I have KD9SV 160 m preamp that I would like to use on 160 m only and have it turned off for all other bands. This unit does not have any computer control – just the old style rocker switch to turn it on/off. So, I decided to keep this manual switch in the on position all them time and install USB relay to control the 12 V power to the unit. This cheap USB relay switch seems to work well (model DSD TECH SH-UR01A):

It sells for $15 on Amazon. It appears as a RS-232 device on the computer (Linux system in my case), which requires baud rate of 9600, 8 data bits and no parity. To turn the relay on, use:

AT+CH1=1

and to turn it off use:

AT+CH1=0

Note that newline or carriage return should not be sent. I use this in my antenna control program (ant.c) where I now have a button to toggle the preamp state.