Controller input protection - Page 1
Forum section: electronics / beginners

author: haaad
author_rank: Newbie
timestamp: July 21, 2023, 10:39:02 am
content: Hi to all,Could you advice/direct me with circuit for protectingcontroller inputs for alarm system I would like to make.Controller is Raspberry Pico. (cheap and powerful).Story behind...Alarm systems are bloody expensive...And since I already have many of the components at hand...Existing designs I have researched are ok and probably would work, but...I would like to have EOL/DEOL capability - so I will be feeding the line to ADC on Rpico - in simple words - monitoring voltage on the line...To expand number of channels I will be using 2x analog switch 16:1 CD74HC4067 multiplexer, 32 zonesWhat I would like to know - what circuit you would use to protect the system from zapping or others...Any other notes?Best regardshaaad

author: electron_plumber
author_rank: Regular Contributor
timestamp: July 21, 2023, 04:47:28 pm
content: The solution depends on your requirements and your constraints.what inputs you're trying to protect (sounds like ADC inputs)what you're trying to protect them fromhow much money you're willing to spend on your solutionare you designing a custom PCB, or are you looking for an inline solution? If the former, how much space do you have on the board?etc, etc,For ESD protection, diodes are a common solution. In some high-volume/low-cost designs, I've seen ceramic capacitors used as cheap ESD protection. The idea there is that when you take an ESD gun and inject X coulombs of charge, an ideal capacitor will develop V=X/C across it. Obviously, ceramics aren't ideal, so you need to know what you're doing here to make this work. In either case, placemen is important to minimize inductance to whatever you're using for ESD protection. Just read the app notes.Sometimes ADC datasheets will allow over-voltage conditions as long as you limit the max fault current, which you can do with an inline resistor. Just keep in mind that any series resistance on an ADC channel increases the impedance that the channel "sees" on the net it's monitoring. This can impact bandwidth and can cause errors due to leakage currents and minimum settling time violations. If you google those terms, you can read all about it.If money and space are not a concern, you can add all sorts of fancier protection schemes. E.g., you could go hog wild and add an op amp to every analog channel that's designed for high voltage withstanding in rough environments. That wouldn't be a very cost-optimized solution, but just pointing out that there are lots of options and the best solution depends on your requirements and constraints.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 21, 2023, 05:47:38 pm
content: Are your alarm inputs from simple current loops?Twisted pair?Four dirt cheap things.1. Put a resistor R2 in parallel with the switch- this helps to stop the loop becomming an antenna when the switch opens.2. Bypass RF and transients to ground with the cap.3. The MUX IC will have internal protection diodes. A series resistor R3 limits the diode current to a safe value.You can add external didoes if you feel like it. I don't see the benefit, others will differ on this I'm sure.4. Fuse the loop. Some fool may short it to mains or a phone line by accident or out of spite.

author: haaad
author_rank: Newbie
timestamp: July 21, 2023, 07:20:47 pm
content: Hi there,Thank you for the answers.I will study them...Meanwhile I would like to clarify - saying alarm systemI assumed everyone will know that I will be using standard burglar alarm components like PIR, bellbox, door contacts,...Sorry for not clarifying this. And even if you are good in electronics you may not know the specifics...I will try with some visuals now (hold on to something).Ready?Alarm PIR inside is like 2 switches (simplified)- tamper- pirImage shows DEOL configuration (switches should be NC)At main board we would have voltage dividerHope this explains it.And I hope the pictures will pop up where they should - preview does not work?Best regardshaaad