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?
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?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: momentary contact switch circuit?
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?
-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?
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RE: momentary contact switch circuit?
Dan - Owner

http://www.Hi-TecDesigns.com
RE: momentary contact switch circuit?
RE: momentary contact switch circuit?
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?
Bowden's Hobby Circuits web pages have a wealth of such circuits using everything from discrete transistors, to 555 timers.
h
RE: momentary contact switch circuit?
The 555 timer Mono stable (one shot) circuit looks like exactly what I need!
Thanks,
-Capel
RE: momentary contact switch circuit?
Best of luck, and don't let a skeptic like me run you off.
Me wrong? I'm just fine-tuning my sarcasm!