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Thread: high voltage input ... maybe?

  1. #1
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    high voltage input ... maybe?

    Hi all,
    Feeling pretty noobish with this question. Clearly showing my lack of solid electronics education here

    Building a controller board for an arcade cabinet with teensy 3.2 as the brains. Controls LED's, monitors coin drops, allows clients to make settings via a buttons and a pop-up UI in the game software, and so on. It also spits out tickets, and this is where I need help.

    Two wires. One is the enable wire, where anything over 2V turns on the motor to dispense tickets. The other is an input from the ticket dispenser to the teensy. It goes low every time it detects a notch in the ticket (a little light sensor that senses when that little half circle notch in the corner of the ticket passes over). Using the wires together you deliver and count tickets. The unit is powered by 12V DC. My current head scratching comes from the notch sensor. In the docs (for both of the most commonly use ticket dispensers) mentions the line is pulled high on the dispenser's PCB to a max 30V. I've measured this with a multimeter and consistently read around 5V (I unfortunately don't have a scope so can't see quick peaks). Originally I was planning on a voltage divider since 30V and teensy's don't usually mix well. But that circuit with a 5V input would create a very low input (going from ~3.2V to 0.5V for a HIGH signal). Given the uncertainty in the voltage, should I use another component instead of the voltage divider, e.g. a transistor? Or do I not need to worry about the potential 30V since it's just a pull-up voltage? Here's the dispenser we are using: https://na.suzohapp.com/images/pdf/entropytd_manual.pdf

    Thanks in advance for any advice on this.

    Cheers,
    David

  2. #2
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    Perhaps someone could comment as to whether 100K in series plus the internal diodes would adequately protect the input.

  3. #3
    Senior Member PaulS's Avatar
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    Hi David,

    The 30V mentioned in the datasheet can be a bit confusing.
    Since the output circuitry of the Notch is basically an open-collector output, someone could add an external pullup voltage up to 30V depending on the circuitry attached to the Notch pin. But for Teensy use, no external pullup voltage/resistor is necessary.
    The schematic:
    Click image for larger version. 

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    Your measured ~5V is explainable: a green LED has a forward voltage of ~2V. Plus the zener voltage of 5V2 gives you a voltage on the Notch pin of 12-5.2-2 = ~5V [if the transistor is in high impedance state, meaning open].
    I would connect a simple voltage divider on the Notch pin of 200Ω and 330Ω [notch pin - 200Ω - teensy input pin - 330Ω - GND].
    But test/measure this first without Teensy connected...I'm not an analog expert.

    Paul

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    Perfect! That all makes a lot of sense.
    Thank you for your help with this!
    David

  5. #5
    Senior Member PaulS's Avatar
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    After a night's sleep, I would suggest to use resistor values of 2kΩ and 3k3Ω first and then measure the voltages.

    Paul

  6. #6
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    Your multimeter might be saying 5V because the pulse (which could indeed be 30V) is so short that it gets "averaged." It might be worth it to get an oscilloscope so you can see what's really going on. For a few hundred bucks, you can get a really decent bench-top unit that will show you stuff with nanosecond-scale accuracy.

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