Sorry but somehow I wasn't receiving notifications on this topic's activity.
I sometimes wonder myself if my questions are "xy problems".
And I try to make it simple so it doesn't get people bored just from reading the first post. Maybe others do this too, be it consciously or not.
Okay, let's lay all my assumptions on the table.
To transmit a signal over 40 meters, 5v won't do it, let alone 3.3v, at least not easily, and not with any cable.
The most simple reasons, I think, leaving out losses, impedance, capacitive coupling - I really don't know the theory - are the cable not being shielded, "cross-talk" with the other communication wires, generally the noise from AC cables near it, or EMF (I mean radio noise). Maybe all this counts as capacitive coupling, I wouldn't know. I think that's what you get when you try to learn things over the internet.
And then, a device can only go so far as to drive an output, being able to sustain it over distance, and unprotected from external hazards. This stresses the device, and stress is bad for your health.
Also, I was always led to believe that a higher voltage would be okay to work around distance/interference problems. Work around as in "not having to worry about solving" those issues at all.
- There's a pair of wires with 15V and ground respectively, to feed all or each group of PIR motion sensors. The sensors can be powered with 5-20Vdc, and I thought 15V would be a good value to minimize the overall current over the cable.
- The sensor is provided already fitted in a PCB. It has quite a few components soldered in it.
- The output is indeed 3.3v, triggered whenever motion is detected. Maybe the sensor's internals are all 3.3v, I'm not sure, but it would make sense.
- These signals need to go into inputs on an automation device, which accepts signals from 10 to 30Vdc (the inputs are officially 24V in, but this tolerance is mentioned, and below 8V they're not guaranteed to read HIGH).
So I reasoned that I needed to up the voltage, and since I'm heading there, I might as well do it right next to the output.
Is this explanation any better?
Thanks for your help so far.
- I did like the idea of using an optocoupler, and it looks simple enough.
- Using two FETs and resistors... I never used small FETs so I'd need further help with model numbers and resistor values to make it work. Or a source to learn from. Or a comparison table of sorts.
- @MarkT, open-collector stage - I hope this is the N-channel FET part that you're talking about, or else I'd need further explaining / practical example.
- When I try to explain myself, it can take forever because English isn't my native language :)
Guys... I wish I'd just know where to start learning electronics.
Usually a search engine feeds me the information I need on a day-by-day basis. It doesn't teach a man to fish.
There's so much information repeated, scattered, and without proper foundations to work the way up.