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Z80 Refugee
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Re: connecting multiple magnetic switches

Tue Jul 17, 2018 11:19 pm

pcmanbob wrote:
Mon Jul 16, 2018 9:34 am
Not getting it to an argument over this as I have said before we are not in an industrial or military environment here , this is a hobby environment were people will use what works and is often simple to build.

As for my experience work for BT for over 40 years as a power maintenance technician installing and maintaining control and alarm systems on all sorts of standby generators , air conditioning plants, refrigeration plants , AC-DC & DC-DC power conversion and just about any other building services system, on a range of voltages form 12v DC right up to 11000V AC.

most of these systems used much more complicated interfacing circuits than are ever suggested on this forum, but suggesting they be used here would just be over complicating most if not the replies.
So you think three passives per alarm circuit is overkill?? With a switch local to the RPi I would expect a minimum of two resistors per circuit: one to pull up to 3.3V, and the other in series with the GPIO pin to protect against programming errors (which could otherwise fry the RPi). This isn't military grade engineering, it's basic sense. Put the switch circuit on a long wire and you need suppression - one more component, the capacitor. Where's the overkill?

The three diodes and a resistor provide an alternative to pulling up to 3.3V, which protects the system from accidents again. You only need one of those for all the switch circuits. Cost: 40p. The wire connecting the reed switches will be orders of magnitude more expensive than that.

So I fail to see where anything I have recommended is expensive or unnecessary. What I have provided is a reliable design, safe under all conditions. Surely that's a minimum requirement not to put beginners off - what could be worse than a newcomer killing their RPi? The original proposal used a relatively expensive component (the pot) which could be adjusted so as to fry the Pi, no interference suppression, and no protection if the GPIO pin were programmed as an output. In my opinion, it should never have been put forward.

The only thing I might criticise is that I have provided a conventional circuit diagram instead of a pictorial view of how to put it together. Too bad. The sooner people learn to communicate using the usual "language" of electronics design, the better.
Military and Automotive Electronics Design Engineer (retired)

For the best service: make your thread title properly descriptive, and put all relevant details in the first post (including links - don't make us search)!

chornbe
Posts: 1
Joined: Fri Aug 10, 2018 3:57 pm

Re: connecting multiple magnetic switches

Fri Aug 10, 2018 4:00 pm

What I've learned is that if you put two people in a room both staring at a problem, 99% of the time those two people will argue about solving the problem rather than just solving it using some basic Occam's razor thinking. Sheesh.

Anyway, thanks for the information in this thread; helped me with a few questions I had, too.

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