gpioproximity-sensorraspberry pi
I have an NPN LJ12A3-4-Z/BX proximity sensor that I want to use to detect water usage on a water meter that has a rotating iron disk.
To make sure I don't overpower the 3.3v GPIO inputs I want to use a reed relay SIP-1A05 to keep the 5v circuit separate. But connecting input pin 18 to the relay does not seem to work.
Do I also need to connect the ground? If so, where to? With a resistor to the relay also?
Thanks
Edit with updated circuit from accepted answer.
To use this sensor, connect ground and power (6..36V). The third wire will be connected to ground when something is detected, otherwise it will be floating.
Note that it does not output 300 mA, that figure is the maximum current it can handle (sink to ground).
If you have a 4-20mA input you could use a resistor to supply 20mA to it, and use the output wire to ground the 4-20mA input to ground, so it will get 0 mA (or at least a good deal less than 20) when the sensor detects something.
You could use a serial port if you know how to read a handshake signal in your programming language.
You cannot increase the voltage by connecting GPIO pins in series, as they all are referenced to a common ground.
To drive your relay you should do something like:
simulate this circuit – Schematic created using CircuitLab
Best Answer
According to LJ12A3-4-Z/BX datasheet the sensor should be powered by 6-36V, so 5V may be insufficient ( brown wire = +V, blue wire = -V/GND ). Connect coil of the relay between +V and output of the sensor ( black wire ). Also connect a pulldown resistor ( a few kohm ) between pin 18 and 20.