what I think you mean by a pull up resistor... is this better? I also am using 100 uF caps off the rectifier. I was not sure if I could measure the input voltage off the rectifier with my DRM hand held cause its really transient... if its a little high will it kill the relay? Can add that zener in but as of now I do not have one. – rufessor May 28 '13 at 18:10
• Sorry, I misread the drawing; A 1k to 10k resistor from 0V to the input pin holds the pin at a soft 0V. When the relay clicks in the pin gets pulled-up to 5V by the contact. I might also add that you'll need to account for some contact bounce in the arduino, possibly 10 to 30 milli seconds. – Andy aka May 28 '13 at 18:13
• Ok... THanks. I think I had the pull up resistor correct, see my edit above and it looks like its the same as what you have. But yours is easier to read. As for the zener I will go to my electronics shop and bring this diagram... then put it in. THANKS> I see now that the pull up brings the pin to a true zero rather than floating an some random value when its not energized. Thanks. – rufessor May 28 '13 at 18:29