A little over a year ago, I had a project that required driving an RF single-pole
double-throw (SPDT) coaxial switch either by transistor-transistor-logic (TTL)
pulses sent by an external source or by a local pushbutton. To my surprise,
I couldn't find a suitable circuit.
Many circuits used a "floating" SPST switch for control, so a TTL signal referenced
to common ground couldn't be used. Also, the known J-and D flip-flops first
needed to use an anti-glitch R-S circuit, making the design too complex.
So, I tried a simple circuit using a 7402 quadruple NOR gate (see
the figure). It worked as expected. The only required addition was a 10-kΩ
resistor connected in parallel with the marked 1-kΩ resistor. This asymmetry
contributed to reliable operation. The switching speed is about 1 Hz. Designers
can use smaller capacitors for faster operation.
This circuit took care of all of the project's requirements. We made several
units, and they've operated without any problem for more than a year. The key
was to use the same command input, the pushbutton switch, or an external TTL
command sent through a long cable. The status LEDs send back the resultant setting.
They also send the TTL command from a remote location and to the person sitting
next to the unit.
The 1N4007 diodes protect the PVG 612 power optocouplers from kickback voltage
generated across the RF SPDT switch coils when the current is interrupted.
