momentary contact switch circuit? 1

[cenglish]

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?

 

[itsmoked]

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

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[cenglish]

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.

 

[HCBFlash]

-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.

 

[HCBFlash]

also ref:
faq731-376

 

[MacGyverS2000]

Starting to sound like a student project...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[OperaHouse]

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.

 

[cenglish]

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?

 

[rawelk]

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

 

[cenglish]

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

 

[HCBFlash]

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!