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Picture of Wireless Safety Button for PLC Safety

This project is my proof of concept for using IoT and (eventually) robotics to create an additional layer of safety for hazardous manufacturing facilities. This button can be used to start or stop multiple processes, including the control of signal lights. While I refer to this project as an e-stop button, please note that installing true e-stop controls requires a lot of redundancies and regulations. This project is simply meant to add an additional layer of safety.

Please use safety precautions when wiring and powering this circuit.

Step 1: NODE MCU Server Circuit Wiring

Picture of NODE MCU Server Circuit Wiring

A NODE MCU board will be used as the server, and is the middleman between the button and the PLC. When the button is pushed, the server will receive a signal, which will energize a relay, and send the necessary signal to the PLC to stop all processes.

Board Wiring

To power our NODE MCU, simply use a micro usb wall adapter.

Relay Crash Course

The relay consists of two parts; the coil, and the armature. The coil can be energized, which creates a magnetic field, which will cause the armature to move from the normally closed (NC) position to the normally open (NO) position.

To determine which pin is NO and which is NC, use a multimeter and set it to measure resistance (2k Ohm range). Touch the red lead to the middle pin, and then measure each of the opposite pins. The NC pin will be electrically connected, so you should see a small resistance reading. The NO pin will not be electrically connected, so the reading should be over range.

Once the NO and NC pins have been established, solder or attach two wires to the coil (black wires in the picture above), one wire to the middle contact pin, and one to the NC pin (green wires).

Board to Relay Wiring

We now need to connect our relay to the board. We need to supply 5v to the relay coil in order to engage the armature. Since the NODE MCU board only outputs 3.3v, we need to use a transistor to amplify the signal. Refer to the wiring diagram for circuit connections. Please note that if a different pin is used for the signal, it will need to be changed in the code.

Relay to PLC Wiring

Wire the central pin to the 24v source, and the NO pin to input terminal 1 on the PLC.

garzo3 months ago
In my region, and i'm quite sure that this is true for most part of the world, if it resemble an e-stop (same shape and color) IT MUST BE A PROPER E-STOP! In factories you are not allowed to use a red-mushroom-shaped button if it's not a compliant button with a compliant wiring (o a certified wireless mean of communication). This could be useful as remote input, no doubt on that, but don't consider it a remote e-stop because it doesn't "add an additional layer of safety", quite the opposite.
pben3 months ago
I have a question. Have you looked at the electrical code that requires E-Stops to be hardwired? Wireless controls have been used in industrial controls for at least twenty years that I know of but E-Stops are required by USA national and local codes to be wired in such a way that any break in the wiring will disable the machine under control.

Is there the same requirement in your region?

Wireless controls can be used in the USA if reasonable techniques are used to have reliable control of the machine. So standard start, stop, and other control can be wireless. E-Stops are a special case because of safety.
ZacharyA22 (author)  pben3 months ago
I can't specify codes or regulations, but I'm certain there are many for e-stop installation. This project is not meant to replace wired e-stop buttons and I'm fairly certain I would need a lot of review / certification for it to be implemented. As it stands, this is just a prototype that could exist alongside current safety protocols.

If you or anyone can cite specifics I'd be happy to update my instructions to reflect current regulations.
xivadi ZacharyA223 months ago

Yes, I wouldn't call it an E-Stop or make it look like one. I'd only use it in a situation where you assume it will fail without harming people or equipment (or other life for that matter). :-)
You can't just have this "alongside" a traditional rated e-stop unless you don't call this an e-stop. If it's an e-stop, it must confirm to applicable regs. If you call it a cycle stop, and don't color it like an e-stop, then go wild, just don't call it an e-stop.
Check out NFPA 79 - Electrical Standard for Industrial Machinery. It has loads of info on that sort of thing. I used to refer to it regularly in my machine controls design days, and most panels I work in now are designed to meet those guidelines.
Thanks for the reference. Back when I was a draftsman for a controls company in the 90's I remember having to dig through the big fat national code book a couple of times. There were a few wireless controls just coming out but we still had to lay conduit and wires for the E-Stops. I haven't had to deal with industrial controls since 2003 so my knowledge may be a little out of date now.
FUSIMESTER3 months ago
You need redundant and self-checking circuit for wireless safety devices. There are some existing products:
Charles Su3 months ago
Nice idea for a proof of concept. Wireless e-stops do exist, but involve much more.
To be sure, this is not a safety rated e-stop. For that to be the case, it would need some sort of way for both the PLC and the e-stop to recognize that their communication is active and functioning. ASAIK, Europe tends to be even more strict than the US.