Sensor/PLC interface troubles

Hello,

I was wondering if I could find any help on here. Im brand new and quite frankly haven't a clue what im doing.

Im undertaking a project at the moment that requires me to interface sensors with a PLC. the sensors I'm having problems with currently are OPB 704 Reflective object sensors . I am using a Mitsubishi FXo-MR-ES PLC.

I have done tests to see whether simply putting 24VDC intot he sensor and connecting it directly to the PLC would trigger the input however I have found that the PLC brings voltage from the sensors down to about 4 volts. the PLC needs a minimun of 12VDC at 3mA to detect an input.

I have toyed with Optoisolators as a form of interfacing but this didn't work. i was thinking of putting a sort of driver into the circuit to interface the sensor with the PLC.

this is my first post so I'm sorry if my description doesnt make a lot of sense but I'll be very grateful if someone could shed some light as to what is the best way of interfacing.

Thank you.

If these are the sensors you are talking about: http://optekinc.com/datasheets/opb703-705_70a-70d-b.pdf

My suggestion would be to buy a sensor made for PLC's and save the engineering time.

If you really need to use that sensor, can you give us an idea of how you have it wired right now?

Welcome to the forum!

A quick search for that sensor brings me up a datasheet which looks very...electronic-y. By which I mean, it doesn't look like an industrial sensor at all, it looks very much like a little electronic one you'd put in a children's toy or an arcade machine wired direct to a PCB, rather than a sensor designed to be used with a PLC. So my first piece of advice would be, throw that sensor in the cupboard and buy one that's designed for what you're trying to do. You can get a PE very cheap these days.

But that's not what you asked!

The first thing I'd be checking is that you have a zero volt connection to your input common on your PLC. I'm not familiar with the Mitsubishi range, so I can't point you to a specific terminal, but each bank of DC inputs will need a zero volt connection so that the 24V on your input terminals have a reference point. There might be one zero volt terminal for the whole PLC, or it might be three or four - it depends on the model.

But, if I'm reading the datasheet correctly, your collector/emitter is only a 5V signal (which again makes me think it's more designed for PCB work than PLC work). So yes, you'll need an interface relay. You will need a relay with a 5V coil. You'll connect the anode/cathode of the sensor to +24V and 0V, and then you'll need a 5V supply connected to your collector. The emitter will connect to the coil of your relay, with the other side of the coil connected to 0V. That will mean your sensor will now switch the relay on and off, and you should be able to wire 24V through the contacts and into your PLC input.

But again - by the time you mess about trying to find a 5V supply and a 5V interface relay, and wire it all up, you'd be better off getting the right sensor for the job and making your life, and everyone else's life, much easier.

That sensor can't be directly connected to a PLC.
Not sure how you put 24DC into it. The LED needs 5 Volts with a resistor. 330 Ohms would give you ~10mA. If you connected 24DC directly to it, it's bad now.
The output needs a buffer circuit such as an opamp.

Chose something from here.

My appologies, I have connected a number of load resistors, a 1K and a 30K. I failed to specify that this is actually a project I am doing towards a qualification in engineering so I will have to use the sensors that I have descibed above.
My lecturere was mentioning the use of a transistor as a driver in this set up but they also said that finding a transistor that could take 24VDC would be very hard.

Again I'm sorry as I've only started learning about PLCs about 4 weeks ago, my existing knowlege is next to nothing.

Thanks

Here is someone's wiring diagram for 5V.

https://www.google.co.uk/amp/s/andreweast01.wordpress.com/2014/03/12/opb704-sensor/amp/

I hope it helps you scale up the resistors for 24V. I note that the 704 allows 30VDC reverse bias on the phototransistor, and 2VDC on the LED. Make sure you take the resistor wattage into account when buying resistors. V^2/R and all that.

Use the schematic AustralIan posted. Connect it's output (pin labeled input) to the part labeled Arduino here.
Finding a transistor that could take 24 volts is hard? I don't understand. It would be harder to find one that didn't.