Hi,
I need some way to get a microcontroller to read a signal from a dry-contact relay. Easy enough, but the interface is banana jacks, and this thing will be handled and used by people with no knowledge of electronics or much respect for the unit. I'm not expecting anyone to plug a billion volts into it, but I'd like it to be able to survive 12, and the usual static shock from handling.
The unit is powered by a single lipo cell.
Cost isn't super important here, as long as it's not stupid expensive. The rest of the unit probably costs around $150 in parts and a few hours of labor for me to assemble, so I'd rather not cheap out on input protection and have it bite me later.
So, what's a reasonable way to do input protection on this?
My current guess:
- power the powered output jack via a PNP transistor connected to the battery, not a direct connection to a microcontroller pin
- TVS diode between the two banana jacks
Things I'm not sure about:
- PTC fuse resistors on the banana jack inputs? If I do this, does it go before or after the TVS diode? If there's a constant voltage, I want the fuse to blow up before the TVS diode does, but will the TVS even work to suppress static shocks if it's after the fuses?
- Is there any point in using an optoisolator here? I have a bunch lying around, but considering it's not actually an isolated power source, is there any advantage over just using a transistor?
- Is there any point in adding a fuse here? I figure by the time even a small fuse blows, there's been enough damage to the rest of the unit that the fuse won't matter.
Some ideas:
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Thanks!