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By FLAVIO SPEDALIERI Build this low-cost mini gas laser separate winding on the primary side of the transformer. It oscillates at about 20kHz or so and steps up the 7.2V DC input to approximately 900V AC. This high voltage AC signal is fed to a Cockcroft-Walton voltage multiplier circuit comprising diodes DlD5 and their associated capacitors. The resulting EHT appears at the catha compact voltage quadrupler to proode of D5 and is fed to the anode of duce an EHT voltage of about 5kV the laser tube via two ballast resistors (82kQ and 33kQ). When the circuit is (before the tube strikes). It can be built first turned on, the Cockcroft-Walton into a length of electrical conduit and used as a laser pointer, or built into a voltage multiplier produces a voltage plastic zippy case for experimental of about 5kV to fire the laser tube and work. establish a discharge current of several milliamps through it. Circuit details Once the discharge current is flowing, the Cockcroft-Walton multiplier Let's take a look at the circuit for can no longer function properly bethe Gas Laser - see Fig.1. The voltage inverter module, based on Ql, Q2 & cause its high impedance is heavily Tl, comes ready-assembled on a small loaded by the laser tube. This high PC board. impedance is artificially provided by the lMQ resistor and the 33pF eaTransistors Ql and Q2, together with Tl, form a complementary . pacitor. So what happens is that the inverter circuit supplies the laser tube switching oscillator with feedback to the transistor bases provided via a via the series path provided by diodes Dl-D5. Even so, the voltage across the laser tube is still quite high, at close to 1000 volts DC. The current through tube while it is operating the tube is set by the ballast resistors . Here's your chance to build a small gas laser pointer for $60. It runs off a 7.2V battery power supply & uses a pre-built inverter module. If you've ever been to a conference or to a lecture, you may have noticed the speaker using a laser pointer to indicate something of importance on an overhead projector. Laser pointers are a great idea because they allow so much more flexibility than before. No longer do you have to stand near the screen to point out something, thereby blocking off the view to half of the people in the audience. In the past, however, the price of laser technology has been pretty steep. But now you can build your own laser pointer for about $50. This project uses a small gas laser, a pre-built voltage inverter module and DANGER! (1 ). Looking directly into the laser could damage your eyesight. Although the tube only has an output of 0.5mW, the concentrated nature of the laser light can damage the retina. Never look into the tube while it is operating - ever. (2). The power supply in this project is capable of giving a severe electric shock. It contains a DC-to-DC converter wh ich steps the voltage up to very high levels. Never work on the circuit while power is applied. In fact, don't even trust the circuit after the power has been disconnected, since the high voltage capacitors can retain a substantial charge for quite some time. 70 SILICON CHIP Construction Before starting the construction, there are a couple of modifications to be made to the inverter module. First, the 240Q resistor (Rl) at one end of the board (next to the two transistors) must be removed and replaced with a lkQ resistor. This will reduce the over- ~ / r ---- ------------- --- ----, 1 L1 *LINK SEPERA TE MOD ULE +1.2vo-<fo--L---.J C4 *R1 I I IOUT I 1 01-05 SxBY309 1k ,-----------+---. T1 I OUT 2 c, ovo------"'--------+-----------......._<iov,___ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _-o L - - - - - - - - - - - - - - - - - - - - - - - - LASER POWER SUPPLY Fig.1: the circuit uses a pre-built DC-DC inverter stage to step up the battery voltage to approximately 1000V AC. This signal then drives a Cockcroft-Walton voltage multiplier circuit based on diodes D1-D5 & their associated capacitors. L:j)ER CA T~ODE _j switch, with an additional hole in front of it to accept the neon indicator. The laser tube and its companion PC board are a comfortable fit into the conduit. Before they are installed though, you will need to pack some pliant insulating material, such as corrugated cardboard, around the laser tube so that it is centrally located in the conduit. Important: be sure to install the tube the right way around. The tube specified emits light from its cathode end (ie, from the end opposite the 33kQ ballast resistor), so make sure that this end goes towards the hole in the end plate. Alternatively, you can mount the tube and PC board assembly on the lid of a plastic zippy case, as shown all current consumption of the circuit. Second, capacitor C4 (33pF) must be removed from the board and replaced with a wire link (note: this capacitor can subsequently be used on the multiplier board). Capacitor C3 can be left in circuit, even though it serves no useful purpose. Once these changes have been made, the voltage multiplier board can be assembled - see Fig.2. Begin by installing the five EHT diodes. Note that the cathode end of these diodes is indicated by a purple band on one lead (not the body) and that D2-D5 have to be mounted end on in order to fit. Note also that if you try to check these diodes using a multimeter, you will not get a result. That's because the forward voltage of these diodes is quite high at about 3V. The best way to check one of these diodes is to wire it in series with a 9V battery and a 4. 7kQ limiting resistor and then measure the voltage across it. Don't substitute for the diodes since they are a special high-voltage type which is rated at 12kV. The five disc ceramic capacitors are also high voltage types rated at 3kV and can be installed at this stage. Finally, install the 82kQ ballast resistor, then mount the inverter board on top of the rectifier board by installing three "pinthrough" connections. shown in Fig.2. Note that one end of the 33kQ ballast resistor is soldered directly to one of the tube clips. If you wish to make a laser pointer, the circuitry can be installed inside a 300mm length of 40mm OD electrical conduit. This is fitted at either end with 10mm-thick Perspex end plates. One of these, at the laser end, has a 10mm-diameter hole drilled through it to allow the laser light to escape. The other end plate carries a DC input socket, to match the one on the battery pack (note: Fig.2 shows banana style input sockets, as used in the zippy case version). A hole also needs to be drilled in the conduit at this end for a pushbutton on/off Final assembly Fig.2: before installing any parts, remove capacitor C4 from the inverter (AC drive) board & substitute a wire link. Note that the completed multiplier board assembly should be potted in neutral cure silicone sealant to reduce the possibility if arcing & accidental electric shock. Fig.3 at right is the full-size artwork for the multiplier PC board. The wiring between the board assembly, the tube and the other items of hardware can now be completed as MAY 1993 71 in the photos. To mount the tube, you'll need two cable tie mounts with adhesive bases. Attach these bases to the lid in the appropriate positions, then loop two plastic cable ties through these bases and around the tube. Pull the cable ties up firmly but don't make them too tight or you could fracture the tube. The board assembly can be attached to the lid using a suitable epoxy adhesive. After that, it's simply a matter of wiring up the on/off switch, neon indicator and power supply sockets. A rocker-style on/off switch can be used for this version. Power for the laser can be derived from a 7.2V nicad battery pack (or six 1.2V cells in series), or you can run it from a mains supply capable of supplying 7.2V at 1A. Initial testing Check your work carefully and make sure that the unit is fully enclosed in its case before applying power. The unit should fire up immediately when the battery is connected. If not, disconnect it and individually discharge each high-voltage capacitor using a well-insulated clip lead. Remember, they can give you a substan- tial shock, even ifno power is applied to the circuit. Once the capacitors have been discharged, you can safely check your work and rectify any mistakes. This done, it's a good idea to pot the highvoltage capacitors and diodes in neutral cure silicone sealant. This will help prevent arc over and also provides a further degree of protection against accidental contact with highvoltage components. Finally, be sure to attach a warning label to the unit, advising of the dangers of direct eye exposure (see warning panel). Suitable warning labels are available from the supplier listed at the bottom of the parts list. SC PARTS LIST 1 PC board, code 11104931, 66 x25mm 1 DC-DC inverter module 1 0.5mW laser tube 1 on/off switch to suit - see text) 1 300mm length of 40mm OD electrical conduit (optional see text) 2 10mm thick Perspex end plates (optional) 1 DC socket (optional) 1 plastic zippy case, 130 x 68 x 41mm (optional) 2 banana sockets (1 red, 1 black .,.. optional) 4 rubber feet for zippy box (optional) 6 AA size nicad cells 5 BY509 high-voltage diodes The laser tube is secured to the lid of the case using two cable ties & two plastic mounts with adhesive bases. Note that this tube emits from the cathode end (ie, the end opposite the ballast resistors). Capacitors 2 .01 µF 3kV ceramic 2 680pF 3kV ceramic 1 33pF 3kV ceramic Resistors (0.25W, 5%) 1 1MQ 1 33kQ 2W 1 82kQ 2W 1 1kQ Miscellaneous Hook-up wire, plastic cable ties & cable tie mounts (see text), neutral cure silicone ·sealant. Where to buy the parts Parts for this project are available from Oatley Electronics, PO Box 89, Oatley, NSW 2223. Phone (02) 579 4985. The laser tube sells for $35, while the inverter module, multiplier PC board & high voltage components are $15 (includes warning label). Please add $5 for p&p. 72 SILICON CHIP A laser pointer can be made by sliding the tube & the PC board assembly into a length of 40mm OD electrical conduit. Pack some pliant insulating material, such as corrugated cardboard, around the laser tube so that it will be centrally located in the conduit. Be sure to fit a warning label to the case (see text).