Now that I think about the 0.5W requirement: The standard through hole resistors have 0.5W. Most likely he had those at hand and used them to create his circuit.
I have a box made (when I was a child) of 72 matches boxes and a lot of THT resistors in it (some are hard to close when opened). But there are only few 0.5W. For me standard is 0.1 or 0.125W.
Examples of non-electrical justifications for component selection:
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Use existing stock on-hand
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Maintain parts interchangeability with previous versions
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Avoid proliferation of stocked items in the logistic system
From my memory, 0.5W, through-hole resistors were probably the most common size in the days of vacuum tube circuitry, until the 1960’s or so. (Yes, I first learned electronics on radios that glowed in the dark.) Through-hole 0.25W resistors became the most common in the late 1960’s and 70’s. Lower power devices - 1/8W, 1/10W and less - appeared in the 80’s, especially as SMT became commonplace.
Dale
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Nowhere did I use the word only.
Hi no not at all.
I am just waiting on some more info.
It really is just a standard alarm input 5 volts across the zone normally
That is why I didn’t wrote that you said, but only suggested.
To say less then suggested I would probably had to use the whole sentence to replace that one word.
The whole thing’s underspecified anyway so we’re just second guessing him and each other. Who knows maybe he got 20,000 4.7k 0.5W resistors for half-price. Or any other of a zillion possibilities. No point overthinking his problem.
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It is input which can detect 4 states I think. We (here in Poland) call it parametric input to distinguish from standard digital (two state) input.
Practically if cable connected to those resistors is whole located in one building (common grounding system) you can use any resistors you wont (even 0402 would be OK).
The problems begins when cable goes to next building, or outdoor gate. It could work well many years till one storm strike near the system.
15 years ago I made some experiences with home piezoelectric gas igniter (not sure if I named it correctly). I assumed that spark I get from this $2.5 device is close to spark you get from professional ESD GUN used in EMC test labs. After some pulses went through 0.1W 100 ohm resistor it changed its resistance. Probably to about 85 ohm but don’t remember perfectly. ESD pulses carry thousand times less energy then surge pulses.
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