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Protecting PCB, multiplexers and MCU in vending machines

JuanSys

Nov 15, 2022
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Nov 15, 2022
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Hello everyone, I am working on a low cost vending machine with the ESP32 and as you may know, it is necessary to properly protect the components to prevent users from trying to steal money or damage the components. The PCB has a TFT display and several multiplexers to communicate the ESP32 with the buttons, coin acceptor, bill acceptor, hopper sensor, etc. It's common for people to try to give any of these components a small electrical shock, and multiplexers are the most vulnerable.

That is not my only problem, since the +5VDC and +12VDC power supply is not of very good quality, so I also need to protect/stabilize a bit the current that it supplies to the ESP32 and other components.

So I'm looking for what low cost components I should use to protect everything or at least the essentials. For now it has only occurred to me to add some electrolytic capacitors, some SMD capacitors and to protect against electrical shocks I don't know if I should implement a current limiter or Low-Dropout Linear Regulator or zener diode or some other component. I would greatly appreciate your suggestions. Regards.
 

kellys_eye

Jun 25, 2010
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Jun 25, 2010
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Inductors and ferrite noise suppressors (beads, cores around power leads etc) are commonly used to prevent surges (and interference). Thes might need to be on EVERY lead to/from the ESP32 i/o lines (as well as power - which should really have them regardless).

Capacitors won't help in the short transients periods an 'attack' may deliver.
 

Bluejets

Oct 5, 2014
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Nor will addition of additional devices at uC level to protect against electric shock....Assume you require to protect the user, then use off the shelf safety devices on the mains.
 

Harald Kapp

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Nov 17, 2011
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A good earth connection on all accessible electrically conductive parts will divert attempts to disturb the electronics by short circuiting the electric shocks to earth, not to the electronics.
You can also implement plausibility checks in software (e.g. if events happen in an unusually rapid succession or events seem to happen in implausible combinations).

Nor will addition of additional devices at uC level to protect against electric shock....Assume you require to protect the user, then use off the shelf safety devices on the mains.
I think the issue here is not an electrical shock to the users but an intentional electrical shock applied by the (ab)user to confound the electronics and make the vending machine spit out goods or return money without prior insertion of money.
 
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