Initially I just wanted to replace a few burnt-out lightbulbs in the expression indicator. However, I found that the act of removing those bulbs from the expression indicator caused another bulb to burn out as well. And when I removed that bulb, another one burnt out! I suppose these bulbs were about thirty years old, so of course they were delicate, and I just couldn't handle them gently enough. I then discovered that I couldn't find replacement bulbs. I tried LEDs instead. However, they weren't a good match for the remaining bulbs in terms of brightness and color. LEDs also begin emitting light at a much lower voltage than the bulbs, and so would light up out-of-order when depressing the expression pedal.
The solution, I decided, was to replace all of the bulbs with LEDs. While I was at it, I would replace the driver circuitry as well. Using a microcontroller to control the lights would let me replicate the 'softness' with which the light bulbs turned on. Some advantages of an entirely new circuit are that it requires much less space and power than the original circuit. I had in the back of my mind that I might eventually digitize the organ. That is, I would replace some or all of the control circuitry with (much smaller) modern digital electronics that also provided MIDI signals as output. In the now-free space, I would install a PC to take the MIDI signals and produce I really nice pipe organ simulation. I'd leave original oscillators in place, so the organ could also continue to function (and sound) like it always did. Anyway, the space saved by replacing the expression indicator driver circuits would give me room to work with for installing other circuits.
The schematic for the circuit I came up with is shown at right. In building this I tried, as much as possible, to use components that I had on-hand. Hence the older microcontroller (an Atmel AT90S1200, which is no longer being manufactured) and the LM723's for voltage regulators. This microcontroller does not have built-in ADC, so I built a voltage sensing circuit that was inspired by a capacitance meter from the Electronic Lives Mfg website. (And, by the way, I built my own capacitance meter to that design and it works quite well!) I used amber LEDs in an attempt to match the yellowish color of small light bulbs, but this did not work out as I'd hoped -- when installed in the organ the LEDs appear green! I used resistors to limit the current through each LED to less than 15mA, which allowed me to drive them directly from the microcontroller. I used two LM723's so that the voltage sensing circuit and microcontroller can have their own stable voltage supply.
The AVR assembly code that runs the new indicators is, of course, somewhat buggy. The code senses the input voltage by measuring how long it takes the voltage across C4 to reach the input voltage, once the capacitor is allowed to start charging. However, if the input voltage is negative this never occurs (since the voltage across C4 higher to start with). There should be a check for that. Secondly, there's at least one other bug somewhere that messes up the timing in the LED 'off' period, when one LED is being pulsed to show less than full brightness. I haven't found that bug but it doesn't seem to have a significant impact on how the display looks, so I'm not too worried about it at present.
If I had more time, I would have made a second version of these expression indicators. Because of time constraints, I built the current version on perfboards and used the original expression indicator panels (that used to hold the light bulbs) to hold the LEDs, as can be seen at right. The new version would be an etched circuit board that fits in the place of the original indicator panels, so there would be no need for a second circuit board anywhere. The microcontroller would then have to be surface-mount, so probably something like the ATTiny261. That has a built-in ADC, which would also simplify the circuit. I might also have tried to program an automatic scaling function into the microcontroller to eliminate the need for external adjustment pots. And I would have switched to white LEDs.