SparkFun Forums 

Where electronics enthusiasts find answers.

Have questions about a SparkFun product or board? This is the place to be.
User avatar
By DanV
#189984
It's possible that the "5V Output Power Output 5V, the maximum output current is 20mA. " spec from your laser power supply is not enough current.

That 5V has to not only power the level converter, but also provide the current to drive the TTL level input.

But 'TTL' level says more about the voltage than the current draw, so you could put the meter (on mA setting and probably need to move one lead on your meter) in the "5V to HV" circuit path and see what the current is.