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momentary contact switch circuit?

momentary contact switch circuit?

momentary contact switch circuit?

(OP)
I'm looking for some suggestions on how to activate a PLC. Currently I have timer that outputs +5vdc for about 30ms when it goes off. I use this to close a reed relay connected to my PLC input and start the PLC's cycle.

What I want to be able to do is make a device that closes the relay for less than a second, when someone lifts an object off the table or floor.  So I can use a limit switch on a pad of some sort that is normally closed when say a coffee mug is sitting on it. When you pick up the mug, the circuit is closed and the PLC cycle starts.

My question is what is the easiest/cheapest way to limit the duration of the output to less than 1 second and how would the reset work?  Is there an inexpensive off-the-shelf device that will do this or do I need a custom circuit?

RE: momentary contact switch circuit?

???  Why not just use the PLC with a toggle input? Or a flipflop logic block?

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: momentary contact switch circuit?

(OP)
itsmoked,
You have a valid suggestion, but here is why:
1)I don't want to change my PLC program. It still needs to work with the timer the same way.
2)I was thinking about powering the circuit with a 9vdc battery and don't want to leave it on for long periods of time.

RE: momentary contact switch circuit?

-suggestions on how to activate a PLC?
-start the PLC's cycle?
-the PLC cycle starts?
-easiest/cheapest way to limit the duration of the output to less than 1 second and how would the reset work?
-I don't want to change my PLC program?

Sounds like you need to do a bit more studying about PLCs, so you can at least talk about them in some understandable fashion.  You might even figure out what they're used for.

RE: momentary contact switch circuit?

For simple and cheap, use a relay, capacitor and resistor.  Select a resistor of a high enough value that won't allow the relay to close or stay closed.  A value of 1K-2K is a good start.  This charges up a 1,000uF capacitor and powers the relay through the the microswitch.  When the relay discharges, the relay will open.  Down side is the capacitor requires some recovery time to regain charge.

RE: momentary contact switch circuit?

(OP)
Guys,
I’m not a student. I have a MS in Mech. Engineering, but I don’t have much experience with circuit design.

I'm not trying to control my PLC output.  I'm trying to control the INPUT to my PLC.

Let me see if I can explain my situation a little better.

I'm using a 12vdc Crouzet PLC with relay outputs. The program in the PLC is initiated by momentarily applying 12vdc to an input. Once initiated, the PLC runs through 1 cycle of its program which takes anywhere from 10 to 60 seconds. Today, I can initiate the program in several ways:
1. A momentary pushbutton switch  
2. A motion detector
3. A count down timer

I’ll refer to these 3 as “input devices”.  Each of the input devices is connected to a reed relay mounted on a PCB. When the reed relay closes, 12vdc is applied to the PLC input for the duration of the event. I'm using a reed relay because each of the input devices uses a different voltage. In each of the 3 cases above, the voltage on the PLC input is at 0 when the cycle completes.

I want to add a fourth input device.  This is what I described in my initial post. A pad has an object on it and when the object is lifted, the reed relay is closed for less than 1 second which starts the PLC running.  After the PLC cycle is complete, the system is reset by placing the object back on the input device.

Since I have a number of these systems deployed, changing the program in the PLC is not a desirable option.

OperaHouse, I think you understand where I am going.  The time required to recharge the capacitor should not be a problem. Would you sketch the circuit you are proposing or point me at a similar diagram on the web?

RE: momentary contact switch circuit?

You could also approach this by using a "one-shot" (monostable flip flop). It isn't quite as simple as the relay/RC method OperaHouse mentioned, but worth a look.

Bowden's Hobby Circuits web pages have a wealth of such circuits using everything from discrete transistors, to 555 timers.

http://ourworld.compuserve.com/homepages/Bill_Bowden/page9.htm#mono.gif

RE: momentary contact switch circuit?

(OP)
rawelk,
The 555 timer Mono stable (one shot) circuit looks like exactly what I need!
Thanks,
-Capel

RE: momentary contact switch circuit?

I suppose I came down on you a bit hard,,, but your clarified more detailed posting was faaaaaarrrrr more intelligible.

Best of luck, and don't let a skeptic like me run you off.


Me wrong?  I'm just fine-tuning my sarcasm!

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