Gort Modulation Indicator

Gort - Klaatu barada nikto!

For modulation indication I experimented with ultra-bright flat top LEDs into a 'side view' fiber and Gort is the result.

There are two, parallel mounted, 6 inch long (dictated by the fibers purchased in 1 foot lengths) 6mm diameter fibers, one for the yellow modulation indicator (top) and one for the red cutoff indicator (bottom), both with a matching LED super glued onto each end shooting straight into the fiber. The ends were then heat shrinked together to cover the LED and make the two fibers into one unit. 'Side view' length came to 5.75 inches.

The photograph's amber tint from yellow LEDs is caused by the maroon back panel in dark lighting conditions, and exaggerated by the camera, but shifts more to yellow with normal ambient lighting.

In normal operation audio modulates primarily the yellow bar intensity. Over-modulation causes the red bar to flash on so the two act as a sort of 'optical' PPM.

As indicated by the title, the horizontal 'glow tube' reminds me of Gort from The Day the Earth Stood Still and, so, the name.

Modulation Indicator Electronics

As Built Schematic and BOM

The modulation indicator consists of yellow LEDs that are driven by the positive modulation level and, so, vary in intensity with the audio signal for mainly a visual effect. Positive mod is sensed by detecting average idle voltage (C3) and then measuring the amplitude above that point.

The carrier cutoff detector senses when modulator voltage approaches 80% modulation (~1.1V).

Both positive modulation and cutoff indications are peak detect and decay, simulating a PPM effect although the time constants are different because the circuit is not driving a meter and the "standard" hold time is too long for a pleasing visual indication by LEDs. In a sense, the human eye's sensitivity to light transients performs part of the 'peak hold'.

U1A does triple duty as a modulator cathode buffer and driving both the idle and modulation peak detectors, which precludes using the traditional peak detector feedback from the D9 cathode. D8, then, adds the same diode drop to the idle detector. Since subtracting idle from the modulation signal causes a divide by 2 the U1D LED drive opamp multiplies it back up by 2.

U1B is essentially a threshold detector with D7 biased to around .55V which, since the cathode voltage is divided by 2, sets the cutoff detection around 1.1V. Since U1B will go to the rail on cutoff detection the output is divided by 2 so U1C can regulate the LED current. R17 provides a small amount of feedback so the detector is not quite so aggressive right at the threshold, giving a quick ramp up over a small detection range.

The power supply dictated a number of circuit decisions due to the wide variation in heater supply voltage . In particular, the transformers in these UHF converters have rather poor regulation, since they expected a constant load, so heater voltage rises significantly should a heater fail or the tubes be removed. That, plus high line voltage condition, made a tripler unattractive since it would exacerbate the high voltage seen and increase power consumption, not to mention increasing the cost. So, with a doubler, we have the other end of the scale, full load and low line voltage, dictating the modest 10V power rail, dividing the sensed cathode voltage in half, and running the LEDs off the unregulated supply to obtain enough headroom for the double LED forward voltage. That also reduces the heat load on the series pass regulator so no heatsinking is required.

As Built Gort Modulation Indicator

Gort Power Supply Board

Gort Modulator Circuit Board

The Gort Modulation Indicator was split into two circuit boards because that fit readily available Radio Shack prototyping boards and allowed greater flexibility in placement on the back of the UHF converter dial plate.

Note that the selection of which opamp does what was chosen for a 'wrap around the ends' component connection, which suits the prototype board layout. The yellow modulation section is the left side and the red clip detector is on the right.

There is only one wire jumper on the modulator PCB from R2 and D7, at bottom center, to ground. In other places component leads are used to jumper, such as another ground connection at top center and the pin 1-2 connection. The same was done with R1, at bottom left, to get a 'hole' for the external modulator cathode connection. Similarly, the collector lead of Q1 jumpers to the 'two hole' pad for an external connection point to the red LEDs. Q1's base lead jumpers directly to U1C pin 8, and so on.

The Gort power supply was laid out so component leads could serve as the 'PCB traces' and connection points are component leads looped over the top.

The BOM lists currently available Mouser parts but I used existing stock and, from a purely economic standpoint, two resistors (R5 and R3) were constructed from a series combination of in hand values, as well as a 27nF substituted for C5, rather than pay shipping on just a couple of parts.

The pictures are from testing the completed Gort assemblies on the transmitter breadboard.

The plan was to JB Weld #4 flat heads onto the rear of the dial plate for mounting studs so there are no screw heads on the front side. The circuit boards were used as templates by mounting the screws to them, applying epoxy to the heads, and then 'sticking' the whole assembly in the desired location. Once the epoxy sets the boards are removable for final wiring and can then be reattached. It couldn't have worked better.

Threaded spacers could also be used but my local Sears Hardware wanted more than a buck each for them, which would have made the mounting as expensive as the electronics.


Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player

Gort Modulation Indicator in Action

Audio is from the camera mic picking up a nearby AM radio receiving the transmitter signal but it doesn't fully capture the depth of LED modulation nor crispness of response due to frame rate and lighting limitations. However, light meter averaging exaggerates the LED intensity.

Red bar picks up at around 80% modulation.


As an amusing 'extra', the Gort Modulation Indicator also serves as a power up sequence indicator. Since at initial power up the tubes are not conducting the red clip LEDs come full on. Then, as the tubes begin to conduct, the red LEDs cutoff and the yellow LEDs come full on, then decay down to 'normal' as the 'idle' detector settles.

It will also serve as a 'dead 7AU7' detector since a lack of current kicks on the red LEDs, alone, with no yellow modulation, similar to initial power up. I.E. If the power up sequence hangs at full red then the 7AU7, or associated circuitry, is defective.


(c) 2004-2011 All Rights Reserved