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See in the dark with this low cost INF RED NI This infrared night viewer is simple to build. The viewer itself requires no assembly. All you have to do is assemble a battery powered high voltage supply and connect it to the viewer. You can then see in the night time to your heart's content. By BRANCO JUSTIC Ever wanted to see in the dark, when it's pitch black and you're sure that something sinister is lurking out there? Of course you have. Everyone would like to be able to see in the dark but humans just don't have this capability. Or at least they didn't until infrared night viewers became available. Infrared night viewers were first used by the military at the end of the second world war and were used extensively by the allied forces during the Vietnam war. These days, they are used by police forces around the world for crime detection and also for identifying marijuana crops hidden in otherwise apparently undisturbed bushland. Now you can have your own night viewer for quite a small outlay and with very little electronics assembly required. With the night viewer to be described here you will be able to see in the dark as clearly as in the day - it literally turns "night into day". There are two ways of using it. First, you can use it to observe night time scenes under moonlight. For example, you could use it to observe possums in trees. Possums can be very hard to see at night -you can often only see their shadow but with the night viewer you can see them as easily. Alternatively, you can use the night viewer in pitch darkness, together with a torch fitted with an infrared filter. This is the most dramatic demonstration. You can walk into a room in pitch darkness, turn on a torch fitted with an infrared filter and not be able to see a thing. Then if you look through the night viewer you will be able to see just as clearly as if the room was brightly lit. IR viewers such as this do have serious uses as well, such as examining the output of infrared lasers and IR LEDs in appliance remote controls, fibre optics, medical equipment and so on. By the way, you should never directly examine the output of an infrared laser or fibre optic cable. Shine it on to a flat surface and then you can safely observe the reflected IR light with the viewer. IR night viewers have been described in other magazines in the past but they have generally required some fairly specialised work in adapting lenses to an irifrared imager module. The unit described here requires none of this work. It looks just like a set of binoculars cut in half - which is effectively what it is. It has a length of shielded cable which must be connected to a suitable high voltage supply. So all you have to do is build the battery powered high voltage supply, connect it to the cable from the viewer and the project is complete. How infrared imagers work The image converter tube is powered by a high-voltage power supply (left) which develops about 13kV. This 13kV supply is derived using a single 9V battery & a special voltage multiplier circuit. 62 SILICON CHIP The special tube used in an infrared night viewer is sometimes referred to as an "image converter tube". It is a type of cathode ray tube but it does EHT TO VIEWER 680pF 3kV 680pF 3kV not have a filament. It has a cathode, an anode and a focus electrode. A lens system in front of .the tube focuses the infrared image onto the cathode face. The cathode is coated (on the inside of the glass) with a photoemissive material which responds mainly to infrared light in the region from 800 to 1200 nanometres but they also have some response in the visible light region. When the target materials are hit by infrared light, they emit electrons which are accelerated by the high voltage which is applied between the anode and cathode. The anode of the image converter tube is just like the screen of an oscilloscope. It is coated with a green phosphor which gives off light when it is hit by high velocity electrons. So what happens is that the invisible irifrared image which is focused onto + the cathode is convertT 9V I ed to a visible green ......I image on the anode screen. This image is then observed through the eyepiece lens of the viewer. The focus electrode on the image converter performs the same function as the focus electrode on any cathode ray tube such as an oscilloscope or television picture tube - it brings the image into focus. However, later model infrared image ·converter tubes such as the one used in this monocular viewer are of the prefocused type and therefore do not require an external focus electrode. Since the monocular viewer de- 680pF 3kV Fig.1: the circuit of the high voltage power supply uses a transistor blocking oscillator which produces about 1.3kV peak to peak at the transformer secondary. This is rectified by the Cockroft-Walton multiplier to produce about 13kV DC. The link in series with the 2.7kn resistor allows a small resistor to be added into circuit to adjust the high voltage output but this is not required for the tube used in this project. 680pF 3kV 680pF 3kV 680pF 3kV 680pF 3kV LINK (SEE TEXT) 680pF 3kV 01 1N914 2.7k 680pF 3kV + 10 FOCUS B 680pF 3kV EOC VIEWED FROM BELOW NIGHT-VIEWER POWER SUPPLY scribed here comes complete with lenses and is prefocused, all you have to do is build the power supply shown in Fig.1. This is essentially a one transistor blocking oscillator driving a Cockroft-Walton :voltage multiplier. Let's have a look at how the oscillator works first. It consists of a transis- · tor with its emitter connected to the centre-tap (point 4) of the transformer primary while its base is connected to one side of the primary (point 3) via a lOµF capacitor. The other side of the primary (point 1) is connected to the negative side of the 9V battery. When power is first applied, tranNovEMBER 1992 63 The voltage multiplier "stack" occupies most of the space on the power supply board. Keep all component leads short & don't handle the board after power has been applied - it can deliver a nasty shock. tor so that the base of the transistor is driven harder. The transformer core then saturates and transformer action ceases so that point 3 A special torch can be used to illuminate the collapses to 0V which turns target with infrared light when natural light off Q2. This also causes the levels are extremely low. Often though, lOµF capacitor to be dismoonlight or reflected skylight (in cities) will charged and so all action be sufficient to let you view objects with ceases. reasonable clarity. The lOµF capacitor must then charge up to the point sistor Ql obtains its base current via where the base of Ql again starts to diode Dl and the 120n resistor. The conduct and the cycle recommences. transistor conducts and applies 9V The result is a square wave signal between the centre-tap and the 0V with a frequency of about 1 lkHz apside of the transformer winding (ie, plied to the transformer primary and battery negative). Transformer action this is stepped up in the secondary to then causes +18V to appear at point 3 about 1300 volts peak to peak. This is and this "jacks up" the lOµF capaci- then rectified and stepped up in the The completed PC board is mounted "edge-on" inside a plastic utility case. Note the plastic barrier strip that's used to isolate the board from the battery & the power switch. 64 SILICON CHIP Cockroft-Walton multiplier stage to around 13,000 volts DC. Cockroft-Walton multiplier The Cockroft-Walton multiplier consists of 20 high voltage silicon diodes and associated high voltage capacitors. For those who wonder where this rectifier circuit came from, it has been around for a long time. It was first used to generate voltages as high as 200,000 volts to drive an atomic particle accelerator developed by Cockroft and Walton at Cambridge University in the 1930s. These days it can be used in a pressurised vessel containing silicon hexafluoride to develop voltages as high as two million volts. For everyday use, the CockroftWalton rectifier configuration is suitable for any application which .requires very high voltages at low currents. To understand how the CockroftWalton multiplier works, let us consider just the first few stages. Look first at the circuit consisting of diodes D2 and D3 and the two associated capacitors. The rest of the diodes and capacitors in the ladder above D3 can be ignored for the moment. In the first negative half cycle from the transformer, diode DZ conducts and charges the 680pF 3kV capacitor in series with the transformer secondary. In the next positive half cycle, the charged capacitor's voltage is added to the peak of the transformer voltage to cause diode D3 to conduct and charge up its associated capacitor to 1.3kV. So far then, what we have de- PARTS LIST Fig.2: the component wiring diagram for the PC board. Make sure that you get all the diode polarities correct otherwise the high voltage output will be reduced. scribed is the classic "halfwave voltage doubler" or "diode pump". Subsequent negative and positive half cyles from the transformer cause this step-up pumping action to continue so that each capacitor in the ladder is eventually charged to 1.3kV. After twenty or so cycles from the transformer, the voltage at the cathode of diode D21, at the top of the stack, reaches about 13kV DC. Also shown on the circuit of Fig.1 is a point marked "focus" at the junction of diodes D3 and D4. This is intended to supply the focus electrode on IR imager tubes that require it but, as previously mentioned, it is not relevant to this project. Construction The circuit of Fig.1 is built onto a PC board measuring 125 x 41mm. This is designed to fit into a standard plastic utility case measuring 130 x 70 x 45mm. The assembly of the PC board is a · repetitive job but it needs concentration otherwise you will find that some diodes are in the wrong way around. The high voltage diodes are BY509s which are normally potted into triplers for colour TV sets. The ferrite potcore transformer is supplied ready wound and the pins are marked. All you have to do insert it into the board and solder it. A battery snap needs to be soldered to the board and the job is all but finished . To finish the job, the board must be installed in the plastic case and the cable from the IR viewer connected to it. You will need to cut a square hole for the rocker switch (Sl) and drill a hole for a grommet to fit around the coax cable from the IR viewer. With these holes cut and drilled, fit the rocker switch to the case (it just snaps into place), then pass the cable from the IR viewer through the grommeted hole and solder it to the appropriate points on the PC board. This done, slide the PC board into place inside the case, as shown in one of the accompanying photographs . A sheet of 2mm thick plastic cut to size is then placed in slots above the PC board. This provides a separate compartment inside the case for the battery. This is necessary because the metal case of the battery must not come into physical contact with the high voltage capacitors. A piece of self-adhesive foam stuck to the bottom of the battery compartment will stop the battery from rattling around inside the case. The 9V battery should be an alkaline type for long life. A clip fitted to the lid of the case will allow you to fit the supply to your belt for convenient carrying. Testing With the battery fitted and the lid of the case screwed on, you are ready to try out the viewer. This can be done in a dimly lit room. Turn on the power -you should hear a high pitched w histle (provided you can hear 1 lkHz) from the inverte'r - and then look through the viewer. You should see a greenish image. Try focussing the image for increased clarity. And now for the pitch dark test. While objects such as human bodies do emit infrared radiation, the wavelengths are too long to be observed WARNING! This project generates an output voltage of 13kV & is capable of delivering a nasty shock. Always make sure that the capacitors have discharged before handling the board after switching off the power & never handle the board while power is applied. 1 monocular IR viewer assembly 1 plastic case, 130 x 70 x 45mm 1 PC board, 125 x 41 mm 1 sheet of 2mm thick plastic, 125 x 41 mm (insulating panel) 1 prewound potcore transformer 1 belt clip (with adhesive back} 1 9V alkaline battery 1 battery snap connector 1 rocker switch (S 1) Semiconductors 1 2N2219A NPN transistor (01) 1 1N914, 1N4148 diode (D1) 20 BUY509 high voltage diodes (02-D21) Capacitors 2 10µF 16VW PC electrolytic 20 680pF 3kV ceramic disc Resistors (0.25W, 5%) 1 2:?kQ 1 1kQ 1 120Q Where to buy the kit The complete kit for this project, including monocular IR viewer, parts for the high voltage power supply and 75mm infrared filter, is available from Oatley Electronics, PO Box 89, Oatley, NSW 2223. Phone (02) 579 4985. The price is $279 plus $6 for packing & postage. Oatley Electronics can also supply the power supply separately and other types of IR image converter tubes. Note: copyright of the PC board associated with this project is retained by Oatley Electronics. with the night viewer. So in a pitch dark room it is unlikely that you will see anything at all. In order to see with the IR viewer, you will need an infrared torch. "Where do I get that?" you might ask. The answer is that you will fit a supplieq infrared filter to a standard torch and that will do the job admirably. From then on, you really will be able to see in the dark. Just one thing though; after you have finished playing around in the dark, don't forget to turn off your newly acquired torch. Since you can't see the infrared beam it emits, it is all too easy to forget to turn it off. SC NOVEMBER 1992 65