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A Low-Cost Camera Switcher By JIM ROWE L'IL SNOOPER If your security system has more than one CCTV camera but only one monitor, you really need an automatic “camera switcher” (or sequencer) to let you keep an eye on what’s happen­ing in each camera’s field of view. This easy-to-build unit can handle up to four cameras and features variable rate scanning and a pause button, for when you need to study something interesting. 70  Silicon Chip I DEALLY, EACH CAMERA in a CCTV system will have its own dedicated monitor – making it easy to watch and listen to what they’re all seeing and “hearing”. But video monitors aren’t cheap and this approach is just too expensive for many of us. Fortunately, there’s an alternative: use just one monitor, together with a gizmo called an “AV sequencer” or “camera switch­er”. This works like a multiplexer or scanner, automatically cycling around between the cameras so that you get the video and audio first from camera 1, then from camera 2, then from camera 3 and so on. Each camera’s signals are presented for a few seconds in turn, allowing you to keep an eye and ear out for anything of interest. AV sequencers are available commercially, of course, but while they’re much cheaper than additional monitors, they’re still rather pricey. You’ll be able to build L’il Snooper for much less than a commercial unit and you will have the satisfac­tion of knowing that you built it yourself. L’il Snooper can handle the video and audio from up to four cameras and its scanning rate can be adjusted over a range of about 10:1 to suit different applications. It has a row of LEDs on the front panel to show which camera is being presented at any moment and there’s also a “Pause” button. This button lets you stop the scanning and concentrate on just one camera if you spot or hear anything of interest from that unit. Just about all of the parts used in L’il Snooper’s circui­try fit on a small PC board, making it very easy to put together. The completed board assembly fits snugly in a standard low-cost instrument case, with all input and output connectors along the rear. A small DIP switch inside the unit lets you set up L’il Snooper for sequencing the signals from two, three or four cam­eras. The complete unit runs from a nominal 12V DC supply and draws less than 110mA, so it can easily be operated from a small plug­ pack or even a battery. How it works Fig.1 shows a simplified block diagram of the L’il Snooper. As you can see, it’s really very straightforward. Each of the camera video signals is terminated in the cor­rect 75Ω resistance (to prevent cable reflections and ringing) and then fed to separate unity-gain buffer amplifiers. The buffer outputs are then fed to the inputs of multiplexing switch SWA which selects each one in sequence. From there, the selected signal is fed to another video amplifier stage, this time operating with a gain of 2 to compensate for the loss in the 75Ω “back terminating” resistor in series with the output. The audio signals from the cameras are handled in a similar way but with less complication. Here, the inputs are taken directly via coupling capac­itors to audio multiplexing switch SWB, with only a unity-gain buffer amplifier stage required between SWB and the audio output socket. As you’ve probably guessed already, Fig.1: the block diagram of the Li’l Snooper. The audio and video signals are fed to multiplexing switches which are controlled by a sequencing counter. VR1 controls the oscillator to set the scan rate. switches SWA and SWB are driven in tandem to perform the sequenc­ing (or switch­ing). In fact, they’re both driven by a counter which is stepped by pulses from a low-frequency oscillator. The se­quencing or “scanning” rate is adjusted by varying the oscilla­ tor’s frequency. So that’s the basic idea of how L’il Snooper works. Now let’s look at the full circuit, to fill in the details. Circuit details Although we showed the video and audio signals being se­lected by a pair of single-pole rotary switches in Fig.1, the actual circuit (Fig.2) does the same jobs using two groups of four SPST on-off switches. In addition, the switches are elec­ tronic rather than electromechanical and are based on two 74HC4066 quad bilateral switch ICs (IC2 & IC3). Each pair of switches controls the video and audio from one of the camera inputs and we do the sequencing by turning on each pair of switches in turn. This is done by applying +5V to their gates, which are connected in parallel. Only one pair of switches is turned on at any time, so only one camera’s audio and video (AV) signals are passed through. All of the video inputs use an identical input buffer cir­cuit based on an emitter follower stage. Transistor Q1 is the buffer for camera 1, Q2 for camera 2 and so on. The inputs are terminated in 75Ω resistors and are AC-coupled to the transistor bases to prevent damage or signal distortion due to excessive DC levels. The 1MΩ resistors and diodes D4-D7 form simple clamp-type “DC restorer” circuits, setting the sync tip levels of all the video input channels to the same voltage level – ie, to +1.2V as established by forward-biased diodes D8 & D9. This makes sure that the video signals remain in the correct voltage range for correct operation of the bilateral switches. It also ensures that the signals all have the same black level so there’s no undue “flashing” as June 2001  71 Everything apart from the scan rate pot (VR1) and the pause switch (S1) is mounted directly on the PC board, so the unit is easy to build. Check that all polarised parts are correctly orientated and make sure that you don’t get any of the ICs or the voltage regulators mixed up. which form a 2:1 voltage divider from the collector of Q8 back to the base of Q7. Audio circuitry the sequencer switches from camera to camera. The video signals on the emitters of Q1-Q4 are fed directly to video switches IC3a, IC3b, IC2a & IC2b. And as you can see, the outputs of these switches are all connected together, so whichever signal is selected is fed to the input of the video output buffer amplifier (Q6-Q9). As previously mentioned, this simple circuit operates with a gain of two and has a bandwidth of over 10MHz. 72  Silicon Chip Transistors Q6 & Q7 form an input differential pair, with the output of Q6 fed to the base of output stage Q8. Transistor Q9 is used as an “active load” for Q8, presenting it with a low DC load but a relatively high AC load. This is done by connecting Q9 as a constant current sink, with LED5 providing a suitable reference voltage on its base. Negative feedback is used to set the amplifier’s gain to two and achieve the bandwidth we need. The feedback is provided by the two 470Ω resistors, The audio section is even simpler than the video section, as indicated in Fig.1. As shown, each input is connected to ground via a 470kΩ “bleed” Fig.2 (facing page): the circuit uses 74HC4066 analog switches to switch the audio/video signals and these are sequenced using counter stage IC1. Transistors Q1-Q4 function as video input buffer stages, Q5 buffers the audio output signal and Q6-Q9 form a video output amplifier. June 2001  73 Capacitor Codes      Value IEC Code EIA Code 0.22µF  224  220n 0.1µF  104  100n .047µF 473   47n .01µF  103   10n emitter follower, with the output taken from its emitter via a 0.1µF coupling capacitor. Sequencing Fig.3: install the parts on the PC board as shown on this layout diagram. Make sure that you install DIPSW1 the correct way around and that only one switch is in the “on” position. resistor and the audio signals fed via .047µF coupling capacitors to audio switches IC3d, IC3c, IC2d & IC2c. The only extra complication here is that the switch side of each coupling capacitor is connected to a “half-supply” voltage of +2.5V via a 47kΩ isolating resistor. This half supply voltage is provided by two 10kΩ resistors connected between the +5V rail and ground. 74  Silicon Chip This ensures that the audio signals remain in the optimum voltage range for the bilateral switches (for minimum distortion) and that they’re at the same DC level to prevent switching clicks. The outputs of the audio switches are connected together, so that whichever signal is selected passes directly to the base of output buffer transistor Q5. As you can see, this is simply an Now let’s see how the sequencing circuitry works. The sequencing counter is formed by IC1, a 4017 Johnson-type decade counter whose first four outputs (O1O4) are used to drive the four pairs of switches. We use simple feedback from these outputs back to the master reset (MR) input (pin 15) to force the counter to count by a smaller number than 10, to suit the number of cameras being used. This feedback is controlled by switch DIPSW1, which is set to suit the number of cameras used. If there are four cameras, only the “4” switch is turned on (closed), which makes the coun­ter reset each time the O5 output goes high. This turns the counter into a modulo-4 counter, so that all four pairs of analog switches are turned on repeatedly in sequence. On the other hand, if you have only three cameras, the “3” switch of DIPSW1 is turned on instead of “4”, so that the counter resets each time the O4 output goes high. This makes the counter operate in modulo-3 mode so that only the first three pairs of analog switches are turned on in sequence. Similarly if you only have two cameras, the “2” switch of DIPSW1 is turned on to make the counter operate in modulo-2 mode. Only the first two pairs of analog switches are then turned on, in sequence – or alternately, if you prefer. What happens if you turn on only the “1” switch of DIPSW1? That’s right, the counter then resets whenever O2 goes high – so it effectively stops counting altogether, with the analog switch­es for camera 1 turned on continuously. Clearly, there’s no point in doing this because L’il Snooper then doesn’t do anything useful. But if you only have one camera you don’t need a sequenc­er, anyway! I used a 4-pole DIP switch because you can’t buy one with three poles. LEDs 1-4 are used to indicate which camera input channel is selected at any time. As you can see, they are driven from the four switch-selecting outputs of IC1, via inverters IC4c-f. The LEDs can share a common 470Ω current-limiting resistor, as only one of these LEDs is ever turned on. The low frequency oscillator which drives the counter is formed by Schmitt inverter IC4a, connected as a simple relaxation oscillator. The 500kΩ pot is connected as an adjustable feedback resistor, allowing the oscillator frequency to be varied over a range of about 10:1 (from roughly 0.3Hz to 3Hz). The output of the oscillator is fed to one of the two count inputs of IC1, at pin 14. This allows the counter to operate whenever the other count input (pin 13) is held low. And it normally is held low by the output of inverter IC4b, whose input is pulled high via a 100kΩ resistor to +5V. Counting can be paused very easily, simply by pressing the Pause pushbutton switch S1. This shorts pin 3 of IC4b to ground, forc­ing its output high and hence stopping the counter. Pressing the switch again resumes counting. The 0.1µF capacitor across S1 provides the necessary decoupling to prevent miscounting due to contact bounce. Power supply The power supply part of L’il Snoop­ er is very straightfor­ward. As shown The RCA output sockets and the DC power socket are all mounted directly on the PC board, so there’s very little internal wiring. Use insulated wire to prevent shorts between adjacent links near IC2 and IC3. on Fig.2, the nominal +12V DC from an external source (eg, a plugpack) is fed in via polarity protection diode D1 and filtered using a 1000µF electrolytic capacitor. The fil­tered DC rail is then fed directly to 3-terminal regulator REG1 to produce the main regulated +5V rail. In addition, the filtered 12V rail is used to power IC5, a standard 555 timer IC used here as a self-oscillating commutator switch. This drives a Resistor Colour Codes  No.   4   4   2   5   3   5   2   1   1   3   5   1 Value 1MΩ 470kΩ 100kΩ 47kΩ 10kΩ 4.7kΩ 2.2kΩ 1kΩ 680Ω 470Ω 75Ω 47Ω 4-Band Code (1%) brown black green brown yellow violet yellow brown brown black yellow brown yellow violet orange brown brown black orange brown yellow violet red brown red red red brown brown black red brown blue grey brown brown yellow violet brown brown violet green black brown yellow violet black brown 5-Band Code (1%) brown black black yellow brown yellow violet black orange brown brown black black orange brown yellow violet black red brown brown black black red brown yellow violet black brown brown red red black brown brown brown black black brown brown blue grey black black brown yellow violet black black brown violet green black gold brown yellow violet black gold brown June 2001  75 With the sockets all fitted and their pins soldered under­neath, the next step is to fit switch DIPSW1. Watch out here – it must be fitted with its “ON” side towards the rear of the board. The rest of the components can now be installed. As usual, fit the low-profile resistors and diodes first, making sure each diode is orientated correctly. You can then fit the small non-polarised capacitors, followed by the TAG tantalums and the electrolytic capacitors – again watching their polarity. Next can come the transistors. Note that there are eight BC548s and only one BC640 (Q8). You may want to fit the BC640 first to make sure it doesn’t end up in the wrong spot. The next step is to fit the two voltage regulators and the four ICs. Make sure you don’t accidentally swap the regulators – the 7805 is REG1 and the 7905 is REG2. The leads of both are bent down by 90° 6mm from their bodies, so that their metal tabs can be bolted flat against the PC board. Use 10mm-long M3 machine screws to secure them to the PC board before soldering their leads. Watch the static! Figs.4&5: these full-size artworks can be used as drilling templates for the front and rear panels. Drill small pilot holes first, then carefully enlarge them to the correct size using a tapered reamer. simple charge-pump voltage inverter using D2, D3 and two 220µF capacitors, to produce a -10V rail. This is fed to REG2 which produces a regulated -5V rail for the video amplifiers. Putting it together Apart from the 500kΩ pot and Pause pushbutton S1, all of the components used in L’il Snooper mount directly on a PC board coded 02106011 and measuring 120 x 144mm. The only offboard wiring you have to worry about is the four short wires which connect the pot and pushbutton switch to the front of the board. The four indicating LEDs mount directly on the PC board but protrude through 3mm holes in the front panel. Similarly, the input and output connectors are also soldered directly to the PC board and are accessed via 76  Silicon Chip holes in the back panel. Fig.3 shows the assembly details for the PC board. There are 22 wire links on the board and it’s probably best that you fit these before anything else, to make sure you don’t miss any. Most can be fitted using component lead offcuts or tinned copper wire but be sure to use insulated wire for the longer leads, particularly where they run close together (see photo). After the links are in, fit the DC input socket and the double RCA connectors along the rear edge of the board, as these can be a bit fiddly. You may have to enlarge the holes in the board slightly to take the various pins, in each case. Note that each double RCA socket has a “barbed” plastic spigot on each side and these mate with 3mm holes in the PC board to help hold each socket in position. The four dual-in-line (DIL) ICs are all CMOS devices, so take the usual precautions against static charge damage when you’re fitting them to the board. Earth the soldering iron and yourself if possible and solder each chip’s supply pins to their board pads before you solder the other pins. Be sure to fit each IC with the correct orientation, as shown in Fig.3. All that should be left now to complete your board assembly is to fit the five LEDs. These are of course polarised, so make sure you fit them with their longer anode (A) leads to the left. LED5 is the easiest to fit, because it’s simply mounted vertical­ly on the board near Q9. You can leave about 8mm of lead length between the LED body and the top of the board. The four “indicator” LEDs (LEDs1-4) should initially be mounted vertically also but with their leads left at full length. When they’re all fitted, carefully bend each LED’s leads forward by 90°, at a point about 11mm down from the bottom of the LED body. This isn’t difficult to do if you use a pair of long-nose pliers to grip them just below the bend point. Your four LEDs Parts List 1 PC board, code 02106011, 120 x 144mm 1 plastic instrument case, 160 x 155 x 65mm 5 dual RCA sockets, vertical PCmount (CON1-5) 1 2.5mm PC-mount DC power connector (CON 6) 1 4-pole DIP switch (DIPSW1) 1 SPST push-on/push-off switch (S1) 1 small instrument knob 1 500kΩ linear pot (VR1) 2 10mm x M3 machine screws with M3 nuts 4 small self-tapping screws, 6mm long The audio/video input and output sockets protrude through holes drilled in the rear panel of the case. Another hole, at bottom right, provides access to the DC power socket. should all end up pointing forwards in a neat row, ready to mate with the holes in the front panel. There’s one last step to finish the board assembly – you have to connect two short lengths (about 50mm) of insulated twin-lead hookup wire (eg, rainbow cable) for the rate pot and pause switch connections. Bare and tin about 5mm at both ends of all four wires before soldering them to the appropriate pads on the PC board. Fitting it in the case The board fits snugly inside a standard plastic instrument case measuring 160 x 155 x 65mm. However, before installing the board, you have to prepare the front and rear panels (note: some kits may come with these prepunched). In summary, you have to drill six holes in the front panel and 16 in the rear panel (each double RCA socket also attaches to the rear panel via a small self-tapping screw, for added support). The artwork for the two panels is reproduced here and photocopies of these can be used as templates for drilling the various holes. If you’re building your own unit from scratch (rather than from a kit), you might also want to use a clean photocopy of each as a dress panel. Don’t try to drill large holes in one go, otherwise you’ll end up making a mess. Instead, drill small pilot holes first, then carefully enlarge each hole to its correct size using a tapered reamer. When your panels are finished, cut the pot shaft to length (to suit the knob), then mount the pot and pushbutton switch in position. The knob can then be fitted to the pot, after which you’re ready for the final assembly. This is best done in a particular order, to make things easier. First, slide the rear panel into its slot in the bottom of the case, then fit the board assembly so that the RCA connectors pass through their respective holes. Be sure to push the board all the way home so that the connector bodies sit flush against the inside of the rear panel. At this point, the PC board’s mounting holes should line up with the support pillars in the bottom of the case. Once every­thing is correct, secure the board with four small self-tapping screws, then fit the small self-tapping screws which secure the double RCA connector sockets to the rear panel (these go in from the outside). Both the PC board and rear panel should then be securely attached to the bottom of the case. The front panel assembly can now be slid down into its slot, gently easing it down in front of the four LEDs until they locate with the matching holes. Semiconductors 1 4017 CMOS counter (IC1) 2 74HC4066 analog switch ICs (IC2, IC3) 1 74HC14 hex Schmitt inverter (IC4) 1 LM555 timer (IC5) 1 7805 3-terminal regulator (REG1) 1 7905 3-terminal regulator (REG2) 8 BC548 NPN transistors (Q1Q7, Q9) 1 BC640 PNP transistor (Q8) 5 red LEDs (LED1-5) 3 1N4001 diodes (D1-D3) 4 BAW62 diode (D4-D7) 2 1N4148 diode (D8,D9) Capacitors 1 1000µF 16VW RB electrolytic 2 220µF 16VW RB electrolytic 2 100µF 10VW RB electrolytic 1 10µF 10VW TAG tantalum 4 2.2µF 10VW TAG tantalum 4 0.22µF MKT polyester 2 0.1µF MKT polyester 6 0.1µF monolithic ceramic 4 .047µF MKT polyester 1 .01µF MKT polyester Resistors (0.25W, 1%) 4 1MΩ 2 2.2kΩ 4 470kΩ 1 1kΩ 2 100kΩ 1 680Ω 5 47kΩ 3 470Ω 3 10kΩ 5 75Ω 5 4.7kΩ 1 47Ω Finally, the four connecting leads from the board can be soldered to the lugs of the pot and pushbutton switch, to June 2001  77 The remaining possibility is that LED5 glows steadily and one of the others also glows steadily. This would suggest that the power supply is probably OK but the sequencing counter isn’t counting for some reason. Possible causes of this are a short or hairline crack on the PC board in the vicinity of oscil­lator IC4a, pause inverter IC4b or near the counter itself (IC1). Alternatively, the 10µF tantalum capacitor may have been in­stalled with reverse polarity. Even if none of these problems is evident, it’s a good idea to check the +5V and -5V supply rails with a DMM. They should both be within a few tens of millivolts of these figures. If so, you can fit the top of the case and screw it together – your L’il Snooper is now ready for business. Putting it to work Fig.6: check your PC board for defects by comparing it with this full size etching pattern before installing any of the parts. complete the wiring. Your L’il Snooper is now ready for the smoke test. Setting up The first step in setting up is to decide how many cameras you’re going to be using and set DIPSW1 accordingly. Only one of the “4”, “3” or “2” switches should be pushed to the ON position – the others (including the “1” switch) should all be left off. The Scan Rate pot should initially be turned fully clockwise. Now connect your plugpack or other source of 12V DC to the power socket, apply power and check LED5 (on the board, just behind the pot). It should be glowing steadily. The LEDs on the front panel should be glowing in sequence, like a small light chaser. If so, try turning the pot 78  Silicon Chip anticlock­ wise – this should slow things down and if it does, your L’il Snooper is probably working correctly. If LED5 isn’t glowing and/or all of the other LEDs are off, disconnect the power immediately and check for problems. If all of the LEDs are off, you may have a problem in the power supply. Look for a diode or an electrolytic capacitor that’s fitted the wrong way around. Check also that REG1 and REG2 haven’t been swapped and check the wiring polarity to the 12V DC connector plug. These are the most likely causes of a “no-go” situation, apart from a hairline crack or short circuit in the PC board pattern. If most of the LEDs glow (when it’s their turn, in the case of those on the front panel) but one or two don’t, odds are that you’ve fitted those particular LEDs the wrong way around. Putting L’il Snooper to work is easy. Just plug each cam­era’s video and audio outputs into the appropriate input sockets (starting with those for Camera 1) and connect L’il Snooper’s outputs to the AV inputs of your video monitor or TV receiver. When it’s powered up, you can adjust the scanning speed using the Scan Rate control and stop the scanning at any time by pressing and holding in the Pause button. It’s as simple as that. By the way, the picture on the monitor may roll for an instant as each camera is selected. That’s because the cameras won’t be locked together and the switching isn’t locked to any of them either. However, most modern monitors and TV sets lock very quickly, so this shouldn’t be a problem. You may have to find the best setting for the monitor’s vertical hold control, though. If you add extra cameras (up to a total of four) at any stage, you’ll have to open up L’il Snooper’s case again and adjust the DIP switch settings so it scans the right number of inputs. Tweaking the scan rate A final word: if you’re not happy with the scanning rate range, this is easy to change. All you need to do is substi­tute a different value for the 10µF tantalum capacitor connected between pin 1 of IC4a and ground. A larger value (say 22µF or 33µF) will slow the scanning rate range down, while a smaller value (say 6.8µF or SC 4.7µF) will speed it up.