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I have some Davis weather sensors that I want to attach to an Arduino to build a small weather station.

I would prefer to have one power supply, but how would I create a opto-isolated circuit for the rain meter (reed switch) and the anemometer (reed switch). I know there are circuits that use opto-isolators like the 4N35, but how to I split the power? I prefer not to use a battery or something that could go flat. I can use a small solar power circuit maybe.

It is correct that I will need to power supplies, one for the isolated circuit and the other to drive the Arduino.

Any ideas? Suggestions etc. What is the ideal circuit for opto-isolators?

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  • \$\begingroup\$ Reed switches are already a form of isolation. Perhaps a schematic is called for. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Dec 29, 2013 at 10:57
  • \$\begingroup\$ Seeeduino stalker waterproof solar kit ( seeedstudio.com/depot/… ) \$\endgroup\$
    – geometrikal
    Commented Dec 29, 2013 at 11:06
  • \$\begingroup\$ Ignacio, the long wires from the reed switch to the Arduino board would offer no protection to EMF induction and large currents that could be induced during a lightning storm. I need to isolate the Arduino from the sensors. \$\endgroup\$
    – andrebruton
    Commented Dec 29, 2013 at 18:52

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How much power do you need in your isolated circuit? Probably you should use the isolated DC/DC converter. There is quite a big number of them available on the market.

Just as an example: offer from Farnell.

You must also be aware what should be the quality of isolation (for the safety reasons). However in case of lightnings you may need additional surge protection.

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  • \$\begingroup\$ Interesting, did not think of using a DC/DC converter. I did not think the input and output were isolated from each other. \$\endgroup\$
    – andrebruton
    Commented Dec 30, 2013 at 7:17
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It looks like I will end up using a standard opto-isolator circuit to isolate the Arduino from the spikes that would come in via the sensor wires. I will use a normal battery, resister LED circuit for the sensors as the power will only be used when the sensor switches on as the magnet passes the reed switch. Someone says the battery should last about two years.

This is not ideal as I would have liked to have a single power source. I suppose when I do not receive any readings for a few days, I know the battery has died and needs replacing, but then I have lost some weather data.

On the Arduino side, I will use a 4N25 for each isolation circuit. From pin 5 of the 4N25, I will use a 10K resister to the 5V supply, pin 5 to the Arduino digital input and pin 4 of the 4N25 to ground.

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  • \$\begingroup\$ FYI you can use the Arduino to read it's own battery level. \$\endgroup\$
    – HilarieAK
    Commented Jan 30, 2016 at 13:07
  • \$\begingroup\$ I'll investigate! Sounds great! \$\endgroup\$
    – andrebruton
    Commented Jan 31, 2016 at 14:16
  • \$\begingroup\$ Under "Detecting Low voltage" gammon.com.au/forum/?id=11497 \$\endgroup\$
    – HilarieAK
    Commented Feb 1, 2016 at 8:21

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