Circuit Notebook 91 - Video Switch

Video Switch for Portable or Simple Shack Use

The previous edition of Circuit Notebook (No.90) described a video distribution amplifier and suggested its use in a portable ATV station.

Here is an even simpler but practical unit using a single Veroboard distribution amplifier. The unit, shown in Fig.1, allows a video monitor to be connected to a receiver output to view incoming signals, or to the camera (which is feeding the transmitter) to view the outgoing pictures, without having to change over cables or parallel any coax connections. As most stations use a camcorder, the call sign can be keyed into the camera for station identification.

The circuit is shown in Fig.2, the internal view in Fig.3 and the rear view in Fig.4. Video signals from the camera are taken through the CAM input to the changeover switch and to the input of the distribution amplifier. One output from the amplifier is taken to the transmitter TX connection and the other to the changeover switch. The switch selects RX or CAM and routes the signal to the MON, monitor output.

The unit is powered from 12V DC through a 2.1mm connector (centre +) and draws approximately 32mA. A DC-DC converter (Newport /Tresco 1205S) provides the +5V and -5V supplies for the video amplifier. The converter is mounted upside-down and fixed to the case using Superglue.


Hazard warning sign If a 12V plug-top power supply is to be used, it must be a regulated type. The output of a 12V 300/500mA unregulated type will probably rise to 18 - 20 volts when only a small current is being drawn and this could damage the DC-DC converter.


Veroboard Video Distribution Amplifier Circuit Notebook No 90 CQ-TV 215 p.13.

DC-DC Converter 1205S Data Sheet

Newport or Traco Search 1205S


Fig.1. Video Switch

Fig.2. Circuit Diagram

Fig.3. Internal wiring

Fig.4. Rear connections