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By JIM ROWE The on-screen video looks considerably better than this photo indicates. The contrast is better and the Moire patterning, a result of the interaction between the onscreen display and our digital camera, is absent. Video Reading Aid . . . for vision impaired people Do you have a family member with vision problems – like cataracts, or age-related macular degeneration? Here’s a lowcost video reading aid that will make it much easier for them to read a book or newspaper. It combines a small CMOS TV camera with a video processor which boosts the contrast and allows them to select either a positive or negative enlarged image for viewing on a TV set or video monitor. E YE PROBLEMS like cataracts and age-related macular degeneration are all too common in Australia, especially among those of “mature age”. In fact, it was recently estimated that one in every four people over 75 has symptoms of this kind of visual impairment, while one in every 10 lose their central vision. Understandably, those unlucky enough to suffer from these problems can find it very difficult to read a book, magazine or newspaper. This lowers their quality of life dramati38  Silicon Chip cally and deprives them of important sources of news, entertainment and information. In many cases, however, reading printed material can be made a lot easier by using improved lighting to increase the contrast, plus a magnification system to enlarge the type. Optical magnifiers with built-in lighting are available for use as reading aids but they’re fairly pricey. You can also get similar devices using video magnification but these are even more expensive. As a result, such devices are often out of the reach of the people who could benefit from them. Recently, we decided to have a go at a video magnifier ourselves and this project is the result. It combines one of the very small low-cost black and white CMOS cameras currently available from various suppliers with a very compact video processing circuit, and has a switch so you can select one of three image options: high contrast greyscale positive, hard limited or ‘digital’ black and white positive, or digital negative. And the output is siliconchip.com.au standard video so it’s compatible with any normal PAL TV receiver. The camera and video processor are both fitted inside a standard UB3-size project box. Because a person with impaired vision doesn’t want to be fiddling with camera focusing, we’ve mounted it on plastic food container to give it a fixed focal length. In use, this plastic skirt sits directly on the printed page and slides easily over the page, without marking. Basically, it behaves a bit like a giant mouse – you just slide it so that the lens is over the text you want to read. Illumination is provided via four high-output white LEDs, which mount on the underside of the box adjacent to the lens. In practice, the LEDs have to be “doctored” to ensure that their light output is reasonably diffused over the camera’s viewing area but this is easy to do, as described later in the article. The end result is an easy-to-build video magnifier which you can feed into almost any old colour or B&W TV set. You should be able to build it for less than $200. By the way this price estimate is based on using of one of the low-cost B&W cameras with a CMOS sensor that are currently available from various electronics retailers. We’ve tried it out using two of these: the Swann unit sold by Dick Smith Electronics as Cat. L-5877 ($89.86) and the Samsung unit sold by Jaycar Electronics as the QC-3474 ($89.95). These both give good results, although the Swann unit requires a minor modification to disable its inbuilt IR LEDs, so that it runs cooler (more on this later). Of course, other mini CMOS cameras from other suppliers should also be suitable. How it works Refer now to Fig.1 for the circuit details. The output of the CMOS camera is fed through a video processing circuit that’s rather similar to some of our video enhancers but modified to enhance the contrast. The circuit can also generate a negative version of the image, without degrading the signal’s sync pulses. As shown, the video output from the camera is terminated in a 100W load, to provide matching. It then passes through a 1mF coupling capacitor, after which it splits in three directions: siliconchip.com.au The Video Reading Aid skates over the printed page on a plastic skirt (actually an upside down food container). This keeps the lens at the correct focal distance and makes the unit easy to operate. across to CMOS analog switch IC2a, down to the pin 2 input of sync separator chip IC4 (via a 100W resistor and 100nF capacitor) and further down to the non-inverting (pin 3) input of video amplifier stage IC5a. IC4 (the sync separator) is used to extract the sync and “back porch” gating pulses from the video signal. These are then used to provide control signals for video switches IC2a and IC2b. In greater detail, both the back porch and composite signals are combined in gate IC3c (used here as a negativeinput OR gate) and used to turn on switch IC2a, to allow the sync and blanking information to pass straight through. At the same time, IC3a inverts this signal to control switch IC2b. This latter switch allows the processed video through to the output buffer (IC5b) during the “active” part of each video line. In effect, IC2a and IC2b operate in complementary fashion. When IC2a is on (closed), IC2b is off (open) and vice versa. This means that when IC2a is closed, the sync and blanking pulses are fed through to IC5b while the active video is blocked. Conversely, when IC2b is closed, the active video is fed through and the sync signal is blocked. The “back porch” (or burst gating) pulses from pin 5 of IC4 are also invert- The Video Reading Aid is based on a miniature black & white CMOS camera such as this Swann unit from DSE. ed by IC3b and used to control switch IC2c. This forms an active clamp to fix the blanking level of the incoming video to ground potential. The part of the circuit we’ve just described is basically the control section, which ensures that only the active video is subjected to processing. Now let’s look at the actual processing circuitry itself, which involves IC5a, IC6, transistor Q1 and IC2d. IC5a is simply a video amplifier and operates with a fixed gain of two, as set by the two 510W resistors in its feedback divider. Its output at pin 1 becomes the “high contrast analog positive” video signal and is fed to the first position of selector switch S1. This same output signal is also fed to the non-inverting input (pin 2) of October 2005  39 Par t s Lis t 1 PC board, code 02110051, 122.5 x 57.5mm 1 UB3 utility box, 130 x 67 x 44mm 1 mini CMOS B&W TV camera (see text) 2 L-brackets, 15 x 15 x 10mm – see text 1 47mH RF choke (RFC1) 1 3-pole rotary switch (S1) 1 2.5mm PC board mounting DC connector (CON1) 1 RCA connector, PC board mounting (CON2) 1 4-pin SIL header strip 4 M3 x 25mm tapped metal spacers 8 M3 x 6mm machine screws 2 M3 x 10mm machine screws 1 3m length of light figure-8 twin shielded cable 2 RCA plugs, yellow 1 12V/200mA regulated plug pack supply with 2.1mm plug 1 2.1mm concentric DC line socket (to match plugpack) 1 Microsafe plastic food container, 130 x 105 x 60mm (available from Woolworths) 1 1kW mini horizontal trimpot (VR1) 1 5kW mini horizontal trimpot (VR2) Semiconductors 1 741 op amp (IC1) IC6, an LM311 high-speed comparator. This compares it with a reference DC voltage level on pin 6, as set by trimpot VR1, to generate a “hard limited” or rectangular digital equivalent of the boosted video signal. IC6 has positive feedback applied via the 4.7kW, 100W and 33kW resistors, to give it a small amount of hysteresis and ensure clean switching. Trimpot VR2 also allows fine adjustment of this feedback. The output from pin 7 is then fed to transistor Q1, which is connected as an emitter follower to provide buffering. From there, the buffered signal is fed to the second position of selector switch S1, to become the hard limited or “Digital Positive” video signal. This signal is also fed to the control gate (pin 12) of IC2d, used here as an analog 40  Silicon Chip 1 4066B quad bilateral switch (IC2) 1 4093B quad CMOS Schmitt NAND gate (IC3) 1 LM1881 video sync separator (IC4) 1 MAX4451ESA dual video amp (IC5) (www.futurlec.com.au) 1 LM311 comparator (IC6) 1 PN100 NPN transistor (Q1) 4 5mm high-brightness white LEDs (LED1-LED4) 3 1N4148 signal diodes (D1,D2, D5) 2 1N4004 power diodes (D3,D4) 1 1N752 5.6V/400mW zener diode (ZD1) Capacitors 1 220mF 16V RB electrolytic 1 10mF 10V RB electrolytic 3 4.7mF 16V tantalum 1 1.0mF MKT metallised polyester 2 100nF MKT metallised polyester 6 100nF multilayer monolithic 1 2.2nF 50V disc ceramic 1 220pF 50V disc ceramic 1 22pF 50V disc ceramic Resistors (0.25W, 1%) 1 680kW 1 2.2kW 1 33kW 2 1kW 1 4.7kW 4 510W 1 3.9kW 4 270W 1 3.3kW 4 100W 1 2.7kW 1 75W inverter. The inverted video signal appears at pin 11 and is fed to the third position of S1, to become the “Digital Negative” video signal. Limiting circuit The processed video signal selected by switch S1 is first fed through a simple diode limiting circuit involving diodes D1-D3 and a 1kW series resistor. Diode D3 ensures that the negative excursions of the signal (ie, its black level) are clamped at 0.6V below ground, while D1 and D2 ensure that the positive excursions (ie, peak white level) are clamped at 1.2V above ground. The processed video fed to video switch IC2b is thus limited to a fairly normal voltage range, so it shouldn’t cause any overload problems, either in the video output buffer stage (IC5b) or in the TV set. The recombined sync and video signals from switches IC2a and IC2b are fed to pin 5 of IC5b via a simple low-pass filter comprising a series 100W resistor and 22pF capacitor. This removes any switching transients. The signals are then passed through video buffer IC5b, which operates with a fixed gain of two, to compensate for the losses in the 75W “back termination” resistor in series with the output. This is the standard video buffer configuration and is used to allow the output signal to be fed along relatively long video cables with minimal degradation. Power supply Both the mini video camera and the video processing circuitry are powered from an external 12V DC source – either a 12V battery or a regulated plugpack supply delivering 12V at up to about 150mA. The four white LEDs (LED1-LED4) used to provide illumination are powered from the same source. Series diodes D4 and D5 provide reverse polarity protection and also reduce the overall supply voltage to 10.8V, which is necessary to protect both IC5 and the CMOS camera from over-voltage damage. Because IC5 needs a balanced bipolar supply, IC1 and ZD1 are used to give the 10.8V supply an active “centre tap”, which is connected to the circuit’s earth. The two main supply rails thus become +5.4V and -5.4V nominal with respect to ground. The CMOS camera and all of the remaining ICs are connected directly between the +5.4V and -5.4V rails, as are the illumination LEDs. The latter are connected in two series strings, with a 270W resistor in each string to limit the current to around 17mA. Provided high-brightness white LEDs are used, this modest current provides plenty of illumination. Construction All of the video processing circuitry fits on a PC board measuring 122.5 x 57.5mm and coded 02110051. This board has a rounded cutout in each corner, so that it slips neatly inside a standard UB3 jiffy box. The video selector switch is located near the centre of the board, while the DC input and video output connectors at mounted at one end – see Fig.2. siliconchip.com.au siliconchip.com.au October 2005  41 Fig.1: the circuit uses sync separator IC4 plus gates IC3c & IC3a to drive switches IC2a & IC2b in complementary fashion. IC2a switches through the sync signal from the camera when closed, while IC2b switches through the active part of the video signal. IC5a, IC6, Q1 & IC1d are used to process the video signal, to produce normal, enhanced contrast and negative displays, as selected by switch S1. Fig.2: follow this assembly diagram to install the parts on the PC board, taking care to ensure correct component polarity. The four high-brightness LEDs and the MAX4451ESA device are installed on the copper side of the board (see Fig.3). This is the fully-assembled PC board, mounted on 25mm tapped spacers. Note how the highbrightness LEDs hang down from the underside. The CMOS camera module is mounted centrally inside the box (Fig.6). It sits under the PC board with its lens protruding through a 16mm hole in the base and is supported by two small aluminium angle brackets. The adjacent illumination LEDs are mounted on the copper side of the PC board at full lead length, so that the body of each LED protrudes through a matching 5.5mm hole in the box. Fig.2 shows the parts layout on the PC board. Begin the assembly by fitting the 12V DC input and video output connectors, then install the eight wire links. Next, fit the 4-pin SIL header which is used to terminate the leads from the CMOS camera. This goes just below the 8mm hole that the leads feed through. That done, you can begin fit42  Silicon Chip ting the passive components, starting with the resistors and RF choke and following these with the two trimpots, the smaller capacitors and finally the polarised tantalum and electrolytic capacitors. Follow these with diodes D1-D5, making sure you fit each one the correct way around as shown in Fig.2. Also, make sure you use the larger power diodes for D4 and D5 and the smaller glass signal diodes for D1-D3. Zener diode ZD1 can then go in, again taking care with its polarity. At this stage, it’s a good idea to fit rotary switch S1. To do this, first cut its shaft to about 8mm long and carefully file off any burrs. That done, it can be mounted on the board with its indexing spigot at the 12 o’clock position, as shown on the overlay diagram. Push it all the way down onto the board before soldering its pins. The next step is to fit IC1, IC4, IC6, IC3 and IC2, in that order. Note that the last two of these devices are CMOS ICs, so be sure to take the usual precautions to avoid subjecting them to electrostatic damage – ie, don’t touch their pins, make sure the tip of your soldering iron is earthed and solder their supply pins (pins 7 & 14) first. It’s also a good idea to “discharge” yourself by touching an earthed metal object before handling these devices or, better still, wear an earthed wrist strap. The board “topside” assembly can now be completed by fitting transistor Q1. Be sure to orient it as shown, then flip the board over so that you can fit IC5 – see Fig.3. siliconchip.com.au This IC is in an SOIC-8 surface mount package which measures only about 5mm square and has a pin spacing of just 1.25mm. It is just large enough to be soldered in place by hand, provided you take your time and work carefully. This job requires a soldering iron with a very fine tapered bit, which is also well tinned and clean. You should use fine gauge (ie, 0.8mm) resin-cored wire solder, to ensure there are no solder bridges between adjacent pins. The best procedure is to hold the device in position using a wooden toothpick while you carefully solder one of its supply leads – either pin 4 or 8. This involves just touching the outer end of the device lead with the soldering iron and feeding on the solder, so that a tiny drop melts and bonds the lead to the pad underneath. That done, you can quickly solder the other supply lead and then the rest of the leads. So the trick is to make one joint first, to hold the device in place while you solder all the other leads. Doctoring the LEDs Now for the LEDs. These are left until last because, as mentioned earlier, they first have to be “doctored”. As supplied, the rounded end of each LED’s clear body produces a fairly narrow semi-focused axial beam. That’s fine for most applications but not this one, as this would produce very uneven lighting below the camera lens, with four bright spots separated by relatively dark regions. The cure is simple – by sanding four small “flats” on the end of each LED, its light output becomes much more diffused and this gives more Fig.3 (left): use fine-gauge solder and a fine-tipped soldering iron to install the SOIC device (IC5) on the underside of the PC board. Fig.4: here’s how the four highbrightness LEDs are modified to diffuse the light. even illumination. Fig.4 shows the basic idea. It’s quite easy to sand these flats by hand, because the LED bodies are moulded in a fairly soft “water clear” plastic. A small piece of medium garnet paper wrapped around a piece of flat wood will do the job quite nicely and you will only need seven or eight passes to produce each flat at the correct angle (the exact angle isn’t critical, by the way). Don’t try to polish the surfaces after sanding – just leave them with the after-sanding matt finish, as this gives better light diffusion. After all four LEDs have been treated, you can fit them to the underside of the board. They must all be mounted at full lead length (ie, with the shorter cathode leads just entering their matching holes), so that they’ll later protrude through the holes in the bottom of the box when the board assembly is fitted. Before actually installing the LEDs, it’s a good idea to fit 20mm lengths of 2mm sleeving over each lead, to prevent accidental shorts. You can use red sleeving for the anode leads and green or black sleeving for the cathode leads. After the LEDs have been fitted, the board assembly can be completed by attaching four M3 x 25mm tapped spacers (one at each corner), using 6mm long M3 machine screws. Box preparation The box needs to have a number of holes cut in the bottom and lefthand Table 2: Capacitor Codes Value 100nF 2.2nF 220pF 22pF μF Code IEC Code EIA Code 0.1µF 100n 104 .0022µF   2n2 222   NA 220p 220   NA   22p   22 Table 1: Resistor Colour Codes o o o o o o o o o o o o o siliconchip.com.au No.   1   1   1   1   1   1   1   2   4   4   4   1 Value 680kW 33kW 4.7kW 3.9kW 3.3kW 2.7kW 2.2kW 1kW 510W 270W 100W 75W 4-Band Code (1%) blue grey yellow brown orange orange orange brown yellow violet red brown orange white red brown orange orange red brown red violet red brown red red red brown brown black red brown green brown brown brown red violet brown brown brown black brown brown violet green black brown 5-Band Code (1%) blue grey black orange brown orange orange black red brown yellow violet black brown brown orange white black brown brown orange orange black brown brown red violet black brown brown red red black brown brown brown black black brown brown green brown black black brown red violet black black brown brown black black black brown violet green black gold brown October 2005  43 Fig.5: here are the drilling details for the plastic case. It’s best to make the larger holes by drilling small-diameter holes first and then carefully enlarging them to size using a tapered reamer. 44  Silicon Chip siliconchip.com.au Fig.6: this diagram shows how it all fits inside the case. Note that the lid should sit firmly on the switch indexing ring, to keep it in place when everything is screwed down. end of the base, plus two holes in the lid. The positions and sizes of these holes are shown in Fig.5. Depending on the box, you may also have to cut away some of the plastic ribs moulded on the inside at one end, near the input/output connectors. This can be done using a sharp chisel. Mounting the camera Once you’ve drilled all the holes, the mini camera can be prepared for mounting. First, remove the two screws which attach it to its existing U-bracket, then cut the camera’s output cable about 40mm from the body (or its mini connection plug). Remove about 15mm of the outer sleeving from the end, then separate the individual leads. In most cases, the positive power lead has red insulation, while the video lead has yellow insulation. The negative power lead usually either has black insulation or is in the form of a screening ground braid. If the camera also has an audio output (many of them do), this is usually a wire with white insulation. This output isn’t used in this project, however. After separating the various leads, strip about 5mm of insulation from the ends and tin the exposed wire ends, ready for connection to the 4-way header on the PC board. If you camera has a ground braid, this should be siliconchip.com.au neatly twisted together, sleeved and tinned as well. And that’s basically all you have to do to prepare a camera like the Samsung unit sold by Jaycar. However, with a camera like the Swann unit sold by DSE, you also have to disable the inbuilt IR LEDs (originally intended for night illumination). That’s done by removing the back of the case (it’s usually attached by two tiny screws) and removing one of the LEDs – either by cutting its leads with side-cutters or desoldering them from the internal PC board. You don’t have to worry about the others, because they’re usually connected in a series string. The camera can now be mounted inside the box using two small Lbrackets, made from 1mm aluminium sheet – see Figs.6 & 7. The camera mounts between the brackets using the same two screws which held it in its original U bracket. It’s a good idea to fit an M2.5 flat washer on each screw before passing it through the hole in the L-bracket and then fit an M2.5 star lockwasher on each screw before it enters its tapped hole in the side of the camera. This arrangement keeps the camera firmly vertical when both screws are tightened. The camera mounting brackets are then attached to the box using M3 Fig.7: here’s how to make the two L-brackets that support the mini CMOS camera. x 10mm machine screws, nuts and lockwashers. Final assembly Once the camera is mounted, the PC board (with its mounting spacers) can be lowered into position. Feed the camera cable through its board access hole as you go and make sure the four LEDs all pass through their respective holes in the base. The board assembly can then be secured from underneath using M3 x 6mm screws into the tapped spacers. Finally, connect the camera cable leads to their respective header pins on the PC board. The positive power lead (red) connects to the leftmost pin, nearest the 1mF MKT capacitor, while the video wire (yellow) connects to the October 2005  45 Above: this close-up view shows the mounting details for IC5. Below: the plastic skirt has a clearance hole for the camera lens and is attached to the base of the case using double-sided adhesive strips. Above: the CMOS camera is attached to the base of the case and its leads fed up through a small hole in the PC board. rightmost pin. If present, the audio wire (white) is left disconnected – just tape it up so it can’t make contact with anything. If there’s a negative power wire (black) separate from the ground braid, solder this to the second pin from the left and connect the ground braid to the remaining pin - ie, the third pin from the left. Alternatively, if there’s no separate negative power wire, simply connect the ground braid to BOTH of the centre pins. The only other possibility is that your camera may have just a black negative wire and no ground braid. In this case, connect the black wire to both centre pins instead. Switch indexing Before testing the Video Reading Aid, you have to set the rotary switch so that it has only three positions and not four. To check this, fit its knob temporarily to the spindle and try turning it to see how many positions are available. If there’s only three, you can relax. But 46  Silicon Chip if there are four, the switch will need to be reset. To do this, first turn the switch anticlockwise to its end position and then remove the knob. That done, unscrew the mounting nut, and remove both it and the star lockwasher underneath. This will reveal the indexing stop washer, which you then have to prise up using a small screwdriver. The underside of this washer has a small spigot, which sits in one of the matching slots in the switch body. If you look closely you’ll see that there are a series of numbers moulded into the switch body, between the slots. The idea is to find the slot between the numbers “3” and “4” and refit the indexing washer with its spigot in that slot. Check that the switch now has only three positions, then refit the star lockwasher and nut. Fitting the plastic skirt The plastic skirt fitted to the unit is actually an upside-down food container. The recommended unit (see Parts List) measures 130 x 105 x 60mm deep and has an indent in the centre of its base which provides clearance for the LEDs. The unit is also curved towards the sides, which means that it siliconchip.com.au naturally clears the four corner mounting screws that go into the spacers. Attaching it is hardly rocket science – just cut a hole in the centre to clear the camera lens, attach some doublesided tape to its base and attach it to the bottom of the box. If you use a different food container from the one we used, then you may have to also drill holes to clear the LEDs and the mounting screws. Testing Now for the smoke test! First, set the rotary switch to fully anticlockwise (Medium Contrast), set trimpot VR1 to fully anticlockwise and set VR2 to its mid-range position. That done, connect the Reading Aid’s video output cable to the video input of a TV set and apply power. Note: you must use a 12V regulated plugpack or 12V battery. Do not use an unregulated plugpack, otherwise you’ll damage the camera and IC5. If all is well, you should see a bluishwhite glow from the illumination LEDs underneath the Reading Aid box. Now place the unit on some printed material. The image will probably be quite blurry initially – just adjust the lens until you get the correct focus by rotating it clockwise or anticlockwise. This will have to be done by trial and error, since the plastic skirt is in the way when the unit is resting on a surface but it shouldn’t take long to get it just right. You may also have to adjust the brightness and contrast controls on the TV to get a good image. If there’s no image or none of the LEDs is alight, you’ve probably got the power supply the wrong way around. No damage will result from this – just reverse the connections and all should be OK. However, if the image does appear but only two of the LEDs are alight, the odds are that you’ve connected at least one of the dark LEDs around the wrong way. If all LEDs are alight and you have a clear image on the TV, turn the rotary switch to its centre position. The image will probably go very dark but if you turn trimpot VR1 slowly clockwise with a small screwdriver, it should gradually turn into a very “contrasty” but still clear black-and-white image. The correct setting for VR1 will be quite obvious – just set it for maximum clarity and best contrast. If you can’t achieve this by adjusting siliconchip.com.au Fig.8: check your PC board for defects by comparing it with this full-size etching pattern before installing any of the parts. Fig.9: this is the full-size artwork for the front panel. It goes on the lid and can be protected using wide strips of clear adhesive tape. VR1 alone, you may also need to adjust VR2 slightly one way or the other. Once the correct settings have been found, try switching S1 to the third position (fully clockwise). The image should change into a high contrast negative, with black type on a white background turning into white type on a black background, which many people with visual impairment find easier to read. Final assembly Assuming it all checks out, disconnect the power supply and remove the knob, mounting nut and star lockwasher from the rotary switch. The box lid can then be slipped into position over the switch shaft and should rest on the top of the box, with the switch locating spigot passing up through the small hole that’s located just behind the main spindle hole – see Fig.5. Image Washed Out? Depending on the high-brightness LEDs supplied and/or the amount of ambient light at the reading location, you might find that the on-screen image is washed out (ie, over-bright). In that case, try throttling back the LED brightness by increasing their series 270W resistors to around 680W. Alternatively, if you have plenty of ambient light, you may get a better result if the LEDs are taped over (or the unit modified so that they can be switched out of circuit). Be prepared to experiment to get a good picture if necessary. All that remains now is to fit the four lid fastening screws and then refit the star lockwasher and nut to the switch ferrule. Your Video Reading Aid should is now ready for use. SC October 2005  47