Greetings, I need to build a DC

[jwilday]

Greetings,

I need to build a DC voltage sensor to detect if a field device is powered or not. The field device is an optical sensor that runs on 24VDC. The emitter side pulls 10mA and the receiver side pulls 15mA. The problem arises when operators remove a small shuttle with the sensor attached to it and forget to plug the sensor back in when they reinstall the shuttle. Due to a proprietary connector I cannot run a "confirmation" wire back to the PLC.
I would like to build a circuit that will detect when the sensor is plugged in (and under power) and can either operate a solid state relay or directly activate a PLC input. I have found some devices for purchase that go in-line with the sourcing voltage but they cost roughly $100 (way-way too expensive)

Thanks for the help
JW

 

[IRstuff]

Why can't you measure the current draw of the sensor at the PLC? TTFN

 

[jwilday]

IRstuff,

The PLC analog input cards are currently full. It would cost too much to buy a card with more points on it. I have about 300 machines to modify so any cost is multipied by 300. My time/labor cost is cheap so if I could come up with a solution for $10-$15 worth of componants I would be in great shape.
I have 4 digital inputs (ie on/off) which are triggered by being pulled to ground. That is why I thought a solid state relay might be useful.

 

[IRstuff]

If you can find the right relay with right resistance, you can potentially replace or place in series with either the bias resistor for the emitter or the load resistor for the receiver. TTFN

 

[dydt]

hi..
can you not put a small 24VDC lamp or buzzer with relay circuitry right by the connector using same 24VDC supplying your optical sensor? just to remind the operator that they have to plug the sensor back in?

dydt

 

[danielmurray]

Could you tell me more about the sensor and connector you are using. Is it a lumberg of brad harrison type mini connector or something similar? Is the sensor made of metal or plastic? What is the model and manufacturer of the sensor?

 

[ooge]

How about wiring the LED of an optocoupler in series with the supply line? Current flowing in the LED will switch on the optocoupler, and the output can be used to drive your relay/PLC input. A 4N25 or 4N35 should work fine.

A simple resistor network might be a good idea to limit the current through the LED, just in case the supply gets accidently shorted. Bear in mind that this way of doing things will result in a drop in the supply voltage.

 

[jwilday]

Hi Again,

Thanks for all the input and suggestions. I tried using 2 different relays. One a solid state and the other a mechanical reed style. Due to the current limiting in the sensor, there was not enough current to activate the solid state relay. The mechanical relay seemed to work but it took so much power to operate the coil that the sensor was not working.
I have found a solution however!!
I have built a circuit using an LM319N (NTE part # NTE919D) high speed dual comparator. This chip operates at up to 32VDC. I simply pass the negative voltage side through the circuit and the chip pulls its own output to ground when the sensor is powered. By placing the circuit in series with the sensor I have gained an additional signal to the PLC that is active only when the sensor is under power.