How to prevent Arduino Uno digital pin for being damaged using 12V device?

Projects General Guidance
1

I have a project involving a 12V coin injector (TW-130B variant) and a clone Arduino Uno. But I’m focusing only on the coin part.

This is the wiring I use involving coin injector and Arduino:

coin
coin621×490 5.53 KB

And this is the code I used to test the coin part only:

#define coin 13
unsigned int credit=0;

void setup() {
  Serial.begin(9600);
  pinMode(coin, INPUT);
}

void loop() {
  if(digitalRead(coin)==LOW){
    credit++;
    Serial.println("coin detected!");
    delay(1000);
  }
  else{
    Serial.println("insert a coin");
    delay(1000);
  }
}

1K resistor between the coin signal and D13 is for safety, while 10K resistor between D13 and 5V is the external pullup, I used external as the internal INPUT_PULLUP still leaves it floating occassionally. Don’t mind the delay for this testing code, as I avoid that for the actual project.

Previously, the coin injector works functionally, but now, after working well for a while, the coin injector pin (D13) would be damaged. I tried using voltage divider too (10K to GND, 20K to coin signal), but it just doesn’t work, using voltage divider (with or without pullups) won’t ever make the Arduino register the coin injector’s inputs.

What’s the solution to prevent damage on Arduino’s D13 using coin injector like this, but by keeping the external pullup? What I have currently are resistors (5.1K, 10K, 20K, 40K, 82K), and also diodes.

2

Sounds like you think you know what’s wrong… that can lead to overlooking other possible explanations.

Using a voltage divider didn’t work? Used how?

If you put > 5.5V on a 5V pin, the pin is wrecked. A properly used voltage divider would prevent that. 12V through a 1K resistor is still 12V.

3

Looks as though you have a voltage divider between the 5-V supply and the device output.

The device datasheet just describes the signal as a pulse, but no value. Have to assume it's 12-V DC. It can be set, apparently, to either 20, 50, 100 mS

A voltage divider on the device output needs to be between the SIG and GND.

The datasheet also shows more connections than yours, e.g. a Counter output and Coin signal. Which one are you using?

You might have already damaged the Uno.

4

How have you confirmed that the pin is damaged?
I don't see how it could be.
Supplying 5V to the arduino 5V output can be problematic.
I suspect you have a problem somewhere else.

5

Hi, @89jvek

Do you have a DMM? Digital MultiMeter?

Tom.... :smiley: :+1: :coffee: :australia:

6

The outputs of these coin acceptors are open collector / open drain.
This means that the voltage is determined by the user's connections.
Using a pullup resistor to +5V will give a 5V amplitude signal.

There is a selector switch that you can use to select the output transistor being 'normally closed' or 'normally open'.

A second switch selects the pulse width, there is always 100ms between pulses.

You program the coin acceptor to produce the required number of pulses for each coin denomination.

Here are oscilloscope traces showing the types of pulse that can be generated:

The first is a 20 pence coin, output set to 'normally closed' and 'fast'.
The coin mechanism is programmed to produce 4 pulses for a 20p coin (5p base unit).

20p nc fast
20p nc fast1024×600 11.9 KB

The second trace is for 10 pence coin, 'normally open' and 'slow'.

10p no slow
10p no slow1024×600 11.9 KB

The wiring shown in post #1 should work correctly.
I am just using just using the internal pullup resistor on an Arduino Uno R3.

From 89jvek's code, I can see that he has the output set to 'normally open', as a low going pulse is detected as a coin being accepted.

EDIT,
The TW-130B coin acceptor only accepts a single denomination of coin/token.
It will only give a single coin output pulse.

7

D13 is the on-board led LED_BUILTIN.

You should use another digital pin.

If the D13 pin is REALLY damaged it looks as if there is a substantial injection of current from somewhere - maybe EMI? You should put a small capacitor from the digi5tal pin to ground.

1 Like
8

Your circuit looks OK to me, the input protection will protect you from most transients. If my memory is working D13 is the pin used for the onboard LED which will give you some interesting results. Disconnect the SIG line and run the blink sketch, if the onboard LED is working the port should be OK. Connect the SIG pin to another digital input or you can use one of the analog pins and use the digital read function with it. Happy countint!

9

I said I used a 10K to GND, 20K to coin signal voltage divider, and the arduino won’t register the input from coin injector, regardless if I used a pull-up or not.

10

Only the pin 13 is damaged, but I need all the pins in this occassion so I can’t change the pin to other ones. For this post, I don’t use voltage divider, just a pullup to 5V, and also 1K to coin signal is for safety measures. I used the coin signal one.

11

It’s damaged as it begins constantly inserting credit after it being ran for a while. And yes, already tested D13 with other device (e.g. IR sensor) and the input in D13 is permanently LOW, rather than being HIGH when object is detected and LOW if object is detected as it should be.

12

Using another digital pin with the same setup would be safe unlike for D13? Which amount for capacitor?

13

Hi, @89jvek

Have you tested D13 with the coil dispenser disconnected and grounding the input then connecting it to 5V?

Tom.... :smiley: :+1: :coffee: :australia:

14

I guess using the Blink Example code (as it’s the LED_BUILTIN) and it seems to work for now. Still the question is if my setup damaged the pin or it’s just D13 problem.

15

That tests it as an output.
Test is as an input like I desdcribed in post#13.

Write some code to test the other pins.

Thanks.. Tom.... :smiley: :+1: :coffee: :australia:

16

with 10k 1uF the time constant is 10msec . So somewehere between 0.1 - 0.47 should be safe for a 20msec pulse.

OK I know its basic, but have you uploaded a sketch to JUST set D13 as an input and read it with a DMM with the coin acceptor disconnected?

1 Like
17

Great minds think alike.... (But fools never differ...LOL)

Tom.... :smiley: :+1: :coffee: :australia:

1 Like
18

The capacitor must be 1uF? (I have 100nF ones) And the wiring is both 10K and 1uF to the 5V?

I haven’t yet, but what’d you expect for the result of testing D13 as input with DMM?

19

Monitor the voltage on Pin13 with DMM and see that it goes from 0V to 5V and that your code detects this change.

Basic troubleshooting 101.

Tom.... :smiley: :+1: :coffee: :australia:

1 Like
20

0.1uF is 100nF. To GROUND for decoupling.