Photoelectric Sensor (PLC Newbie Question)

Hello,
I have a couple of questions and would appreciate any help from you guys.
Im basically trying to learn PLC's by playing with them at home. I have a simple conveyor made from Vex robotics parts. Its about 2 feet in length and 4 inches in depth. I have 2 simple push button switches that are latching and unlatching the DC motor. I'm using a Allen Bradley Micrologix 1000 with no Analog inputs or outputs - Just 6 relay outputs. I'm looking for a photoelectric sensor to position on my simple conveyor. It would be great if it was cheap. I'm guessing i need to use a through photoelectric sensor. Transmitter & Receiver b/c i have only the digital inputs to use? Im using simple puck like objects on the conveyor. They are made out of wood but i could get simple plastic parts if that would help.
My question is can you recommend a simple, cheap photoelectric sensor for a PLC that only has digital inputs? Am i asking for too much. Thanks again for the help.
-Tim

tim_callinan said:

My question is can you recommend a simple, cheap photoelectric sensor for a PLC that only has digital inputs? Am i asking for too much. Thanks again for the help.
-Tim

Click to expand...

There are so many makes and models to choose from. Since you are a student on a budget your best bet may be to get one on Ebay. Make & model is not important.

Most photeyes supply a digital output of on or off. There is ussally a pot on the photoeye to adjust gains. When picking the photoeye make certain to get the right voltage that will match the voltage of your power supply and the input range of your PLC. The other important factor is to get either PNP or NPN photoeye to match the input type of your PLC. Chances are the Micrologix needs PNP but I believe you can get a Micrologix vesion that needs NPN inputs. PNP mean that the input is normally at 0 volts until the device turns on and pulls the input up to the supply voltage. NPN means the input is normally at the supply voltage until the input turns on and pulls the voltage down to 0 volts.

There are several type of photoeye optics. Thru beam has two seperate units one transmits light and the other recieves light. Relective and retro relective types are self contained units that bouce the light off a relector mounted behind the target or off the target itself. Any of these should work on your conveyor if they are mounted properly.

There are also light operated and dark operated options. Light operated turns on the output of the PE when it sees light and off when it does not. This is not important since it is easy to invert the logic in the PLC program.

You can also look at diffuse photoeyes to sense an object on a conveyor. This type of photoeye senses the light that bounces off the object to be sensed. There is no need for a reflector or a send/receive unit.

Cheap Sensors.

Here is a place that has cheap sensors and take credit cards:

http://www.automationdirect.com

Setting up the sensor should not be an issue. You may want to spring a few extra bucks for some brackets so the only thing you are fighting is your code and not the sensor alignment.

Good luck

Thanks for the education guys.
This is a great site.

Although as previously stated the brand of photoeye for your purposes is not important, on the other hand the voltage rating of the photoswitch is someting to pay some attention. Some of the MicroLogix 1000 PLCs have 120 volt AC inputs (1761-L16AWA, 1761-L32AWA, 1761-L20AWA-5A, and 1761-L32AAA) and all the other Micrologix models have 24 volt DC inputs.

You want to match your photoswitch voltage rating with whatever type of PLC input that you have. Yes, there are AC-rated photoswitches that will produce an on/off input to a AC-input Micrologix PLC.