Why do you feel you need an optocoupler? Do you have a need to isolate two circuit stages, or does the word sound useful?
If it weren't for the optocoupler, I'd suggest you start by looking for a driver chip that'll do what you want all in one package.
But you haven't given enough information. Do you mean a real TTL input that doesn't load its driver any more than a single TTL gate? Or an input that's compatible with TTL output, but may need more current? Or something yet again? What speeds are you going to be operating this at? Temperature? Etc.
A 2N4401 with an appropriate base resistor will turn on adequately at
2mA base drive, and really hard at 10mA. So a circuit like this one would satisfy a set of requirements that includes what you've specified, but may still fail miserably at your task:
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
It sounds like you are after an open collector output. An NPN Darlington would be my preferred approach - the Darlington has two benefits over a regular NPN in that the higher gain means you need less current, and its switching voltage is naturally in the middle of the TTL undefined range. A current limiting resistor is still advisable but its value is not critical.
A BC517 comes to mind with a gain of at least 30,000, meaning even fairly miniscule currents will saturate the Darlington for 150mA. Connect the emitter to ground and the load to the positive supply and the collector. Connect the TTL output to the base via a series limiting resistor. A 22K resistor will limit the current to around
100uA but anything between perhaps 1K and 430K should be fine, however you are pushing things at the extremes of that range.
A Darlington-connected pair, however, only pulls the output voltage down to one volt or so, not the 0.2 volts that a single NPN can do (or that TTL needs). A solution that does this, if you've got the room, is an emitter follower with collector to a power rail, followed by a pass transistor.
Since the OP has not bothered to respond to requests for more information, it's impossible to know what he really needs.
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