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Pulse input from SS switch to create latching Start / Stop button

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I'm curious to see if someone else out there has had a need to do this and if there is a cleaner way to accomplish it.  

Many of our panels use a single mechanical switch housing that we call a "Push / Pull".   It combines the function of a momentary NO Start button and momentary NC stop button, and pilot light, into one physical housing.  You pull it to engage the NO start and push it to engate the NC stop.   Used to use an IDEC product until they discontinued the line.  Currently using an alternate brand however its much more expensive and we are having QC problems with it.   

So my vendor gives me a sample of a Solid State Switch with an integral LED pilot light that can be configured to different colors.   It only has 1 switch contact, however there are no moving parts and seems quite robust.  The sample has both red and green LEDs.    

Since the unit has to drive a PLC input anyway,  I wrote a basic test program to evaluate its functinoality.    Here is a screen shot, took 4 lines of code, not very complicated.   

Bascially on first scan the PLC sets 05 and lights the green LED in the switch, could be considred a "READY TO START".  

Press the switch and its pulse output resets the GREEN LED output and sets the RED LED output.   Cosnider this a "On"  state.  

Press again while its RED is lit and it functions as a stop button.   



Works fine but I needed the two short fractional second timers to keep the PLC scan time from running faster than the pulse transtions.  

While that in of itself is an issue, I would like to avoid the timers, especially since the Samba platform is limited to 32 timers.   

Anyone have a better idea?   


thanks ! 



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After reading the need described in the author's post, I can state that the proposed solution has an 1 contact SS button that, through the PLC, simulates the operation of the original switch.

The main problem of this decision is non-compliance with safety requirements. The switch

2 hours ago, Jeff164 said:

Many of our panels use a single mechanical switch housing that we call a "Push / Pull".   It combines the function of a momentary NO Start button and momentary NC stop button, and pilot light, into one physical housing.  You pull it to engage the NO start and push it to engate the NC stop.   

that you once used had a clearly defined state. In the event of an accident, the User has the skills and understanding that pressing the button will stop the machine.

In your SS button case, it is impossible to predict whether the user will press the button only 1 time and not allow the machine to continue working in a dangerous situation.

At the same time, modern equipment safety requirements require that the production device must have separate on-off buttons and a separate emergency stop button. Simplification is allowed only in household appliances.

I don't know where your solution applies. However, I advise you to familiarize yourself with the safety requirements for this type of equipment.

A possible solution through the PLC is to turn on the system by pressing the button for a longer time - for example, 2s and turn it off immediately when the button is pressed in the working state of the machine. But it still does not meet the safety requirements.

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@kratmel - Thank you for your reply, I truly do appreicate it.   I will need to dig into this further.   Is it possible for you to cite the appliciable code reference? 

Our existing Push / Pull start-stop device is one device, IE while it has 2 functions, its one device, one panel cutout.   The manner in which you operate it defines the outcome.  The issue we are currently having is that the original MFG we used has discontinued the line, and our current supply has some QC issues, IE we get switches that fail to make contact on the NC side and sometimes or fail to open the NC contact.   It has been brought up to the distibutor but beyond credit for faulty product there doesnt seem to be any resolution.  

I started exploring this solid state switch as a means of addressing that issue.   The solid state switch has an integral LED that is color configurable, so the red and green lights in my solution were to drive the green light as in a "ready to start" mode and the red when the switch would function as a stop switch.   The contact output of the solid state switch is a single pulse out, irregardless of how long you hold the switch so a longer press would not help in this instance.   

We do have a seperate, maintained contact Emergency Stop mushroom head type pushbutton on all machines and this will always be a hard contact switch.  

Again - I thank you for pointing this out - I've been workign hard to improve our products and I'll not knowingly skirt safety regulations.  

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Aside from the Safety factors that Kratmel pointed out (to which you responded that you do indeed have hard E-Stops), that sometimes this type of change should be "blessed" by the OEM, and of course, all changes well documented.

I believe this to be an efficient modification to the original design, I have done similar designs while updating older Push-Button-Relay machines to PLC Control, and using single push/push Touchscreen buttons (I.E., a single button that changes color, Green - ready/push to start, Red - running/push to stop). Also I agree with the start delay feature (along with a buzzer that warns others that the machine is starting).

Are the Red/Green LED's separate devices, or polarity change dictates color?

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