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MFJ 432 Voice Keyer Interface

The MFJ 432 voice keyer requires an interface cable to be used with TR LOG. The control port is really for a simple push button accessory box, and ties directly into the voice keyer ISD1020A integrated circuit. This interface circuit uses common Radio Shack parts and will provide DC/RF isolation between the PC and keyer by using buffer logic and reed relays. Relays were used as the MFJ 432 only requires a simple switch closure to activate a message.

Interface schematic for MFJ 432 Voice Keyer

The buffer is the CD4050B non-inverting CMOS. The reed relays are Radio Shack 275-232, 5 volt. Do not use the 5 VDC provided by the keyer for this circuit. It will run on 4 to 5.5 volts provided by three AA batteries for DC isolation.

A handy project box is one of the A/B parallel port switch boxes used for selecting computer printers, etc. They are metal, have the DB25 ports already installed, and can be bought for less then $8. Other options that use the PC parallel buss are CW, PTT, paddle input and foot switch, all easily added to the project.

This simple interface has provided the isolation required for a six transmitter multi-multi with networked PCs. Extensive testing and multiple contests have produced zero failures.

73, --Rich KL7RA

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Last updated: 26 January 2006