RF Sensitive On-Air Light

The original Article can be found in the August 2004 edition of QST on page 56, by Keith Austermiller, KB9STR

On-Air light in place at W1AIR
Here is a picture of the on-air light fixture on top of the shelf in the shack.  It is hard to tell but it was illuminated at the time of this photo.

Underneath the TM-251a
(above) Here the outboard detector circuitry and antenna sit underneath the 2m radio.  The antenna can be seen at the left of the cigar box.  I've temporarily put the circuit board into the box until I can get a better enclosure.  (left) Just a larger picture of the box itself without the radio on top.

An inside look at the RF driven on-air light control box.
(above) It was a bit hard to get a decent picture of the internals of the control box.  You can see the piece of perf board with the major components mounted to it.  The clip lead is going from one leg of C1 to the antenna, which came out of a portable phone base.  Coming through the back of the box are the wires going to the on-air light and the 12vdc supply. (right) just another interior shot of the control box... or control 'cigar box'.

Close-up of the on-air light fixture.

(above) Another tough shot because of the white interior of the fixture.  You can see the 12v auto lamp (on the left) that I'm using to light the fixture which has been tie-wrapped to the middle of the existing 120vac lamp harness.  This way I didn't have to chop up the fixture itself.


From start to finish the project took me six hours.  The hardest part of the project was seeing the small parts when soldering, especially the pins on the op amp chip and the 10k variable resistor.  (Glasses on.  Glasses off.  The curse of being in desperate need of bifocals).  I plan to get a decent project box to mount the detector circuit in at some point and I'll probably stick with the whip antenna as it seems to work well with it and won't require extra expense in getting and mounting another. 

The unit performs much as Keith, KB9STR described it in the article.  You can adjust the sensitivity by changing the 10k variable, originally it was so sensitive that simply touching the dc supply 'wall wart' caused the light to go on.   I've tested it using my HT on 2m, 220 & 440.  From a distance of about six feet I can cause the light to come on at 5 watts on both 2m & 440, any lower power doesn't trigger the circuit (which is probably good).  The circuit doesn't seem to be very sensitive to 220, I have to have the radio right next to the box in order to get the light to come on.  On HF it worked fine when I tested it on 20m, bringing on the light when you begin talking.  There is a 1000pF capacitor  used to keep the relay coil energized during SSB or CW use.   The original circuit also called for a pair of 12v auto lamps to be used, I used only one because two in parallel didn't light as brightly and one seems just right.