I want to make a a line of sight sensor. I want to drive a laser diode with constant current and aim it at a photodiode, then using a transimpedence amplifier I will monitor any changes in light intensity. The reason I am using a photodiode and a laser diode is because I want fast response times.
I have in my hands 2 OPV382 laser diodes that I fail even to turn on. I am using an LM317T to provide constant current to my laser diode. In the datasheet it states that the diode starts conducting at less than 2mA and that the maximum continuous current should be 12mA. I decided to try with a low current first just to be on the safe site but nothing happened.
My circuit was a resistor between the adjust and output of the LM317T to set the current and then the laser diode connected to the output and to ground. I used various resistor values from 220 Ohms to about 70(and connected the laser diode only momentarily but still nothing).
I also added another resistor from adjust to ground to set the voltage to 2.2V that is the maximum forward voltage and again nothing. Can you explain to me what I'm doing wrong? The supply voltage is 5V(measured 4.9V)
The only thing I can think of is the series resistance of the laser diode which is 20-55ohms according to the datasheet, could this be affecting the rest of the circuit?