Output Buffer for 7I73

Greetings...!!!

I am working on HBM project and for Operator panel interface i am using a mesa 7i73 and mine illuminated push buttons have 24V pilot lights and to operate that i was thinking to use opto-isolator circuit using PC817 and ULN2803as shown in below schemetics.
 


But PC817 would required minimum 5ma current sustaining output and mesa 7i73 only provides 4ma on output so as noob in electronics i have confused that unl2803 could work directly with mesa 7i73 output as isolator or it required greater current then 4ma and is safe to use directly with 7i73 output pin ?

5 MA is OK for 7I73 outputs, they just wont meet their high level output voltage spec

@PWC

is any other opto that could work flawlessly with 7i73 output ?

If you are using a ULN, you are on the edge of the specifications because the ULN2803 has a TTL (5 V) input and the outputs of the 7i73 are 3.3V. It needs 2.4 - 3.0V depending on the output current. as you have only an optocoupler connected to the output, it should work.

The question is if you need or would like to have a galvanic isolation between the 24 V and the 7i73 ground.
If not you could use a ULN2003 which has open collector outputs, so you can connect the 24V LEDs directly. But this one has also a TTL input, so not ideal.
Or maybe better find a 3.3 V compatible chip.


But you can also connect the optocouplers directly to the 7i73 like Peter mentioned.
The controller on the 7i73 has three different types of outputs, I don't know how they are mapped, but take the most weak ones.
They have a maximum rating of 15 mA, and can deliver 2.4 V at 6 mA. So should be fine for the optocouplers. Just calculate the resistor based on a voltage somewhere between 2.4 and 3.3 V.

 

 

@HansU Thank you so much for your valuable response the reason of isolation of both ground is to make a safe circuit. I am also preferring to use UNL2803 direct with 7i73 if only it's a safe way. Hence I have ordered some optos(pc817), unl2803, resistors and also bc547 to try some schemetics on breadboard

Forget about the objection regarding the thresholds. I have read your example from the right to the left thinking the right hand side connector was the 7i73.

But you should remove LED1 .. LED8 - would be too much drop for the 3.3 V signal. And you can dimension R1 .. R8 that you have a current of e.g. 2 mA through the optocoupler.
At 2 mA you have CTR of 100 which means that you can draw 2 mA at the output. That should be more than enought fpr the ULN inputs.

Yes i was also thinking about to remove led at IO pin and put after UNL side. I would definitely give it try want to simulate in Proteus but don't know why it's not working may be due to demo version so i have to wait till mine bread board and resistors kit arrive.