Adding TTL input to a DDL driver

Hey LPF,

I've been working on a CNC laser engraver lately.
Everything is up and running for the laser itself, and for that to happen it needs to gain a TTL input.
I'm using a LM317 DDL driver (LM350 to be precise) to supply the diode with power.

As far as I have understood TTL basically switches a port from high to low.
So looking around how to add TTL to a DDL driver I found a blog from somebody who used a DDL driver and added a optocoupler/-isolator and a NPN transistor to the circuit to achieve a TTL input.


Now I do have a 6N138 optocoupler and a TIP122 NPN transistor laying around.
Can i use the setup from this schemtic to achieve a TTL input for my laser engraver/cutter?

I would advise you to change the transistor from the ground to the 9v

So instead of the optocoupler and transistor being where they are in the schematic I should move them in between the power source and the IN pin on the LM350?

And I can use a TIP122 and a 6N138 for this, or would a TIP122 suffice?

Omitting the optocoupler is okay if you are okay with your signal being common with the 9V ground. It depends on your setup.

There are more elegant ways of supplying TTL constant current, but this will work.

marcuspeh said:

I would advise you to change the transistor from the ground to the 9v

Click to expand...

That won't work. Not unless the ON signal is 15V+. This is an NPN - not a PNP or PMOS. It's fine where it is.

Cyparagon said:

Omitting the optocoupler is okay if you are okay with your signal being common with the 9V ground. It depends on your setup.

There are more elegant ways of supplying TTL constant current, but this will work.

That won't work. Not unless the ON signal is 15V+. This is an NPN - not a PNP or PMOS. It's fine where it is.

Click to expand...

So in this schematic the transistor can remain where it is right now?

And having the common ground being 9V is not a problem for my controller, it can handle up to 12V so i'm safe there.

You did say there were more elegant ways of doing a TTL driver, do you have an example/schematic?

This for example:
http://laserpointerforums.com/f51/simple-linear-mosfet-dimming-circuit-pwm-70812.html#post1020909