I'm doing a big project, where I will connect a lot of sensors to my Arduino Mega 2560. Eagerly, I bought boards of this type, but I can't get them to give me any voltage output! I've connected them with +24V, and my 2 wire 4-20mA sensor, a pressure transmitter, to the other end. There's no documentation to be found about the board, despite it looking like a popular product.

I have Vout measured with a voltmeter, it says 0.13. It's no different than the other cards, where there is no sensor attached at all. I was expecting there to be +24 between I+ and I-, but there is nothing. It's 100ohm resistance between them internally in the card. I have 16 of them, and they all behave the same, so there must be somehing I'm not doing right Could it be that they are made for 3-wire sensors? How can I wire it differently in order to make it work? Here's a photo of the card: Photo of the card

  • \$\begingroup\$ Have you measured the voltage developed across the current sense resistor by the sensor output? The output is going to be near zero with the input below whatever the zero trimmer is set to. \$\endgroup\$ – Phil G May 3 at 14:13
  • \$\begingroup\$ Show us a sensor datasheet. \$\endgroup\$ – G36 May 3 at 14:55
  • \$\begingroup\$ This device is designed to be used with a sensor that outputs 4 to 20 mA. Is your sensor putting out current? I would try to design a system that just uses a precision resistor to convert the current to a voltage and handle the offset in software. \$\endgroup\$ – Mattman944 May 3 at 14:59

Your loop powered (two wire) transmitter + sensor has two leads, call them S+ and S-.

You need to connect one end of your two-wire sensor to the +24V (S+) and the other side (S-) to the + current input. No connection to the - current input (it's common with the power supply ground).

The two-wire sensor needs a power supply and you are not giving it any.

enter image description here

You should adjust the pots to ensure that you don't get more than 5V out of the outputs before connecting it to the Arduino. It would also be a good idea to clamp the output in case there is a fault with the sensor but that's outside the scope of this answer.

  • \$\begingroup\$ That worked very well, thanks for your time! Now I just need to calibrate it, which isn't easy, because I don't know how to simulate a 4-20mA sensor. \$\endgroup\$ – Sir Vegard G May 10 at 9:33
  • \$\begingroup\$ I use a calibrator which is very precise, however there are cheap simulators on the market from the usual suspects. I have not examined them but they’re probably okay for casual use, particularly is you have an accurate milliammeter to put in series. \$\endgroup\$ – Spehro Pefhany May 10 at 9:56

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