Read open-drain signal already pulled-up at 24v from 3.3v Arduino

[ilanErlich]

Hello,

I would like to read the LED ON/OFF status from a UV Module with a 3.3v Arduino board.

Schematic:

The open-drain output is already pulled-up at 24v and has a sink current < 10mA as stated in the datasheet of the module.

Could you please help me to read the digital state of this output?

Thanks,
Ilan

[DVDdoug]

You can use a Voltage Divider

If the resistors that sum-up to about 10K, that's 4.8mA. The Arduino itself has in input resistance of around 100 Megohms so essentially no current flows into the Arduino.

[dlloyd]

Using an opto isolator:

The output pin 2 is clamped to 24V by ZD24. If an opto's IRLED is connected from 2 to 8, the current through the IRLED is (48-1.2)/47K ≈ 1mA.

Here the Arduino input will read LOW while the LED status is OFF
and the Arduino input will read HIGH while the LED status is ON

[ilanErlich]

Hi DVDdoug,
thank you for your suggestion!

[ilanErlich]

Hi dlloyd,

Thank you for your comment.

From Kirchhoff's law I understand IRLED calculation but don't see what you mean by pin 2 is clamped to 24V.

What's the purpose of the Zener here ?

[ilanErlich]

Also, could I use the same 4N35 opto to drive the LED ON/OFF signal from the Arduino (3.3v) ?

How to dimension R1 here?
As high as possible to limit power losses?

[dlloyd]

@ilanErlich

but don't see what you mean by pin 2 is clamped to 24V.

The zener has no effect when an opto is connected, but I just mean that with nothing connected to pin 8 and with the MOSFET being off, pin 8 will read 24V even though internally this signal is pulled to 48V through the 47K resistor.

@ilanErlich

Also, could I use the same 4N35 opto to drive the LED ON/OFF signal from the Arduino (3.3v) ?

In this case, the opto seems redundant. Could we see a link to your product's datasheet?

[ilanErlich]

That's clear to me, thank you again!

[ilanErlich]

Sure:

UV Module TDS.pdf (462,3 Ko)