Solid State Tuning Indicator

Many radios in the 1930's and 1940's used a "tuning eye" to assist the user in properly tuning in a radio station. Common tubes included the 6E5 and 6G5/6U5. To me it was more of a gimmick because many radios didn't have them and tuning can successfully be done simply by listening. The tuning eye was visually appealing though with its' green luminescent glow. I won't go into the theory but essentially the automatic volume control (AVC) circuit of the radio controlling an electron field/beam that strikes a phosphorescent target and causing it to glow. With a weak signal, AVC voltage was less negative, causing the eye to "open up" such that approximately one quarter pie shaped area of the target was not illuminated. On a strong station, the pie shaped non illuminated area decreased and sometimes disappeared completely. The following is a picture of a working tuning eye tube in my Zenith chairside:

The problem though with this visual delight was longevity of the tube, or lack thereof. I've heard they had a life of 500, maybe 1000 hours. After that, the target pretty well lost all of its' illumination/luminescence. Indeed, many I came across were dim. Supply of NOS tubes is limited, causing prices to increase. That set up the need for something that would last longer, much longer, while retaining some of the "visual flavor" of the original eye tube. Back in 2008 I started searching the net and came across a schematic by Y. Kondo. His circuit used 10 LED's driven by a TA7612AP chip. The circuit also contained a TL071 op amp that used the AVC voltage to provide a voltage to the LED driver. The circuit also contained a power supply to provide +/- voltages for the op amp, supplied by the filament voltage. The whole thing was intended to be a "plug in". While functional, the fun part is always in the design and construction. I determined 10 LED's was not enough and didn't mimick the display properly. I decided to use 20 small rectangular LED's that would be driven in pairs and mounted in a circle resembling the dimensions of an eye tube target. The following is a picture of the LED layout used in my prototype design:

Satisfied with the display, I moved on to the circuit board. I built the prototype on a 1"x2" project board as this is the dimension that would end up with something that was very close to an original eye tube. I switched the LED driver to an LM3914 as they were more available. I also used a TL071 but had all kinds of fun supplying the positive and negative voltage it needed. I just couldn't seem to get Hondo's power supply to work. I shelved the project.

Who knows why but my interest was revived in 2014. I hauled the prototype back out and figured out the power supply issues. I designed a PCB for the circuit and built another unit. This time...success! Here are some pictures of the second unit; construction and in operation in my "frankenradio".

As can be seen with the unit in service, it mimicks a strong station nicely but differs in that the eye opens up more than the original 90 degrees on no signal. I couldn't figure out an easy fix for this so decided to live with it. Also, I still need to fabricate some form of light diffuser in front of the LED's to soften the glow and make it look more like the original. These LED's come in red, yellow and green.