Argon maximum pin input


#1

I am trying to sense an input at a pin. I was wondering what the max voltage is that I can safely send to a pin to sense. I would like to send 24VDC for an extended period of time, say 1 hour. Basically when I sense a change at a pin, do code. But the voltage I want to sense is 24VDC. If I need to step it down what is the best solution for that, just a resistor? And what do I need to step it down to, to safely sense it? I really am not interested in what the voltage is on the pin but that there was a change from 0. I guess I would need a “high voltage” relay but in reverse. I would not control the relay with a low voltage but sense a high voltage, engage relay and send low voltage.

Any help would be great.

Thanks


#2

Hey @jhill1440

The maximum voltage you can expose for a Mesh board would be 3.3V.

There are a bunch of ways you can accomplish this.

  1. If you’re checking for a binary 0/1 you can isolate it with a MOSFET. An example circuit is here

This will not isolate your low voltage circuitry completely. Without knowing more about your system it’s not 100% safe solution.

  1. You can use an opto-isolator.

These guys are more forgiving. All you need is higher rated resistor and to make sure you’re not over driving the circuit. The H11L1S is a great first stop.

Hopefully that helps!


#3

I recently had this issue. I was using a Photon to sense high/low from an alarm system. I didn’t know high was 12V until I plugged the argon in and it reported an SOS at power on. The Photon didn’t care but I’m sure it’s not supported and may not work long term.

I went with a simpler option of using a voltage divider. A quick Google and the use of a voltage divider calculator will have you on your way :slight_smile:


#4

Hello, thanks for the quick replies. I was afraid that would be the case to see only 3.3v. Ok now to step it down to something readable. And yes I am really only looking for a binary 0/1. Voltage or not. Thanks


#5

You can always use a simple voltage divider, to step down the voltage safe enough for the analogue input. So design the voltage divider for 24 V so that it steps down to 3.3V, and then you can build the software logic and set thresholds accordingly.


#6

A quick trick I use (a bit more expensive but if its only a once off need…) is to use a low cost voltage regulator like this. It deals with surges and normally just shuts down if anything is out of range. Add a small capacitor (10n) on the input side and a 1uf (or so) on the output side.

https://www.digikey.com/product-detail/en/stmicroelectronics/L78L33ACZ-AP/497-16175-3-ND/1663444