@Externet: Back in the day, neon lamps were ubiquitous and cheap. Just about anything that operated from a convenience outlet had a neon lamp, with a built-in current-limiting resistor, to indicate when power was on. NE-51 with a bayonet base was the popular choice for socketed replacement reasons, but sometimes little wire-leaded NE-2 lamps were used in consumer equipment. These, of course, were not intended to be replaced by the consumer.
Sometime in the early 1960s, IIRC, Popular Electronics magazine published a neon "light chaser" circuit. We happened to have NE-51 lamps in abundance as part of our flight-line bench stock where I served in the Air Force. I thought this strange because I couldn't think of anywhere on the B-52H bombers that they were used. All of the lamps on the equipment I serviced used 28 V incandescent filaments, with a series rheostat on the control panel to adjust their brightness while in flight. Perhaps some of the other A&E shops used the NE-51 in great abundance.
It didn't take long before just about every airman in our Armament and Electronics (A&E) maintenance shop was "borrowing" neon lamps, resistors, capacitors and rectifiers from bench stock supplies to make copies of the PE circuit. I don't know why the "powers that be" never seemed to notice the sudden surge in NE-51 usage, but apparently they didn't. However, this "maker" fad died a fairly quick death after just one season because everyone who wanted a neon light chaser soon had one. I never was interested in making chaser lights, neon or otherwise, and eventually devoted most of my off-duty efforts to assembling an amateur radio station.
Years later, while working at UDRI, an engineer buddy needed some way to know when his office phone was ringing when he was working in another laboratory some distance away. We had the AT&T Centrex system installed on campus, and most offices had multi-line telephones with call buttons that illuminated when an incoming call was ringing. Karl salvaged the rubber boot from a largish alligator clip after he discovered the large open end was an almost perfect fit around the square call buttons on his desk-set telephone. He fitted a CdS photo-cell inside the rubber boot (with its two wires extended with twisted-pair wiring) exiting the small hole normally occupied by a single test lead. Those two wires were then remotely connected in series with a bell transformer and a small doorbell, both installed in his lab. So, whenever the phone in his office rang, the resistance of the CdS cell dropped and the doorbell in the lab rang. Karl would then drop whatever he was doing and race back to his office to answer the telephone, which was invariably a call from his wife. Why he didn't just ask the University to parallel a circuit in the lab phone with his incoming line in the office phone I will never understand. Too complicated, maybe, or he didn't want to wait. Neither of us were what, at that time, were called
phone phreaks, although I probably knew a little more about the POTS (Plain Old Telephone Service) network than Karl.
A few years later I got assigned to a 70 GHz microwave propagation study project. This involved mostly COTS microwave equipment, including a reflex klystron that was used as a local super-heterodyne oscillator. My mission (if I chose to accept it) was to control the repeller plate voltage of the klystron, which affected its oscillation frequency, to allow demodulation and detection of a phase-modulated 70 GHz signal. Problem was, the repeller plate was biased at a high negative voltage with respect to ground and only needed a few dozen or so change in voltage to complete the frequency control feedback loop. IIRC, I solved this problem with a CdS cell, a dry-cell battery, and a voltage divider, all floating between the repeller plate and the negative bias supply. The CdS cell was illuminated on the low-voltage side with an LED driven by an op-amp that completed the negative feedback loop that maintained the local oscillator operating frequency. Since almost no power was consumed by the repeller plate circuit, the small voltage swing we created with the CdS cell and the high-resistance voltage divider did not tax the capacity of the dry-cell battery at all. So, it was left connected, more or less permanently, "on" 24/7/365. AFAIK the battery was never replaced during the life of this project, which only lasted a couple of years.
I still re-visit the CdS photo-cell from time-to-time, mainly because I think it is a way-cool component that is easy to interface with. However, for serious light intensity measurement applications, I always choose a reverse-biased diode photo-cell. If temperature stability is a potential problem (as it was when I used a photo-diode to control the light output of an incandescent Nernst glower, used as a calibrated and intensity-stabilized broadband infrared light source), I mount two diodes to a common heat sink (so their temperatures will be the same) and either "blind" one of them in a partially drilled hole or cover it with opaque tape to exclude ambient light. Then I reverse-bias one diode with a negative power supply (typically -15 VDC) and the other with a positive power supply (typically +15 VDC), connecting both diodes to the summing point of an operational amplifier with a feedback resistor selected to convert the photo-diode current to an appropriate output voltage. The temperature-dependent "dark" current of the active photo-diode is canceled by the "dark" current of the "blind" photo-diode, yielding an output voltage from the op-amp that is a linear function of the light intensity "seen" by the active photo-diode. This circuit works with just about any PN junction photo-diode over at least six decades of intensity variation... microvolts to volts. That rivals many photo-multiplier circuits, although PMTs can't be beat for low-light-level sensitivity, capable of counting individual photons with high quantum efficiency.
Point of all this: every electro-optical component has its place, even so-called "obsolete" devices. Get to know their capabilities and limitations before choosing an appropriate device for
your application. Just make sure it is really still available for purchase! I haven't seen vacuum photo-diodes for movie projector sound system use in more than fifty years... <sigh>
@(*steve*) would probably be able to restore this to its former glory, but he would need a 16mm projector with sound capability to complete the project. Heaven (not me) knows where you would get film for it. The last time I watched one was in grade school in the 1950s...
Sergeant York starring Gary Cooper, IIRC. VHS (and short-lived Betamax) VCR tapes came along shortly after that and motion picture projectors vanished almost overnight. It used to be quite a "perk" in high-school to be a motion picture projectionist at commercial theaters because that meant the school called on you whenever they needed to show a 16mm film. Digital distribution has pretty much eliminated the projectionist job: theaters just need someone to change out the xenon projection lamps now.
