Reading isolated multichannel input with voltage drop on all channels

Projects General Guidance
1

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:

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

As I'm used to opto coplers are responding to current, a few, say 5 - 10 mA. Voltage dividing looks like waisting components.

3

In theory your voltage divider supplies 0.5V, too low for an opto coupler. In fact it limits the current to 0.3mA, also too low for an opto coupler.

As already mentioned a single current limiting resistor is sufficient instead of the voltage divider.

4

One 4.7k series resistor would limit the LED current to about 4.8mA, the PC847 has 4 opto couplers per chip.

After getting the signals down to low voltage you might use a port expander like:

5

A voltage divider for the opto LED is just wrong.
They (LEDs) are current driven, so all you have to do is reduce the current to a safe value.

A single 22k resistor in series with the opto LED would do.
That limits opto LED current to ~1mA.
More than enough to pull a digital pin with internal pull up LOW.
Leo..