The Simplest Circuit

Nowadays a lot of intercom units are  equipped with video cameras so that you can  see as well as hear who is at the door. Unfortunately, the camera lens is perfectly placed to serve as a sort of support point for people  during the conversation, with the result that  there’s hardly anything left see in the video  imagery.  One way to solve this problem is to install two cameras on the street side instead only  one, preferably some distance apart. If you  display the imagery from the two cameras  alternately, then at least half of the time you  will be able to see what is happening in front  of the door. Thanks to the video switch module described  here, which should be installed on the street  side not too far away from the two cameras,  you need only one monitor inside the house and you don’t need to install any additional video cables.

 

Along with a video switch, the circuit includes  a video amplifier that has been used with  good results in many other Elektor projects,  which allows the brightness and the contrast  to be adjusted separately. This amplifier is  included because the distance between the  street and the house may be rather large, so it is helpful to be able to compensate for cable attenuation in this manner.  The switch stage is built around the well  known 4060 IC, in which switches IC2a and  IC2d alternately pass one of the two signals to  the output. They are driven by switches IC2b and IC2c, which generate control signals that  are 180 degrees out of phase. The switching rate for the video signals is  determined by a clock signal from an ‘old  standby’ 555 IC, which causes the signals to  swap every 2 seconds with the specified com ponent values.

 

Naturally, this circuit can also used in many other situations, such as where two cameras are needed for surveillance but only one video cable is available.

This circuit is a video line driver specifically intended for use with a single-ended power supply. As a matter of fact, the synchronised outputs of a line driver for composite-video signals go negative with respect to ground. In order to be able to process these negative signals in a circuit powered from a single-ended supply, it is necessary to AC-couple the input of the opamp as well as level-shift the signal in the positive direction.

The input is terminated into a 75 Ω resistor (R1). From here, the signal passes through AC-coupling capacitor C2 and is applied to potential divider R2-R3, which provides the necessary DC-offset. The shift into the positive direction amounts to +1.7 V, with the values shown in the schematic. To avoid any misunderstandings we should add that this value is fairly critical. Deviating from the values shown can lead to distortion in the complementary input stage of the opamp that has been used here, and this of course, has to be avoided.

Because we provided the circuit with its own voltage regulator circuit (IC2), just about any mains adapter will suffice for the power supply. The current consumption is less than 20 mA. The construction of the line driver using the accompanying printed circuit board layout is no more than a simple, routine job.

Parts List :

Resistors:
R1,R7 = 75Ω
R2...R4 = 4kΩ7
R5,R6 = 1kΩ

Capacitors:
C1,C4,C5,C7C10,C12 =
100nF
C2 = 47µF 16V radial
C3,C11 = 10µF 6 V radial
C6 = 220µF 6 V radial
C8 = 1000 µF 6V radial
C9 = 100µF 16V radial

Semiconductors:
D1 = 1N4001
IC1 = OPA353UA
IC2 = 78L05

Miscellaneous:
PC1-PC6 = PCB solder pin
Case, e.g., Hammond type
1590A