Photocell Circuit

modulator circuit is shown in figure 47. In the

74. Photocell Circuit

(figs. 47,48, and 49)

simplified circuit, the 1,800-cycle carrier signal

from the fork oscillator unit is fed to points A and

Photocell tube V24 is the heart of the photocell

C of the resistance-capacitance bridge. Output

bridge modulator circuit, which is used only on

from the bridge is the voltage developed across

transmitting. Tube 1645 is used as the photocell

R50, between points D and B. This voltage is

in Facsimile Transceivers TT-1/TXC-1 through

the grid to cathode or input voltage of V10. A

TT-1C/TXC-1. Tube 5652 is used as the photo-

small capacitance (Ct) exists between the cathode

cell in Facsimile Transceivers TT-1D/TXC-1,

and active anode of V24, and this is effectively

TT-1E/TXC-1, and TT-1F/TXC-1. The photo-

balanced out by the neutralizing, capacitance

cell and its associated circuit (fig. 47,48, and 49)

(Cn), which is the capacitance existing between

cause amplitude modulation of the 1,800-cycle signal

the cathode and the second or neutralizing anode

supplied from the fork oscillator unit. Amplitude

within the photocell. When the bridge is per-

modulation follows the lights and shadows on

fectly balanced, there is no output of 1,800-cycle

that portion of the transmitted copy being

signal to be applied to V10; but when increased

scanned. After modulation of the 1,800-cycle

light shines on the photocell, it causes increased

signal is accomplished in the photocell bridge

electron emission from the cathode, producing a

modulator circuit, the modulated signal is fed to

resistive unbalance of the bridge and a greater

the grid (pin 6) of V10, the first signal amplifier.

1,800-cycle signal.

Tube V10 and the following signal amplifiers not

b. Optical System. The light that reaches the

only amplify the modulated signal, but use a

photocell can be determined at any instant by the

series of differentiating circuits (small coupling

shade (from white to black) of the small area of

capacitors and large grid resistors) to remove

transmitting copy being scanned. Light from the

undesirable low-frequency signal components

constant source of illumination (the exciter lamp

generated in the modulator circuit. Photocell

shown in the transmitting block diagram (fig. 31))

tube 5652 in Facsimile Transceivers TT-1D/

is concentrated onto a small portion of the copy

TXC-1, TT-1E/TXC-1, and TT-1F/TXC-1 per-

by the condenser lens system. The illuminated

mits the use of larger values of coupling capacitors

area is approximately the size of the filament in

than photocell tube 1645, which was used in

the exciter lamp. The objective lens focuses an

previous models. The circuit that uses tube 5652

image of the illuminated area of copy onto a

is inherently balanced for the modulating fre-

screen having a small hole or aperture that permits

quency, and therefore requires no discrimination

only the light from an area approximately one

against low frequencies in its amplifier strip as

ninety-sixth of an inch square to pass onto the

required in previous models.

photocell in back of the aperture. As the drum

a. Simplified Bridge Modulator Circuit. A

and copy move during transmission of a picture,

simplified equivalent of the photocell bridge

each scanned element of the picture reflects a

different amount of light back to the photocell.

The photocell and bridge modulator circuit change

each level of light into its equivalent electrical

signal.

c. Schematic Diagram:

(1) Facsimile

Transceivers TT-1/TXC-1

through TT-1C/TXC-1 (fig. 48). The

output of the bridge (the voltage across,

R60) is a varying dc signal that consists

of the keying or modulating frequency

which is superimposed on the 1,800-cycle

carrier frequency. This varying dc sig-

nal is always of such polarity that the

grid end of R50 is positive with respect to

the other end. This is caused by the in-

creased emission from the photocell when

increased light strikes it. The conven-

Figure 47. Simplified bridge modulator circuit.