Variable Power Supply for Laser

In normal every-day use the batteries in a laser pointer will last a very long time because you are only switching on the laser for a second or two at a time when giving a presentation. When used for interferometry, however, you will likely have the laser switched on for long periods at a stretch, which will rapidly drain the two AA, AAA or button batteries powering it. An external 3-volt power supply solves this problem.

It is useful to be able to reduce the voltage powering the laser diode to the point where the diode stops lasing and simply becomes a bright round collimated beam of light. This has been found to yield better interferrograms. Indeed it is not necessary to use a laser - an ultrabright LED would serve just as well as long as the light was collimated into a narrow parallel pencil beam using a small lens.

Below is a simple circuit for building a Variable Voltage Power Supply which may be powered by any wall-wart transformer (up to about 25v) you may have lying around. I use an old 9-volt radio transformer. (Note: the transformer has to deliver at least 6-volts if you want a top voltage of 3-volts. See the datasheet for the LM317 voltage regulator for more info.)

Farnell Components supply the following:
9489380          LM317AT variable voltage regulator é2.98 - (Datasheet)
9708405          1uF 25v Tantalum Capacitor E1.10 (for 5 - min order)
9708480          0.1uF 35v Tantalum Cacitor E1.10 (for 5 - min order)
489876            240R 0.5W 5% resistor E0.85c (for 50 - min order)
9353801          5k Variable Resistor E1.95
You will also need a (very!) small piece of Vero-board (about 1" x 2") to solder your components on to.
You will also need to order a suitable female power connector to accept your transformer's output jack.
You will also need a multimetre to measure the output voltage of your variable power supply while you are adjusting it to 3 volts or less. Without a multimetre to check the voltage you will almost certainly fry your laser pointer as they are very suseptible to a surfeit of electrons!