Need advice about polling a coin meter

[Duffy Moon]

Hi,

I need some advice. I’m building a device that controls and monitors a coin meter attached to another machine. I need my device to be able to add credits and also monitor how many coins have been inserted and I’ve run into a snag. First, a bit on how the coin meter works:

Inside the coin meter are two wires, ground and 17V. When a coin is dropped in, a small switch connects them and the 17V drains to ground. A sensor attached to the 17V wire sees the voltage drop to 0V and registers a coin being inserted. The sensor is embedded in a PCB inside of the machine.

Using a micro-controller I have a 3V control signal going to the base pin of a transistor that is attached to the two wires, ground to emitter and the ground of the micro-controller, and 17V to collector. When the signal is sent it registers a credit on the coin meter. This is working perfectly and does not affect coins being dropped in either. For the coin counting portion I’d like to poll the 17V wire with my micro-controller in order to detect when it drops to 0V but the max voltage I can send to it is 3.3V. I decided to attach a simple voltage divider to the 17V wire, yet when I do the coin meter is rendered useless and won’t register anything when a coin is dropped or from the transistor. It appears that there cannot be anything attached to the wire for it to function properly. My two questions are:

• Is there any way to branch off the voltage of the 17V wire down to 3V without introducing resistance or load to the 17V side?
  
• Any thoughts as to why adding a voltage divider would affect it this way?

I’m really at a loss here and would appreciate any advice.

Thanks!

 

[Gryd3]

Hi,

I need some advice. I’m building a device that controls and monitors a coin meter attached to another machine. I need my device to be able to add credits and also monitor how many coins have been inserted and I’ve run into a snag. First, a bit on how the coin meter works:

Inside the coin meter are two wires, ground and 17V. When a coin is dropped in, a small switch connects them and the 17V drains to ground. A sensor attached to the 17V wire sees the voltage drop to 0V and registers a coin being inserted. The sensor is embedded in a PCB inside of the machine.

Using a micro-controller I have a 3V control signal going to the base pin of a transistor that is attached to the two wires, ground to emitter and the ground of the micro-controller, and 17V to collector. When the signal is sent it registers a credit on the coin meter. This is working perfectly and does not affect coins being dropped in either. For the coin counting portion I’d like to poll the 17V wire with my micro-controller in order to detect when it drops to 0V but the max voltage I can send to it is 3.3V. I decided to attach a simple voltage divider to the 17V wire, yet when I do the coin meter is rendered useless and won’t register anything when a coin is dropped or from the transistor. It appears that there cannot be anything attached to the wire for it to function properly. My two questions are:

• Is there any way to branch off the voltage of the 17V wire down to 3V without introducing resistance or load to the 17V side?
  
• Any thoughts as to why adding a voltage divider would affect it this way?

I’m really at a loss here and would appreciate any advice.

Thanks!

The resistance of the voltage divider may be too low.

17V - Sensor - /Switch/ - Ground
When the switch is 'open' the '17V' side is 'floating'.
This may be floating simply as an open circuit, or may have a 'pull-up' resistor.
What you have done is bypassed the switch with your voltage divider. If the resistance is too low, the sensor won't actually rise back up to 17V.

My advise would be to put your new sensor and voltage divider on the Ground side of the switch instead, or to rebuild your voltage divider with larger value resistors.