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Night viewers are not new but those presented in maga­zines in the past have really needed an infrared source to enable them to see well in the dark. With the design presented here, it is possible to see just by the light of stars. In fact, you can even use the tubes featured here for astronomy. By LEO SIMPSON Lift the veil of darkness with an Image Intensified Night Viewer This project employs a 25mm or 40mm 3-stage fibre optically coupled image intensifier tube. The resultant cascaded tube has a typical luminous gain of over 50,000. If all that seems a mouth­ful it is because it embraces some pretty fancy technology which is still not available to some countries. Image intensifier and image converter tubes operate on the same principle. An image is focused by an 34  Silicon Chip external lens onto the photocathode target of the tube and this cathode emits electrons in response to the incident photons. The electrons are then accelerated by a cone-shaped electrode to strike a luminescent screen. Because each photon landing on the photocathode target ultimately gives rise to many photons from the luminescent screen, the result is a gain in luminous intensity. In the tubes being discussed for this project, the photo­ cathode responds to the infrared region of the spectrum while the luminescent screen has TOP OF PAGE: at night, vegetation which is in complete darkness can be seen in intricate detail on the green screen of the Night Viewer. This simulated shot does not show the distortion at the edges of the screen which is really only apparent as you pan the Night Viewer across the scene. This is a 25mm version of the Night Viewer, with all components sprayed black. Note that the Night Viewer must only be used in the dark. green phosphor (much like the screen of an oscilloscope) and so the image is green. Hence, this sort of tube is also referred to as an “image converter”. The photocathode is actually spherical in section, deposit­ed on the inside face of the window. Ideally, the luminescent screen should have the same radius of curvature as the photo­cathode but in practice, a flat screen is used. This results in image distortion (stretching) at the edges of the screen. The cone shaped electrode is held at the same high voltage as the screen and the voltage between the photocathode and the screen is typically 12kV to 15kV. Typical single stage image converters give a luminous gain of about 75 and in practice, if they are to be used for effective night time viewing, they need an infrared light source to illu­minate the scene. For an image converter to be really effective, it needs two or three stages, as depicted in Fig.1. Here, the screen of the first converter is used as the image input to the photocathode of the second converter and so on. The coupling between stages is in the form of plano-concave fibre optic lens­es. The tubes supplied for this project are used and may have some small but negligible blemishes. Due to the complexity of the cascaded tube construction, small blemishes were acceptable even in new tubes. Several different tubes are available for this project, 25mm and 40mm in screen diameter. Whichever one is used, an input lens fits at the front of the case, arranged so it focuses the scene onto the cathode of the imaging tube. Therefore, the lens serves the same purpose as in a camera and the type of lens is chosen for the application, just as it would be for a camera. You can have a wide-angle lens for close-up work, a tele­photo lens for distance viewing or even a zoom lens. In order to maximise the light being transferred from the scene to the tube, fast (large aperture) lenses should be used. The prototype unit pictured in this article employs a 100mm f2 lens. The prototype unit also employs a low cost dual lens magnifier. If the eyepiece is removed, the screen of the imaging tube can be photographed or videotaped. These tubes don’t need much light to operate. 100 millilux (one tenth of a lux) is the maximum recommended exposure. The life of a tube is reduced with prolonged exposure to this light level. The tube can produce a useful output down to 500 microlux (0.0005 lux). Starlight is about 0.001 lux. In comparison, modern colour TV cameras need about 2 lux and most sensitive monochrome TV cameras need at least 0.1 lux. SCREENS PHOTOCATHODE CONE-SHAPED ELECTRODE FIBRE-OPTIC WINDOWS Fig.1: a 3-stage image converter tube has each section linked together by fibre optic couplings to markedly increase the luminous gain. August 1994  35 C3-C14 220pF-.001 5kV D3-D14 BY509 D2 1N4007 C14 T1 R1 150  D14 NE1 R2 22k S1 Q1 2N2219 C1 100 B1 9V B C2 .047 350V NE2 C SCR1 C106D A R3 22k E C13 G D12 C11 D11 D10 C10 D9 C9 +13.5kV T2 C3 C4 D3 K R4 22k D1 1N4148 D13 C12 D4 C5 D5 C6 D6 C7 D7 D8 C8 -13.5kV PCB GND B E C VIEWED FROM BELOW PASSIVE NIGHT VIEWER POWER SUPPLY KAG Fig.2: the EHT supply for the Night Viewer has three sections: a ringing choke inverter involving transformer T1, a capacitor discharge converter involving SCR1 & transformer T2, & the voltage triplers. Just to give some idea of how sensitive these tubes are, we used the prototype to look at the night time sky and found that it really accentuated the detail in the Milky Way! 65mm plastic tubing is used to house the 25mm image con­ verter tubes, while 90mm stormwater plastic tubing is used for the 40mm units. These plastic tubes, their matching joiners and end caps, are readily available from plumbing suppliers. The resultant dimensions of the fully constructed night viewers are approximately 70mm outer diameter and 180mm long for the 25mm version, and 90mm diameter and 280mm long for the 40mm version. Required circuitry As already noted, an image converter requires a high vol­tage supply of about 12-15kV but the current needed is low, less than 10 microamps. The circuit for the supply produces an EHT voltage of around ±13.5kV when powered from a 9V battery. Current drain is about 14mA, giving a useful life of around 20 hours from a standard battery or about 60 hours from an alkaline battery. The circuit of Fig.2 has three sections: the inverter, the converter and the voltage triplers. The inverter section is a ringing choke oscillator consisting of transformer T1, R1, D1 and transistor Q1. Resistor R1 provides bias current to make the oscillator start, and also supplies feedback to maintain oscilla­tion. Diode D1 protects the base-emitter junction of Q1 when the base voltage swings negative. The oscillator operates at around 120Hz, set mainly by the transformer. C14 C13 C12 D13 C11 D11 C10 D9 +13.5kV C9 D2 T1 100uF NE1 T2 22k NE2 D1 Q2 22k C3 .047 22k 150  B1 D10 D12 D14 S1 The resulting AC voltage at the primary of T1 is stepped up by the secondary and is rectified by diode D2. This diode charges capacitor C2 via the primary winding of transformer T2. When the voltage across C2 exceeds the breakdown voltage of the two series-connected neon lamps, NE1 and NE2, (around 150V) the neons turn on. This triggers the C106D SCR, and C2 is quickly discharged through the SCR via the primary winding of T2. Once the capacitor is discharged, the neons go out, the SCR turns off and the charge cycle starts again. During the discharge cycle of C2, a high-voltage pulse with a peak-to-peak voltage of 4.5kV is produced at the secondary of trigger transformer T2. This pulse is applied to two separate 3-stage Cockroft-Walton voltage tripler circuits. The tripler made up by diodes D4-D9 and capacitors C4-C9 produces -13.5kV. Another tripler made up by diodes D10-D15 and capaci­ tors G A K SCR1 D3 C4 D5 D4 C6 C5 D7 D6 C7 D8 -13.5kV C8 GND 36  Silicon Chip Fig.3: everything except the voltage triplers is mounted on a small PC board measuring 50 x 28mm. The triplers are hardwired & then potted in neutral-cure silicone sealant although this step does not take place until after the circuit is tested & connected to the image converter tube. This photo of the Night Viewer shows the wiring inside the plastic case before the triplers are potted. C10-C15 produces +13.5kV. Voltage regulation is achieved by the neons, as the voltage applied to the primary of T2 is constant at 150V peak. The circuit will produce a relatively constant output for a DC input voltage from 7-12V. Circuit construction Everything except the voltage triplers is mounted on a small PC board measuring 50 x 28mm. The wiring diagram is shown in Fig.3. This board is mounted, together with the triplers, in a standard plastic utility box measuring 130 x 68 x 43mm. The triplers are hard-wired and then potted in neutral-cure silicone sealant although this step does not take place until after the circuit is tested and connected to the image converter tube. Several points must be watched during assembly of the PC board: (1) Make sure the metal side of the SCR faces towards the centre of the board; (2) Make sure that the polarity of diodes D1 and D2 and capacitor C1 is correct; and (3) Resistor R1 is in­stalled “end-on”. The two triplers are hard-wired as shown in Fig.3. Their wiring should be kept as compact as possi­ble. A lead length of 5mm for all components is OK. Note that the polarity of the diodes is different in the two triplers. With the circuitry complete you can proceed to a test, before any connec- tions are made to the image converter tube. The test can be done before the tripler sections are potted but note that only one tripler section can actually be connected at a time in this no-load condition. Having a supply produce ±13.5kV is possible in free air but a total of 30kV is not. When you switch on the power with either of the triplers connected, it is most likely that there will be some corona discharge around the tripler diodes. This won’t damage anything but keep a safe distance from this part of the circuit. The current drawn from the battery should be about 14mA. If a wire connected to the circuit earth is placed close to the relevant EHT output lead, you should be able to obtain sparks up to about 5mm long. After the battery is disconnected, you should connect the earth wire directly to the EHT output in order to discharge all the capacitors. Note that when the circuit is working, you will not see the neons light. This is because of the short duty cycle – you will only see the neons glow when you look at them in the dark. If the circuit does not work try measuring the AC voltage at the base of Q1; it should be around 0.45V RMS measured with a digital multimeter. The AC voltage measured at the cathode of D2 is about 45V RMS measured with a digital multimeter. Don’t try measuring the EHT voltages unless you have a suitable EHT probe, otherwise you will damage your meter. If all is well with the preliminary tests, you can proceed to finish the project. As supplied, the image converter comes prewired. You will need to mount the image converter in the The 3-stage image tube is supplied pre-wired. Note that all of the tube metalwork is connected to the EHT supply and therefore must be fully isolated so that no user contact is possible in the finished Night Viewer. August 1994  37 PARTS LIST This little jig, made of two pieces of scrap PC board, simpli­fies the hard-wiring of the two triplers. The hot melt glue is used to pot the triplers after they are wired into circuit. 1 prewired 3-stage image intensifier tube 1 objective lens (see text) 1 eyepiece lens (see text) 1 PC board coded OATLEY JM 2 neon lamps (NE1, NE2) 1 inverter transformer (T1) 1 trigger transformer (T2) 1 9V battery and snap connector 1 miniature SPDT toggle switch 3 22kΩ 0.25W 5% resistors 1 150Ω 0.25W 5% resistor Semiconductors 1 2N2219 NPN transistor (Q1) 1 C106D SCR (SCR1) 1 1N4148 signal diode (D1) 1 1N4007 1A diode (D2) 12 BY509 or equivalent 8kV 3mA diodes (D3-D14) Capacitors 1 100µF 25V electrolytic 1 0.47µF 350V polyester 12 220pF to .001µF 5kV ceramic Miscellaneous Silicone sealant, epoxy adhesive, hot melt glue, hookup wire, solder, plastic tubing & fittings, galvanised steel tubing. A close-up view of the completed EHT supply, showing how the triplers are potted with hot melt glue in a compartment at the end of the plastic box. plastic tube, as shown in Fig.4. This shows the image converter tube suspended in the plastic tube which is fitted with a sleeve of galvanised steel, held in place with the plastic fittings. These fittings will ultimately be glued into place using plumbers’ PVC glue (as used for gluing plastic sewer and stormwater fittings). At the ends of the plastic tube, the image converter tube should be sealed into place using hot melt glue or silicone sealant. This will do two things: support the tube mechanically Where to buy a kit This project is available in kit form from Oatley Elec­tronics, PO Box 89, Oatley, NSW 2223. Phone (02) 579 4985; Fax (02) 570 7910 A kit comprising a 25mm prewired 3-stage image converter tube, plastic pipe and fittings for the tube case, metal X-ray shield sleeve and the EHT power supply kit is available for $290 plus $10 for postage and packing. The same kit for a 40mm image con­verter is $390 plus $10 for postage and packing. Also available is a suitable eyepiece lens for $18 and an objective lens for $75. The plastic box for the power supply is $4. Payment may be made by cheque, postal money order or credit card. Oatley Electronics also have cheaper kits with single stage image intensifier tubes. For further details, contact Oatley Electron­ics. Note: copyright of the PC board for this project is owned by Oatley Electronics. 38  Silicon Chip and prevent any user contact with exposed metal which is connect­ed to the outputs of the triplers. However, before you do any work with glues or sealants, you must drill holes in the plastic tubing, the metal sleeve and the plastic case, to allow the three wires to pass through and con­nect to the outputs of the triplers. Naturally, these holes must line up precisely. After the holes have been drilled, the plastic box is glued to the plastic ring at the objective lens end of the tube hous­ing. You can use plumbers’ PVC glue for this job but the box will need to be held in place temporarily with strong adhesive tape. After the PVC glue has set, some hot melt glue can be used to fill the join between the plastic box and the metal sleeve. That done, connect the triplers and mount them at one end of the plastic box and partition it off with a piece of plastic as shown in one of the photos. Then fill the tripler compartment with hot melt glue. -13.5kV GND +13.5kV 2mm FOR PROTOTYPE 40mm FOR PROTOTYPE TUBE 1 TUBE 2 PROTOTYPE EYEPIECE "PEAK" 10x PLALUPE No.2032-10 OBJECTIVE LENS EPOXY TUBE 3 65mm STORM WATER PIPE POLYURETHANE FOAM BLOCKS 65mm STORM WATER PIPE END CAP SHORT TUBE CUT FROM A 66mm STORM WATER PIE JOINER EPOXY 65mm STORM WATER PIPE END CAP METAL X-RAY SHIELD Fig.4: the image converter tube is suspended in a plastic tube which is fitted with a sleeve of galvanised steel which functions as an X-ray shield. -13.5kV +13.5kV GND TUBE 2 TUBE 3 SCREEN END OF TUBE ANODE END OBJECTIVE LENS END OF TUBE CATHODE END TUBE 1 Fig.5: this diagram shows how the tube sections are wired to the EHT supply. Mount the PC board and the battery in the plastic box – they can each be secured simply by pressing them into a small blob of Bostik Blu-Tack adhesive. Remember to connect the switch in series with the 150Ω resistor and the battery snap before mounting these two components. Finally, you will need to glue your objective lens and eyepiece lens to the PVC end caps and these can be secured to the completed tube assembly with small self-tapping screws. You are now ready to use your completed Night Viewer. Remember to avoid the temptation to test or use the Night Viewer during the day time. Using it under daylight or in brightly lit rooms will damage the tube. Keep a lens cap on the objective lens when not in use as this will also protect the lens from damage. The prototype was sprayed with black paint to finish it SC off. WARNINGS HIGH VOLTAGE: The EHT power supply used in the Night Viewer is not capable of delivering any significant current continuously but it can provide a nasty electric shock. Make sure that the capacitors are discharged before working on the circuit. LIGHT EXPOSURE: An imaging tube can be damaged if it is exposed to bright light. Do not store or use the tube in daylight. The tube will not be damaged if it is exposed to normal room light during construction, but with no power applied. For longest life a completed viewer should be stored in the dark and used in the dark. Normal night time street lighting levels and street lights are not a problem. Using it in daylight can damage it. X-RAYS: Low level X-ray radiation is emitted by most imaging tubes. This radiation is mainly emitted around the sides of the tubes and not from the screen or the cathode of the tube. The radiation level is reduced to very low levels by housing the whole tube assembly in an outer tube made of galvanised steel – see Fig.4. EXPORTING: exporting these tubes to some countries may be pro­hibited, or may require special export permits. Do not export these tubes to any other country prior to consulting the appro­priate authorities. August 1994  39