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Yesterday’s Photodiodes

As we saw in a previous post, phototubes (1), which date from around 1890, were used in rapatronic cameras to detect photons from nuclear blasts. Here is some background from RCA’s 1940 Phototubes Form PT-20R1:

“A phototube consists of two electrodes in an evacuated glass envelope. The cathode emits electrons when its sensitized surface is exposed to light. These electrons are drawn to the anode because it’s operated at a positive potential. The number of electrons emitted by the cathode depends on the wavelength and the amount of radiant energy falling on it. The phototube thus provides an electric current whose magnitude can be controlled by light.” “There are gas types and high-vacuum types of phototubes. A small amount of inert gas in the phototube increases the sensitivity since the presence of gas increases the amount of current passed for a given amount of cathode illumination.”

The phototube’s cathode is photosensitized and it is specified by a spectral sensitivity characteristic. For example, the “S1” photosurface has a peak sensitivity around 750nm and an “S2” photosurface peaks around 800nm. S1 photocathodes can be made to work within the range of wavelengths from about 500nm-900nm and S2 photocathodes from about 500nm-1,000nm. (For comparison, the human eye responds to photons in the range from about 400nm-700nm, peaking at around 550nm in daylight.) S3-S11 photosurfaces have their own characteristics, which makes them suitable for applications in colorimetry and measurement circuits.

A common example is a gas-type phototube, the 927 (with S1 photosurface). The 927 uses a 3-pin socket (e.g. Amphenol 78-PCG3). The sockets are somewhat rare and in a pinch, the 927 pins accept solder nicely.

Before a simple demonstration, it’s interesting to recall an important application of phototubes and an inventor. Theodore Case was born in Auburn NY and studied light-sensitive vacuum tubes in the 1910’s. (The Cayuga Museum and Case Research Lab are still open to visitors). Case became interested in telephonic circuits and began working on a sound-on-film process in the early 1920’s. Case’s subsequent relationship with de Forest gave rise to significant inventions in “optical sound” and “talkies” in addition to plenty of business intrigue (2-3).

“Talking pictures” were made by encoding sound-on-film as a modulation of optical density in a region of the film adjacent to the “motion picture.” In this manner, the sound could be synchronized to the frame. Figure 1 below gives a summary of the system (1). Devices were invented to record sound as variation in the developed film’s optical density and suitable playback systems were created.

Fig. 1 (A) Position of sound track on motion-picture film. (B) Variable-area sound track. (C) Variable-density sound track. (D) Block diagram of sound-reproduction system.

At playback, a light source illuminates the sound track on the movie film. The photons transmitted through the film are directed to the phototube’s sensitized cathode. These photons liberate photoelectrons, which become anode current that is proportional to sound. Using a suitable audio amplifier and loudspeaker, the modulated optical density is decoded and reproduced as sound for all to enjoy.

While designing an Antique Wireless Association Radio Fab Lab youth workshop on RF communications, I thought that the children might be interested in communicating information (e.g. music) between two distant points using a modulated light source propagating in free space. The children loved it (!) and they built their own transmitting and receiving circuits using modulated LEDs and solar cells. A couple of us wanted to see what could be done with modulated light bulb filaments and phototubes.

Fig.2 A photon transmitter, which uses a filament modulated by an audio signal.

Figure 2 is a picture of the first transmitter, which is based on the schematic shown in Fig. 3 (below). An audio signal was fed into the low-voltage side of a 6.3v/6A filament transformer (Stancor P-4089). The filament transformer’s primary is placed in series with a DC power supply (Triad N-68X power transformer, <325ma through the primary, and a filtered full wave bridge) and light bulb. I used a 60w incandescent bulb and a 50w spotlight (as shown in Fig. 2). The DC voltage was limited to about 60v, to avoid abusing the P-4089. The input audio signal was generated using a sine-wave signal generator or MP3 music player to drive an 8w/8ohm audio amplifier, whose output was connected to the secondary of the P-4089. In this fashion, the light bulb photons dance to the music.

Fig. 3. Schematic of photon transmitter shown in Fig. 2.

The light receiver was designed to eliminate the need for high voltage and operate at only 12vDC and is shown in Fig. 4 and its schematic is shown in Fig. 5. It’s based on a similar phototube circuit that was used in 16mm film projectors to read and demodulate the sound track. This version uses a 927 phototube, a 12BA6 preamp, and an audio amp to power loudspeakers so we can hear the encoded sound. Voltage gain is about 8 and is flat from 80Hz-25kHz. Half voltage gain extends from 20Hz-100kHz. Output is tens of millivolts to hundreds of millivolts, depending on the level of incident illumination.

Fig. 4 927 phototube (background) and its 12BA6 amplifier (foreground) all operating at 12vDC including the 12BA6 heater.
Fig. 5 Schematic of receiver shown in Fig. 4.

The initial tests of the photon transmitter shown in Fig. 2 were to investigate the frequency response of the light bulbs. It was found that the 60w incandescent bulb and the 50w spotlight performed from <10Hz to 10kHz! To old ears, that is nearly HiFi using the modulation of big and heavy filaments. I also tried a small 4w incandescent bulb like that found in small flashlights and nightlights and it worked well too. After these tests, I used the transmitter/receiver pair as a line-of-sight free-space broadcaster to pipe music around the lab with great results.

Fig. 6 A photon transmitter using a modulated LED.

To inch out of the 1930’s into the space age, I built the LED photon transmitter shown in Fig. 6. I used a bright ~30ma white LED (V=5vDC, R=220ohms, C=1uf) driven by the MP3 music player. Wiggling the light from an LED is much less fun, but it is closer to HiFi and it works well when coupled with the low-voltage 927-based receiver shown in Fig. 4. The phototube experiments were well worth the time.

Phototubes remained relevant into the 1950’s when they were eventually replaced by solid-state photodetectors. Photomultiplier tubes are still in use today, although their remaining days are numbered.

References:

1. A. Schure, “Phototubes,” Library of Congress 59-8632, 1959.

2. S. Przybylek, “Breaking the Silence on Film: The History of the Case Research Lab,” ISBN 0-9673366-1-9, 1999.

3. A. K. Colella and L. P. Colella, “Now We’re Talking: The Story of Theodore W. Case and Sound-on-Film,” ISBN 1-4107-9515-2, 2003.