Protecting PCB, multiplexers and MCU in vending machines

[winSOW32]

TL;DR Summary

It's common for people to try to give any of these components a small electrical shock, and shift registers are the most vulnerable. So I'm looking for what low cost components I should use to protect those components.

Hello everyone, I am working on a very low cost, small and basic vending machine ($300) with wooden cabinet for small neighborhood stores. 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 shift registers 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 TVS diodes, 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.

 

[berkeman]

Welcome to PF.

multiplexers are the most vulnerable.

What's a multiplexer in this context?

 

[winSOW32]

Welcome to PF. What's a multiplexer in this context?

Sorry, I got a bit confused. They are shift registers. It is a small, very basic and cheap vending machine. The PCB includes several shift registers to control the peripherals and I'm looking for a way to protect them very cheaply. It is not a large or very expensive business equipment, so I look for something cheap. Many people have recommended me to use TVS diodes :)

 

[berkeman]

A protection scheme for static discharge and RF immunity begins with the metal chassis of the machine. I assume it uses a metal chassis that is Earth grounded through the power cord?

 

[winSOW32]

A protection scheme for static discharge and RF immunity begins with the metal chassis of the machine. I assume it uses a metal chassis that is Earth grounded through the power cord?

It is a very economical and basic machine made for low-income places, so the cabinet is made of wood and at most aluminum sheets or an aluminum mesh can be added. But my work is not the cabinet, but the PCB. So I want to see ways to protect the components, especially the shift registers because they are the most vulnerable to those little current sparks.

 

[berkeman]

Standard ESD testing (via EN 61000-4-2) involves injecting static discharge shocks into all user-accessible stuff, including keyboards, displays, any metal points on the chassis, etc. The normal way to protect the internal electronics from such external shocks is to use diode-type clamping to the chassis (via TVS as you say or regular diodes clamping to the power supply rails with good bypassing to ground). All the external sparks will be generated with respect to Earth ground, so you want to return those transient currents to Earth ground (usually the metal chassis) as quickly as possible, without the ability to flow through any electronic circuits.

So the PCB traces to the keypad, for example, will all have diode clamping to Earth ground, since that is the most common place for human shocks to be introduced. Displays will be covered with clear conductive membranes that are Earth grounded.

If you really cannot use a metal chassis, then you will need to run wide strips of Earth ground conductors from the power cord entry point to the human interface area, in order to do the diode clamping.

Standler is the standard reference book for learning more about this subject (use the Amazon "Look Inside" feature to see the table of contents):

https://www.amazon.com/dp/B00A736SOM/?tag=pfamazon01-20

 

[berkeman]

BTW, if this enclosure will be made out of wood, what is to prevent somebody with a modest size hatchet from just opening it up?

 

[winSOW32]

BTW, if this enclosure will be made out of wood, what is to prevent somebody with a modest size hatchet from just opening it up?

That does not usually happen because the machines are installed in small businesses that are always supervised by people and contain very little money inside, it is not worth going to break a machine to steal money. However, many people carry spark plugs or devices that can burn/alter components and cause the machine to pay them money or give away credits.

 

[winSOW32]

Standard ESD testing (via EN 61000-4-2) involves injecting static discharge shocks into all user-accessible stuff, including keyboards, displays, any metal points on the chassis, etc. The normal way to protect the internal electronics from such external shocks is to use diode-type clamping to the chassis (via TVS as you say or regular diodes clamping to the power supply rails with good bypassing to ground). All the external sparks will be generated with respect to Earth ground, so you want to return those transient currents to Earth ground (usually the metal chassis) as quickly as possible, without the ability to flow through any electronic circuits.

So the PCB traces to the keypad, for example, will all have diode clamping to Earth ground, since that is the most common place for human shocks to be introduced. Displays will be covered with clear conductive membranes that are Earth grounded.

If you really cannot use a metal chassis, then you will need to run wide strips of Earth ground conductors from the power cord entry point to the human interface area, in order to do the diode clamping.

Standler is the standard reference book for learning more about this subject (use the Amazon "Look Inside" feature to see the table of contents):

https://www.amazon.com/dp/B00A736SOM/?tag=pfamazon01-20
View attachment 317204

Very useful and interesting, I will take a look at it. What you say is true, here everyone protects their machines by putting metal meshes on the displays, aluminum sheets on the pcb (especially the controllers) and some connect the power supply cord to ground.
That is another big problem. A lot of people don't connect their power supplies to ground, in fact I don't know of almost any machine that is grounded with the power cord. So I have to find a way that these small current discharges do not damage the components. Many competitors have already implemented protections that temporarily reset or shut down the PCB upon detection of the attack, although many times it is not effective or is not detected at all. We are talking about PCBs that are sold for only $60, so they are very basic and the protections must be as cheap as possible. Again, I'm only a PCB vendor, so I can't get my customers to take the necessary action, many won't do it simply to not spend an extra $1.

 

[Baluncore]

@winSOW32
Maybe you need to detect the static discharge, then sound an alarm and lock the system for one minute, maybe with a count-down in seconds on the screen. That will quickly teach people not to shock it.

 

[winSOW32]

@winSOW32
Maybe you need to detect the static discharge, then sound an alarm and lock the system for one minute, maybe with a count-down in seconds on the screen. That will quickly teach people not to shock it.

The solution to detect current surges and activate the alarm is the one used by more than 90% of manufacturers. In theory it is useful, but in practice I know it is of little use. People already know that and when they try to attack the machine, the alarm goes off and they quickly shut down the machine so the manager won't hear the alarm. Then people turn on the machine and try again but adjusting the intensity or trying different points on the machine.

Basically the alarm helps them regulate their devices to help them find the sweet spot hahaha. I was thinking of making those devices just have no effect (by installing the diodes) or lock/shut down the system for several minutes. The owner of the machine doesn't even think of blocking access to the power switch or plug, but then they come back sad saying that someone burned their PCB or stole their profit.

So the only thing I can do is implement protections on the PCB and a good manual with useful tips. And by implementing good protection on the PCB, I will have a huge advantage over the competition.

 

[Rive]

Insulating the sensitive parts and providing ground near the sensitive entry points to 'catch' the sparks is the first solution I can think of. This is also likely the cheapest solution.
Then comes the serial resistors and TVS diodes near the PCB ports. This can cost some decent money depending the amount of ports protected, and while it helps, it just can't solve every kind of problem.

One more: the circuit and software should be built so that EMP-ing it can't bring it to any undesired state. This one requires some delicate engineering, but without this it just won't work right.