I am building a circuit to read atleast 22 input channels (24 vdc) using an Arduino board which essentially works on 3.3 vdc or 5 vdc.

My current setup makes use of 22 PC817 optocoupler. I am sharing the most basic circuit diagram below with only one channel reading capability to keep it simple to understand.

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To drop the voltage and keep the optocoupler safe, I've used a voltage divider with a 68K and 1.5K Resistance (±5%/±10% tolerance) that would drop the voltage to around 3.3 vdc, that I think would be safe for PC817 (please correct if I'm wrong).

Now since there are 22 channels to read, using 22 x PC817, 22 x 68K and 22 x 1.5K resistance adds a lot of complexity to the circuit, I wanted to know if there's a better way to do it.

I read about 8 channels Digital Isolators like Si838x, and have following questions:

  1. From what I understand I can feed 24 vdc directly to this IC and will be able to handle?
  2. If not, is there any other alternative to this digital isolator were I can handle preferably more channels on one IC?
  3. If it is a no for both 1 and 2 above, what are the best options I have for a multi-channel Opto-coupler?
  4. Any other suggestions to keep the final circuit small?

1 Answer 1

  1. Yes, they are designed to work with 24 VDC (maximum safe voltage). With 8 per tiny SMD package, you need 3 of these ICs to solve your interface problem.

  2. These are the most space efficient parts I know of. Not often interfaces have more than 8 channels.

  3. You can pack them end-to-end, 3 in a row. Be sure to follow manufacture specs for board layout and decoupling capacitors. Use a linear voltage regulator so power supply is quiet. Not enough current consumed to justify a noisy switching power supply.

  • \$\begingroup\$ Will regulating with a 24 v zener work? That's the best and cheapest option I can think of as a regulator. Though, I am pretty sure that the 24 v input is well regulated, since the same is actually an output from another micro-controller board, which has a regulated power supply of its own. \$\endgroup\$ Aug 11, 2020 at 7:31
  • \$\begingroup\$ Zener diodes have poor regulation. Test before connecting the IC. \$\endgroup\$
    – user105652
    Aug 11, 2020 at 7:37

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