How to set up a Dual Directional Detector as a band switch safety sensor
Let's say you change bands and forget to also change the band switch to the correct band segment; if you moved to a lower band, there is little risk of damaging anything...you just won't have much harmonic suppression. However, if moving to a higher band, all of the forward power will be reflected back to the RF deck, and that's where we can create damage. Ideally, we want the board to ignore normal harmonic power being reflected back from the filter; this could be as much as -10dbc (110w or so at 1100w out). We also want it to be sensitive enough to recognize when those reflected power levels are much higher than that.
I tried various ways to set the board up, and found the method shown below to be the most dependable. Since we are already using the negative input latch on the control board to sense high load vswr using the other detector board, we can utilize the remaining positive input latch. That way, one input will not be affected by the other; each can independently trigger a lockout...the first one for a potentially damaging band switch position, and the second one in case of a high load vswr.
Note: Tie together the FWD and REV pads. R8 and R9 replace L1 and L2; also, use the J3 pad on the control board that is nearest the center of the board, not the one closest to the edge.
The lockout function should be tested prior to transmitting. This can be done by temporarily connecting about 1v (positive) to the positive trigger port on the control board. An easy way to do this is to use a standard 1.5v flashlight battery; negative to ground, and just touch the positive to the trigger port. This should produce an instant lockout.