Build A Video Switcher - Glolab

1 Build AVideo SwitcherReprinted with permission fromElectronics Now MagazineSeptember 1997 issue Copyright Gernsback Publications, Inc.,1997 Build AVIDEOSWITCHERW atch several cameras on one monitor with this simple deviceWith the cost of security cameras goingdown, adding a surveillance system for yourstore, office or home is becoming morepractical all the time. However, you mightbe dismayed at the thought of having to buya monitor for every camera that s a single monitor to a singlecamera also runs the risk of burning thecamera s image into the phosphor screen ofthe you prefer a single monitor instead of the NASA-Mission Control look, you could buya special monitor that has a Video switcherbuilt in.

2 That type of monitor canautomatically switch between severalcamera inputs in sequence. With that typeof arrangement, you d have to watch onlyone screen instead of having to scan a wallof CRTs. Switching between severalcameras would also prevent image burn-inon the monitor. Those types of monitors,unfortunately, are also very expensive,offsetting the cost savings of even thecheapest surveillance switchers are also available, but thecost of a Switcher and a monitor could be asexpensive as a monitor/switchercombination unit. A viable alternative for avideo Switcher is to Build your own. Thanksto some recently introduced ICs, the costand effort of designing and building such aunit has become both quite affordable Video Switcher described here candisplay the output of two, three, or fourcameras on a single monitor.

3 The number ofcameras is set by a DIP switch on the circuitboard. That feature avoids blank displays ifless than four cameras are used bysequencing through only the inputs that areconnected to a camera. In the automaticmode, the cameras are switched at a ratethat can be varied with a panel mountedcontrol. The switching rate can be set fromabout once per second to about once every20 seconds. In the manual mode, onecamera output is displayed continuously. Amomentary-toggle switch is then used tostep through the various it worksThe heart of the Video Switcher is a MaximMAX454. That integrated circuit contains afour-way Video multiplexer and an amplifierthat operates as a low-impedance linedriver.

4 The resulting Video output is highquality with very low phase distortion. Thevideo inputs are selected by applying abinary number to the address inputs. Thebinary number is also used to light a seriesof LEDs that indicate whichh camera inputis currently selected. The circuit is poweredby a 9-volt AC wall-adapter transformer, twodiodes, and two voltage descriptionFigure 1 is a schematic diagram of thevideo Switcher . Multiplexer IC1 has fourvideo inputs, two address inputs, one videooutput, one external amplifier input, and andthree power terminals. The Video camerasFRANK MONTEGARI connect to the Video inputs through inputs are terminated with 75 ohmresistors R1-R4.

5 The gain of the internalvideo amplifier is set by a feedback networkconnected to pin 13 of IC1. That feedbacknetwork consists of R5-R8 and C3. The gainis set to 2 in order to compensate for anyloss through the 75 ohm terminator resistor,R9. The resulting net gain is 1 at output binary addressing circuit is built aroundIC2, a CD4017 decade counter. That chipproduces one positive output at a time oneach of its ten outputs in sequence for everyclock pulse. The first four outputs at pins3,2,4,and 7 are connected to transistors Q1-Q4. Those transistors drive LED1-LED4through current limiting resistor R15. Theoutputs from IC2 (pins 2,4, and 7) are alsodecoded into binary logic by diodes binary logic is sent to the address inputlines of number of cameras connected to thevideo Switcher is set with S1.

6 Each switch inS1 is connected to an output from IC2. If,for example, there are only two camerasconnected to the Video Switcher , S1-a isclosed. That connects the third output toIC2 s reset line. When IC2 advances to thethird count, that output passes through S1-ato the reset, and IC2 resets to zero,activating the first camera. The sequencewould be camera 1, camera 2, then back tocamera 1. Closing S1-b or S1-c instead ofS1-a will let the Video Switcher cycle throughthree or four cameras, pulses for the counter are generatedby IC3, an lmc555 CMOS timer . The pulserate and pulse width is controlled by C4,R10, R11 and potentiometer R12. Byadjusting R12, the output frequency of IC3can be controlled between 1 Hz and 1/20Hz.

7 The clock pulses from IC3 areconnected to IC2 through S2, a threeposition toggle switch. Switching S2 to theauto position lets the pulses from IC3 selectthe next camera at a rate set by R12. WhenS2 is in its center-off position, no switchingtakes place, and whatever camera input isselected is passed through to the select position on S2 is a momentarycontact. That position raises the clock inputof IC2 to 5 volts, which increments thebinary count and selects the next S2 is released, it springs back to itscenter-off position. The clock input of IC2 isthen held at a low-logic level by MAX454 requires 5 volts while theother ICs require only +5 volts.

8 Power issupplied by AC adapter T1, rectifier diodesD5 and D6, regulators IC4 and IC5, andfilter capacitors of the high frequency Video signalsinvolved, the Video Switcher should be builton a printed circuit board. The circuit issimple enough to fit onto a single-sidedboard with only two jumpers needed. A foilpattern is included for etching and drillingyour own board. Alternatively, an etchedboard can be purchased from the sourcegiven in the parts list. A feature of thatboard design is ground traces that runbetween all of the Video signal traces inorder to keep induced noise and crosstalkbetween the signals to a you etch a board from the foilpattern or purchase one from the source inthe parts list, use the parts-placementdiagram in fig.

9 2 for component is easiest to install and solder the resistorsand diodes first. Once those componentsare in place, scrap component leads can beused for the two jumper wires. Next, installS1 and sockets for IC2 and IC3. Do not usea socket for IC1, the MAX454 installing J1-J5, hold the connectorstight against the board while soldering thecenter pin. The assembly can then beplaced on a heat-resistant surface and theground pins soldered. Because of their sizeand mass, a larger soldering iron might beneeded to solder J1-J5. Otherwise the boardmight be damaged if heat is applied the connectors are soldered in place,Q1-Q4, IC4, IC5, and all the capacitors canbe installed. The LEDs should be installednext, leaving their leads long so that theycan be bent to reach through the front panelof the enclosure.

10 Double-check theorientation of the polarized components, sothat they are not installed backwards byaccident. Once a component is soldered inplace, removing it becomes much two 3-inch long wires onto the twoterminals of R12 that are clockwise whenviewing the potentiometer from the those wires to the holes for R12 onthe board. Three additional 3-inch longwires are soldered onto the terminals of center terminal connects to the holenear C5 and R13. The momentary-contactterminal connects to the hole near R14. Theremaining terminal connects to the holenear IC3 and IC1 directly onto the circuit will result in the shortest possible leadlength for the Video signals.