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#8384 03/20/02 03:21 PM
Joined: Dec 2001
Posts: 50
I have a control setup with a photo eye's output controlling a solenoid valve. This system is set up to control the tracking of a wide sanding belt. this belt ossilates from side to side on it's drums (as it is supposed to do). The photo eye is set up so that when the belt ossilates over so that the photo eye cannot "see" it, it activates the solenoid which activates an air cylinder which sends the belt moving the other way. My problem is, this photo eye does not always read real clear and therefore does not send a solid signal to the solenoid and therefore causes the solenoid to "chatter".
Finally, here's my question: Is there a device available that I can install between the photo eye and the solenoid to "smooth out" the signal the photo eye receives?


Joined: Jan 2001
Posts: 1,044
Tom Offline
You could put in a timing relay, wired to latch upon the first signal from the photocell. Set the time so that the solenoid will operate long enough to achieve the desired opeation. When the time runs out, timer de-energizes itself & the solenoid.

I use a similar timer for a diesal fuel pump that the truckers just can't seem to turn off. I think it cost around $40 or $50, has selectable time ranges from seconds to minutes to hours.

Maybe someone else has a more elegant solution.

Few things are harder to put up with than the annoyance of a good example.
Joined: Mar 2002
Posts: 6
Junior Member
I suggest you should use a micro PLC, you can connect the photo eye's output as an input signal of the PLC and the PLC can control the solenoid valve. the PLC can filter the fake signal of the photo eys (by programing).I suggest you can use the Siemens LOGO!.
you can mail me for more detail infomation.

Joined: Sep 2001
Posts: 806
A PLC for a single solenoid valve? That seems like killing a fly with a bazooka, no? smile

A few simpler/cheaper options that might work:

A sharper light source (maybe even a laser diode), to focus the light onto a smaller area of the target.

A corrective lens in the optical path (for the same reason)

A "delay on make" time delay relay or a "one-shot" timer between the photocell and the solenoid.

Use 2 photocells spaced an inch or so apart, and a relay that only fires the solenoid when both of them "agree".

If the output of the photocell is DC, a capacitor hung across the output may provide some "damping". Check this with a scope.

Joined: Jul 2001
Posts: 599
JBD Offline
I agree with njwirenut, you need to use a one-shot timer. This type of timer takes a momentary or maintained signal and converts it into a timed signal. The output contacts close immediately and then open after the time delay, whether the input is still there or not.

The Seimens Logo is better refered to as a smart relay and not a micro PLC.

For the others,
Smart relays (also available from Square D, GE and AB) have just a few inputs/outputs, several internal timers, and they are programmed from the front like a thermostat or lighting controller.
They are best when you want to use several timers in an application. They sell for only $150 to $180.

Joined: Feb 2002
Posts: 14
a micro switch maybe on each side in suitable inclosure..?

Joined: Dec 2001
Posts: 50
Thanks for the response, This belt cycles back and forth about 60 times a minute. I've also done some more research since I posted. While I liked all the suggestions, My thinking is also as NJwirenut and JBD suggested, the single shot timing relay. Most machines of this type are set up similar to what NJwirenut described with 2 photocells facing each other. I guess this manufacturer decided to cut corners [Linked Image]

Joined: Aug 2001
Posts: 31

There are 3 basic types of photo-eyes on the market. Through beam, retro-reflective, and diffuse. You always need an emitter and a receiver. Through beam requires two seperate sensors (a seperate emitter and receiver.) Retro-reflective and diffuse have both emitter and receiver onboard the same sensor, the difference is how the reflection or lack of reflection is handled internal to the sensor.

The reason I point this out, is that all sensors are not created equal. You might find that a different type has better accuracy and less bounce without changing anything other than that.

NJ wirenut is right on the money. I would add a redundancy with a second sensor to make an AND gate....i.e. both have to be on.

As far as timers go. Once you add up the amount for timers and the like. A micro-plc might be just as cost effective. Cost's have came down quite a lot over the last few years.


Carl Lee Tolbert
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