Latching Wave Guide Relay Driver

de N6CA August 09, 2002 updated October 30, 2003

-No heat sinking required

-coil drive pulse width set at about 100 ms

-most 28 v relays will drive ok at 24 v

-33v zeners eliminate positive (>33V) spikes and negative spikes from coils

-use a shielded box with good feed-thru capacitors.....always.

-resistors 1/4 w

The CD4093 and its input shaping circuit eliminate possible contact bounce and the potential for multiple trigger problems from relays etc that generate the T-R command. Latching relays usually require only a pulse to actuate them. 100 ms pulse width seems to be adequate to drive most relays. The dual one shot CD4098 provides the transmit pulse on the leading edge of the TR command and the second half provides the trailing edge triggered pulse to drive the relay back to receive. To assure both coils are not simultaneously driven the Q bar output of the Rx one shot drives the reset pin of the Tx section. This feedback/interlock has another advantage. Upon turn-on or re-turn-on of primary power the 4093 outputs a short pulse (glitch). The positive edge should initiate a transmit pulse but the falling edge triggers the Rx one-shot which makes its Q bar low. This terminates/inhibits the transmit pulse. The result is a full duration reset pulse to the Rx relay coil regardsless of the last used state. That means you can leave the latching relay in transmit, remove power and the driver will reset the relay to receive when power is reapplied. I planned it that way....not, We got lucky.....a freebie. The disadvantage (not really) of the feedback is for very short TR commands the transmit pulse will be short as the receive pulse will terminate it. It's no problem as you can't say very much in a few milliseconds anyway.

Hall effect device waveguide relay position indicator

Org/red & green LEDs used for similar output. Very bright 20 mcd for daytime and dimmed for night. Output of Hall devices can be used for interlock on TX if necessary or desired.

back to N6CA homepage