Using a Scantron Reader Sensor

Hello I need some help with an issue I'm having!
I have a Scantron test reader that grads scores. It is an 888P+ OMR Test Scorer

They have a sensor that reads the scantron as it slides through. That sensor PCB has 7 pins/leads/wires which connect to somewhere on the main PCB.

(the sensor has the 7 solder points)

(The main PCB. The unplugged wires are coming from the sensor.)

I am trying to get some kind of signal from those 7 wires to read on an Arduino as an input. I am kind of at a loss on how to actually approach this especially because I can't find any info on the sensor. I am assuming one of those 7 wires is a ground? And 5 or 6 of them are to read what row of the scantron it is and if they marked A B C or D. But I am unsuccessful in getting any reading from the 7 wires on my Arduino when scanning a scantron with the machine on.

Here's how I've tried sending the sensor wires to the Arduino:
I've tried sending all seven of them to Arduino inputs with a pull-down resistor. I also tried grounding what I suspect is the ground wire of the sensor with the Arduino's ground and sent the other 6 to the inputs with a pulldown. I'm wondering if I also have to send power to one of those 7 wires to power the sensor?

I measured the circuit of the scantron reader to be powered at 25V, I'm not sure if the sensor is too though, there were some areas at 15v.

Unfortunately, I cannot find and provide any information on the sensor! So I'm wondering if there is any advice or guidance on how to move forward with trying to get a signal from these wires. Such as using my multimeter or circuit or what I could research in order to try and figure this out? I'm a hobbiest when it comes to this stuff so I'm a bit stumped!

Thank you very much!

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Sorry, can't help you without information. You could show images of how you connected the DMM for test...

Thanks for your reply! I guess I am seeking advice on how to approach testing these 7 wires with the DMM, I'm not really too sure!

  1. Find ground. Turn the machine off. Set the DMM to resistance, continuity beep enabled if it has one. Put one lead on the machine frame, put the other lead on each connector pin until you find zero ohms.

  2. Now measure signal. Turn the machine on. Set the DMM to measure voltage, put one lead on ground, put the other lead on each of the other connector pins.

Please take another photo of the scanner head PCB, showing better detail of the traces. The pins that are shown there appear to all be connected together, and to the PCB common. So those pins are likely all at the same voltage potential. That is a bit of a mystery, suggests that maybe it isn't the sensor at all. You can confirm that with the ohms readings.

We need a lot more photos, to get to the bottom of that aspect... it would be good if you could show how the apparatus sits in relation to the card it's reading...

Thanks so much for this advice! So I got some better photos of the sensor. The leads I believe are not all connected which you can see in this photo (it is the other side of the sensor)

Here's an overview shot too if that helps

I have a question about your previous reply. For part #2 do I disconnect the 7 sensor pins so I can test them while the machine is running? If so, then do I need to ground the sensor to the machine while I am doing the signal measuring? And wouldn't it need power (+) too?
Or do I keep all 7 pins connected to the machine and try to probe them with the DMM while connected?

My goal is hopefully to actually have the sensor not connected to the machine at all and to ground it to Arduino and send the values to it (and I guess power the sensor? As I don't see how else I'd get a signal from it unless I can with pullup resistors??).

Thanks!!

From the look of the main pcb, it looks possible to follow the tracks from the connector on the board and identify the components and hence the circuit. Most likely there will be power pins and i/o. There are appears to be many identifiable chips which you can get the datasheets for and hence solve your problem. It does take a good eye to do this and a bit of deduction. Annoying sometimes manufacturers obscure the part number to make this hard.

No. The whole point of that is to collect data on its operation when it's working. So everything has to be connected.

and try to probe them with the DMM

I doubt that will work. The signals are likely changing too fast for the DMM to display (also too fast to read, even if it could...). Use an oscilloscope.

Unstated, implied I guess, also a lot of hands on electronics experience.

Ah I see, wish I had a scope... wait then what exactly did you mean by:
"2. Now measure signal. Turn the machine on. Set the DMM to measure voltage, put one lead on ground, put the other lead on each of the other connector pins."

I think I misunderstood something! Is this instruction just to find the operating voltage of the sensor? I think I misunderstood and thought it was to measure the signals of the sensor pins as they scanned the sheet. I agree that that would go too fast!

Thanks for all the advice. I'm not so advanced or experienced at any of this stuff! I have time today so I'll give it a shot.

I found the ground and when turned it on and measured the signal It was around 13.5volts for almost all the connector pins. I think the pin next to the ground is power cuz it caused the machine to beep when I touched that with ground versus the other pins. So I guess I'll try grounding and powering the sensor with those two pins and see if I can get any readings from the other pins as I scan sheets.

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