Things to check before you start testing
1) Is it wired correctly?
2) Is there enough voltage? (Rule of thumb 6v for reds 9v for blue)
3) If supply is via batteries, are they fresh? (Used ones may sag and effect your measurements)
Now to test it out......
4) Attach the 4 diodes and resistor to where LD will eventually connect. If your reverse diode and capacitor are already connected to your LD you can connect your test assembly to the output from your fixed resistor and the neg of the battery.
5) Turn the pot to maximum resistance (then back off the pot a turn if it is a 25 turn one to ensure the wiper is not right on the end of the resistor track)
6) With your DVM set to volts attach red and black leads as shown at either end of the diodes. Turn on supply, now wind your pot right up to see if the voltage doesn't exceed your required voltage.
7) Turn off the supply, change range on DVM to mv and attach red and black as shown either end of resistor. Turn on supply and the mv reading will be equal to the maximum milliamps that your LD would see, now wind the pot resistance up to check that the reading reduces smoothly down to the lowest mw, which equals the lowest ma your LD will see.
8) You can also connect your DVM, set to volts across the Vout and Vadj pins of the regulator, you should read 1.25v (thanks rckstr)
9) If all seems OK leave the pot at max resistance, disconnect the supply and remove your test assembly.
10) VERY IMPORTANT! If your capacitor is not connected directly to your diode (in other words it is already in the circuit with your regulator) you MUST short out the + and - at the output with a piece of wire to discharge the capacitor. Failure to do so will blow your LD as you connect it up!
The rest of the set up is up to you, the above only checks that the circuit works correctly.
