1050 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER, 1953 



MODULATING 

 DISK 



PHOTO- 

 TRANSISTOR 



LAMP 



Fig. 7 — Optical section of channel circuit. 



than dc amplifiers and also since the difference between the light and 

 dark currents of the phototransistor is the important factor rather than 

 the absolute value of either. The modulated light is collimated by a lens 

 to minimize the loss as the beam passes through the holes in the card. 

 Unless interrupted by a dropped card, this light beam will bass through 

 all of the cards and fall on the lens which focuses the light on the sen- 

 sitive area of the phototransistor. The light intensity at the lens of the 

 phototransistor is 34-foot candles minimum. This is equivalent to about 

 12 millilumens at the phototransistor, a figure relatively small when 

 compared to the light intensity that is required by conventional photo- 

 electric cells. 



The electrical part of the channel circuits starts with the photo- 

 transistor and is shown in Fig. 8. The light acts as the emitter of the 

 phototransistor. The collector is of the conventional type for point 

 contact transistors. As is normal in grounded base transistor circuits, 

 the collector of the phototransistor is biased in the high impedance di- 

 rection. A variation in the light intensity causes a variation in the col- 

 lector impedance of the phototransistor. The type used has an impedance 

 of about 10,000 ohms when dark which is reduced to approximately 

 3,000 ohms when illuminated. The output of an illuminated photo- 

 transistor when coupled to the amplifier ranges from 1.3 to 12 volts 

 positive peak at 400 cycles depending upon the age and condition of the 

 transistor. 



Since the discrimination by the channel circuit between a clear or 

 blocked light path depends upon the presence or absence of an ac output 

 from the phototransistor, noise of sufficient magnitude, if present when 

 the channel is dark, would cause a false indication. To guard against 

 such false indications each phototransistor is checked during manu- 

 facture for dark noise. During a five minute interval the dark voltage 

 must not exceed 75 millivolts. 



