The video switcher described here can display the output of two, three, or four cameras on a single monitor. The number of cameras is set by a DIP switch on the circuit board. That feature avoids blank displays if less than four cameras are used by sequencing through only the inputs that are connected to a camera. In the automatic mode, the cameras are switched at a rate that can be varied with a panel mounted control. The switching rate can be set from about once per second to about once every 20 seconds. In the manual mode, one camera output is displayed continuously. A momentary-toggle switch is then used to step through the various cameras. The heart of the video switcher is a Maxim MAX454.
That integrated circuit contains a
four-way video multiplexer and an amplifier
that operates as a low-impedance line
driver. The resulting video output is high
quality with very low phase distortion. The
video inputs are selected by applying a
binary number to the address inputs. The
binary number is also used to light a series
of LEDs that indicate whichh camera input
is currently selected. The circuit is powered
by a 9-volt AC wall-adapter transformer, two
diodes, and two voltage regulators.
The binary addressing circuit is built around
IC2, a CD4017 decade counter. That chip
produces one positive output at a time on
each of its ten outputs in sequence for every
clock pulse. The first four outputs at pins
3,2,4,and 7 are connected to transistors Q1-
Q4. Those transistors drive LED1-LED4
through current limiting resistor R15. The
outputs from IC2 (pins 2,4, and 7) are also
decoded into binary logic by diodes D1-D4.
The binary logic is sent to the address input
lines of IC1.
The number of cameras connected to the
video switcher is set with S1. Each switch in
S1 is connected to an output from IC2. If,
for example, there are only two cameras
connected to the video switcher, S1-a is
closed. That connects the third output to
IC2’s reset line. When IC2 advances to the
third count, that output passes through S1-a
to the reset, and IC2 resets to zero,
activating the first camera. The sequence...