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In the past there have been a number of designs for battery operated and mains operated lasers but this is the first design using a visible laser diode. Using this device, a small PC board and three AA-size cells, you can make a very effective laser pointer. By LEO SIMPSON Every year, laser diodes are produced in quantities which run into the millions. They are used in CD and video disc players, CD ROM drives and laser printers. The vast majority of these laser diodes produce their output in the infrared region, at around 780 nanometres. The laser diode used in this project is normally intended for use in bar code readers, as used in supermarkets and libraries. It produces its output as red light, at around 670 nanometres. The laser diode is, of course, a spe60 SILICON CHIP cial type of light emitting diode. And since light emitting diodes do not normally produce a very narrow beam, the laser diode is fitted with a collimator lens assembly which also doubles as a heatsink. The laser diode assembly is mounted on a small PC board together with its drive circuitry which is a constant current source. The PC board and AA batteries can then be packaged in a short length of electrical conduit to make a handy laser pointer. This can be used at lectures and slide shows and has the advantage of being a little more bulky than commercial units which are more expensive. Since when is bulk an advantage? It makes the unit easier to point more precisely and the larger size means that it is less likely to be misplaced. Besides being used as a laser pointer, the unit could also be used for a range of optical experiments, including data transmission. It could even possibly be used as the basis for a surveyor's level, although higher power gas lasers are usually used in this last application. The circuit Now let's talk about the circuit which is shown in Fig. l. The laser diode is connected to the battery supply via a constant current source comprising transistors Q1 and QZ and diode Dl. The battery supply is shown as having of range of3.5-5V. It could be supplied from three 1.5V AA alkaline cells or four 1.2V nickel S1 +3.5-5VC>------e _ _ _ _...__....__ _ _____,....__ _ ....__-o 22ll ~· 100 B2n 3.9pf OLLIMATING LENS LOOSEN SET SCREW ANO SLIDE TO ADJUST SET 60-100mA 01 2N2219A E GND<:r---il>----------------u LASER POINTER VIEWED FROM BELOW cadmium rechargeable cells. You might expect that a laser diode circuit would be little more than a battery and a resistor since it is essentially just a light emitting diode. However, the operating current is a good deal higher than for typical LEDs. Whereas LEDs typically operate with a current of 10 milliamps (depending on their application), a laser diode usually requires somewhere between 60 and 100 milliamps. This means that a current limiting resistor is impractical and a constant current source is required. The circuit works as follows. Q1 and D1 work together to provide a constant voltage of close to 1.2V between Q1 's collector and emitter. This voltage will remain constant for quite large variations of the bat- Fig.1 (left): the circuit uses Ql & Q2 as a constant current source so that the laser diode is never overdriven. Fig.2 (above) shows the laser diode assembly. tery supply voltage. The voltage from the collector of Q1 is applied directly to the base of Q2 which functions as an emitter follower. This means that the voltage at the emitter of Q2 will be constant at around 0.6 volts. Because the voltage at the emitter ofQ2 is constant and because its emitter load resistor is fixed, it follows that the emitter and collector currents of Q2 are constant. Hence, the laser diode current, supplied from the collector ofQ2, is constant. Trimpot VR1 is used to set the current to about 80mA. We'll talk about this setting later in the article. Note that there are two diodes inside the laser diode 3-lead package. The laser diode itself is between pins 1 & 2 while another diode, a photodetector diode, is connected between The Laser Pointer is powered by three AA cells installed in a 4-cell holder (one battery position is shorted out). Keep the parts leads as short as possible. pins 2 & 3. This is shown with a lOOQ resistor connected across it although in this circuit it is superfluous. Its real purpose is to monitor the light output of the laser diode and to use the voltage signal to control a feedback loop which ensures that the laser light level is constant over a long period of time. This is important if the laser diode is used with optical fibres or barcode scanners, or in other data transmission applications. Construction The Laser Pointer circuit is assembled onto a small PC board measuring 85 x 26mm. The laser diode and the collimating lens assembly are supplied separately and need to be assembled together. The first step is to short all three leads of the laser diode together to avoid the possibility of damage to it via static discharges. The assembly is as shown in Fig.2. Remove the end plate by undoing the two screws, taking care not to lose the two spring washers (not shown in Fig.2). This done, insert the laser diode and reassemble the unit. While it is possible to do so, we do not recommend that you try to adjust the collimating lens - it is not an easy job and the unit comes preset anyway. Now that you have finished the laser diode assembly, put it aside and proceed to assemble the PC board. Do not install the laser diode on the circuit board until it has been powered up and the trim pot set for the correct current. If you connect the laser diode into circuit before you are sure it is working properly, you could easily DECEMBER 1991 61 Fig.3: install the parts on the PC board as shown here. Note that pin 2 of the laser diode is connected to the metal case & thus to the collimating lens assembly. The entire assembly slides into a conduit housing as shown at right. damage the dio de - a very costly exercise. No special order needs to be followed in assembling the board. Just install the com ponents as shown while making sure that their values are correct and that the transistors and electrolytic capacitors are correctly oriented. Check your work carefully When you have finished assembling all the components onto the board (with the exception of the laser diode). You now need a source of 4.5V DC or thereabouts. You can use a DC pow er supply or the battery pack. If the latter, you will need to wire it up. If you intend using alkaline cells (wh ich put out a nominal 1.5V), you will need to wire the 4-cell holder so that one cell PARTS LIST 1 Toshiba ROLD9200 red laser diode 1 collimating lens assembly to suit laser 1 PC board, 85 x 25mm 2 2N2219A NPN transistors (Q1,Q2) 1 1 N4148 signal diode (D1) 1 SPST pushbutton momentar·y contact switch (S1) 1 4-cell AA battery holder and snap connector 2 100µF 16VW electrolytic capacitors 1 3.9pF ceramic disc capacitor 1 100 trimpot (VR1) Resistors (0.25W, 5%) 1 1kQ 1 100Q 1 82Q 1 22Q 2 10Q 1 1Q Miscellaneous Electrical conduit for laser housing , foam rubber, insu lated hookup wire, black spray paint , laser warning label, solder. 62 SILICON CH IP position is shorted out. Once this has been done, insert the three cells and measure the output voltage across the snap connector. It should be about 4.5V. Connect the supply to the board and short the collector of Q 2 to the +4.5V rail. Now connect a digital multimeter across the lQ resistor and adjust trim pot VRl for a reading of 85mV. This corresponds to a constant current of 85 milliamps. Now you are ready to connect the laser into circuit. Mount the laser assembly onto the PC board and secure it with the supplied screws and spring washers. Solder short leads to pins 1 & 3 of the laser dio de and sleeve them to avoid shorts to pin 2, then solder the two leads to the PC board. Now apply power and the laser diode should, but may not necessarily, put out laser light. If the current through the laser diode is not quite high enough , it will emit red light in a focused beam but it will not be laser light because the energy input is insufficient. If this is the case , you will have to change the setting of VRl to increase the current through Q2. To do this, reconnect your digital multimeter across the l Q resistor and rotate VRl so th e reading increases. By the time the reading is 90m V, corresponding to a laser diode current of 90mA, the light should have that characteristic "speckle" appearance of laser light. Do not , under any circumstances, increase the laser current beyond lO0mA, otherwise there is a strong chance that you will damage the laser diode. Having confirmed that the laser diode is able to "lase", you can finish the assembly. The laser housing is made from a couple of lengths of 40mm OD electrical conduit joined together with a short length of 40mm ID conduit. One end of the resulting tube assembly is blocked off with a disc of wood or plastic while the other end has a similar disc with a 6mm hole drilled in it for the laser light output. The photos show how the tube assembly goes together. The battery pack is mounted in one half of the tube assembly, together with a piece of foam rubber packed in so that it does not slide about. Similarly, the laser PC board assembly goes in the other half, again with a piece of foam rubber so that it does not rattle about. Finally, a momentary contact pushbutton switch (Sl) is mounted in the laser tube assembly to function as the ON switch. It is simply wired in series with the positive lead from the battery pack. SC Where to buy the kit The complete kit for this project, including laser diode, collimating lens & laser warning label , is available from Oatley Electronics for $179 plus $6 for certified postage and packing. Oatley Electronics can also supply infrared laser diodes and gas laser tubes. Their address is PO Box 89, Oatley, 2223. Phone (02) 579 4985. Note : copyright of the PC board associated with this project is retained by Oatley Electronics.