driving an opto-isolator (pc817 0r 6n137) with a cd4050.

[crosslakeguy]

trying to keep it simple...... controller I have made uses cd4050 buffer/driver. the driver bought has on the inputs both pc817 and 6n137 opto's

I think the mistake I made is the cd4050 is to drive a TTL circuit at a lower voltage ( I thought it was similar to a UNL2003) please correct me if I am wrong in my assumption, the reason I come to this conclusion was, when hooked up to the driver I bought, I am only getting around 2.5v to control driver from the cd4050's, when circuit is open its 5v, when hooked up to driver board it drops by half or more.....

the driver part can either be hooked up as positive or negative triggered. if neg you put 5v on the opto, if pos you ground the opto. from where I hook up from my controller which is 5v trigger. to the board I bought. On their board there is a 270 ohm resistor before the opto.....

so the circuit is as such, if I have you totally confused yet cd4050 --- 270 ohm resistor--- pc817 opto . th question is are my assumptions correct about my mistake? what would be the best solution? I was thinking about using a UNL2003 between my controller and the driver, or removing the 270 ohm resistor

I don't want to damage the driver board, and would like to keep it as simple as possible . I cannot read the data sheets well enough to understand them, hence my dilemma..... any suggestions would be appreciated ( I would really like to sit down and decode a data sheet with someone sometime)

 

[ChrisP58]

AT 5V, the CD4050 has an output rating of 5mA. But the 270 ohm resistor (if feed 5V) would set the current at about 15mA, so the 4050 is being loaded down beyond what it can deliver.

How much current do you need to run through the LED of the opto? And I don't mean what is it's aceptable rating. I mean how much LED current do you need to get the transistor to do whatever it is that is needed?

Look at whatever the circuit that the output of the opto is connected to, and see how much current it needs. Scale that by the CTR (current transfer ratio) of the opto (be generous with a fudge factor) then decide how much LED current you really need.

Post your schematic if you need more help.

 

[alec_t]

Do you have any spare gates in your CD4050? If so you can connect them in parallel to boost the drive current for the opto.