Homemade video switcher

With the price of many surveillance cameras now falling, it is becoming more practical to install a surveillance system in your store, office or home. However, you may also have to consider equipping each surveillance camera with a monitor, or buying a dedicated monitor that can display multiple surveillance images on a single screen. All of the above methods will require a relatively large investment. Here is a video switcher that you can make yourself.

It uses a video switching ASIC, which can display the surveillance images of two, three or four lenses on one monitor in sequence. The number of lenses to be switched is set by the DIP switch on the circuit board. In automatic mode, the lens switching speed can be adjusted from 1 to 20 seconds by the knob on the panel. In manual mode, the surveillance lens images of one channel can be fixed on the monitor, and the surveillance lenses can be switched one by one by manually triggering the switch.

The circuit principle is shown in Figure 2. The core of the circuit is a video switching circuit MAX454 (see Figure 3). It has good quality output images and very low phase distortion. The circuit contains 4 video inputs (IN0~IN3) and a low input impedance line amplifier and driver, two address inputs (A0, A1), a video output and two power terminals. The monitoring camera is connected to the switcher video input terminal through J1-J4. The 75Ω resistor constitutes the input terminal resistor. The gain of the internal amplifier is set by the feedback network connected to the 13th pin of IC1. The feedback network consists of R5-R8 and C3. Its gain is set to 2 to compensate for the consumption on the terminal resistor R9 (75Ω). Finally, the gain at the output terminal J5 is 1.

Figure 3

IC2 (CD4017, decimal counting and distributor) forms a video switching circuit. When a clock pulse is input from pin 14, Q0~Q4 will output high level in turn (see Figure 3). D1~D4 and the three output terminals of IC2 (pins 2, 4 and 7) form a decimal to binary conversion circuit to provide address code for IC1. At the same time, T1~T4 are connected to pins 3, 2, 4 and 7 of IC2 respectively to drive LED1~LED4 as lens switching indication. R15 is the current limiting resistor of the light-emitting diode. The number of monitoring lenses is determined by DIP switch S1. For example: If only two monitoring lenses are connected to this switcher, S1-a should be turned on, and the 4th output terminal of IC2 and the reset terminal (pin 15) are short-circuited. When IC2 counts to the output terminal of Q4 (pin 10), this high level resets IC2 through S1-a, and the switcher returns to the first monitoring lens. In this way, the circuit will cycle between lens 1 and lens 2. Similarly, switching between 3 or 4 lenses can be achieved by connecting S1-b or S1-c.

IC3 (LMC555) constitutes a clock pulse generator, providing a counting pulse signal for IC2. Adjust R12 to output a frequency of 1 to 1/20Hz. S2 is a single-pole triple-throw switch. When it is in the automatic position, the counting pulse output by pin 3 of IC3 enters the clock pulse input terminal (pin 14) of IC2 through S2, so that the circuit automatically counts and switches in a cycle. When S2 is turned to the manual position (S2 is a switch with a spring reset), the power supply voltage gives IC2 a counting pulse through S2, realizing manual switching of each lens. When S2 is in the middle position, the counting input terminal of IC2 is set to a low level by R13, and the counter does not work.

The power supply part consists of T1, IC4, IC5, D5, D6 and C6~C9 to form a ±5V power supply.

Since the circuit is used to process high-frequency video signals, we should pay attention to the use of printed circuit boards when making them. If you design the circuit board yourself, please pay special attention to the need to use grounding copper foil around the signal terminals to avoid introducing noise and crosstalk. When installing components, it is recommended to solder resistors and diodes first, and then use the leads cut from the previous parts as two jumpers. After that, there are sockets for S1, IC2, and IC3, but IC1 should not use a socket. When installing J1~J5, it will be better if you use a slightly more powerful soldering iron. After that, there are Q1~Q4 and capacitors and LED1. Finally, solder IC1 directly on the circuit board and shorten the pin leads as much as possible to facilitate signal transmission.
In addition, there are integrated blocks with similar functions to MAX452, such as MAX453 (two-way) and MAX455 (eight-way). For more information, you can directly visit the MAXIM company website. Enter MAX454 in the search column to obtain all relevant information and apply for free samples for experiments.