When you say 2v, what is the reference point for your measurement?
Most LED's need more than 2v to work.
If you are contemplating connections to the wiring of that machine can you be 1000% certain that there is no risk of dangerous mains voltages getting to the Arduino.
Would it be an option to mount a Light Dependent Resistor (LDR) that could "see" if the LED is lighting without any electrical connection to the machine? That would be very much safer.
What do you meen by Reference point? Wich wires i have measured?
I have used a multimeter and stuck the pins of it in the red en white socket utill connected to the wires.
When when the LED went on the mulitimeter read 2 V and when the LED went off 0.
Your idea of LDR has crossed my mind. But i thought first to do it hard wired for the stability of the system.
If hard wiring does not work and i burn 1 or 2 arduinos i wil try the LDR.
But for now the question remains. If there are no dangerous high voltages on the LED wires of this machine, the normal way to read these voltages id to put the red in a analogpin and the white to the ground of the arduino?
Maybe it is better to use my original post. and ask al my questions there? isn't it?
Kumalix:
What do you meen by Reference point? Wich wires i have measured?
I have used a multimeter and stuck the pins of it in the red en white socket utill connected to the wires.
When when the LED went on the mulitimeter read 2 V and when the LED went off 0.
Do you KNOW that the white wires are connected to GND?
If hard wiring does not work and i burn 1 or 2 arduinos i wil try the LDR.
I'd consider using an opto isolator eg 4N25 together with a series resistor connected across the existing led.
That way, you do not have to connect the grounds of the arduino and the binding machine together, and the risk is much smaller.
On some mains powered devices (probably not this one though) the device "ground" can be at mains potential.
Kumalix:
Your idea of LDR has crossed my mind. But i thought first to do it hard wired for the stability of the system.
Stability of which of the two systems?
Your Arduino ground is probably not connected to the machine's ground. Whether that's even possible depends on the machine's internal circuit, which you probably don't know.
Without having grounds connected, you don't have a shared reference and all kinds of unexpected things may happen, including one or more of the parts involved releasing their magic smoke.
If you really nwant to connect to the wires, go the optocoupler route. I'd also go for a rather large current limiting resistor, to not upset whatever it is that drives that existing LED. After all, all you have to do is pull down a very small signal.
Actually, by placing an LDR against the existing LED you create your own optocoupler.
You all seem cautious about connecting arduino directly to the machine.
Without that much of experience with this kind of stuff i will go for your advise and do it Robin's way: LDR.
You need to use the LDR with another resistor to make a voltage divider. Connect the middle of the divider to the Arduino I/O pin. It should be perfectly possible with suitable choice of resistor to arrange things so that the lamp can be detected by a digital I/O pin.
Kumalix:
You all seem cautious about connecting arduino directly to the machine.
Connecting two different electrical systems together can often be done - but only if you know both systems well enough. You don't know the machine in question well enough, which is perfectly normal as you didn't build it yourself. That's why it's not a good idea to start connecting external electrics to the machine's internals.
Basically the only way to connect an Arduino to another machine is if the other machine is also powering the Arduino, so you can be absolutely sure that the ground level of the Arduino is at the same potential as the ground level of the machine (which generally is NOT mains ground!). That's your reference voltage, and what we call 0V. Then you can measure the potential of the leads of the LED in reference to that ground, and build your own circuit based on it.
On top of that, you have problems with say warranty when messing around inside a machine. Or if it's your boss' machine, you may lose your job. Soldering indeed gives a stronger connection but a piece of duct tape should be enough to keep that LDR in place without making irreversible changes to the machine.
