Using an optocoupler + NPN transistor to drive a 24V PLC digital input from a 5V logic signal — is this topology correct?

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1

I'm building a small isolated interface board that takes a GPIO output from an Arduino Uno and needs to ultimately drive a 24V digital input on a PLC. I'm using a digital optocoupler (Broadcom ACNT-H61L) for isolation, followed by an NPN transistor stage to step the signal up from the optocoupler's 5V logic output to the PLC's 24V input level. I want to confirm whether this overall topology is electrically sound before I finalize the PCB. Can someone please review the schematic.

The optocoupler I am using needs its own power supply on the output side, so the 5V_Isolated and GND_Isolated are from a separate DC - DC converter. I connected the PLC_GND and GND_Isolated because without them being connected, the loop in PLC Digital Input won't be completed, is that right? However, the optocoupler does isolate the MCU side from the Transistor side.

Thanks.

Screenshot 2026-07-30 135305
Screenshot 2026-07-30 135305916×647 7.9 KB

2

If you connect grounds, it is not isolated anymore...
No problem, since you seem to want to isolate the left hand side.
Your schematic seems ok. Labeling is not.
An opto without 5V feed would have been an easier choice (but may need inverted logic).

3

Before discussing whether the grounds should be connected, could you tell us what PLC input you're driving? A part number or the input wiring diagram from the PLC manual would help. PLC digital inputs are implemented in different ways, so whether PLC_GND and GND_Isolated need to be connected depends on how that particular input is referenced.

4

Since the transistor base is driven from the opto, grounds need to be connected.

5

@build_1971 You're right. In this circuit the common ground is required because the optocoupler output directly drives the NPN transistor base. Without a common reference between GND_Isolated and GND_PLC, the transistor cannot be driven properly. My previous comment was therefore misleading.

6

I can't see a problem with your circuit except that it seems a little over-complicated.

It seems odd to have resistors on both the anode and cathode, but I did see the same thing done in the datasheet for the ACNT-H61L.

A more common opto-isolator like PC817 has a max emitter-collector voltage of 80V, which might make the separate npn transistor unnecessary.

Are there reasons you choose these components that you haven't mentioned yet?

7

PLC DI's are usually externally powered, so your setup looks okay once the grounds are connected, but obviously it wouldn't work if it's an internally powered PLC DI.

8

The topology is sound but the circuit will never work.
Did you read the opto datasheet?
What is the min and max LED current?
Have you met those requirements?