Patent GB-794258-A (GB)
Title: Improvements in or relating to detectors for physical phenomena
IPC: H03D1/18
Filed: 1954-08-23
Abstract: 794,258. Electric composition control. GENERAL ELECTRIC CO., Ltd., BRIDGES, D. H., and JONES, D. D. Nov. 23, 1955 [Aug. 23, 1954], No. 24499/54. Class 40 (1). [Also in Group XL (b)] A detector for a physical phenomenon comprises an electric circuit element adapted to vary in resistance and/or to produce a variable unidirectional voltage in accordance with variations of a characteristic of the phenomenon the element being connected in a transistor relaxation oscillator circuit in such a manner that the nature of the output of the circuit is dependent upon the value of the resistance and/or voltage. As shown in Fig. 1, a point contact transistor 2 is connected in a negative resistance relaxation oscillator circuit comprising a resistor 3 connected between the base of the transistor and earth and a condenser 6 connected between the emitter of the transistor and earth. The collector of the transistor is connected through the primary of a transformer 4 to the negative terminal of a battery 5. A photo-electric cell 1 of the selenium rectifier type is connected to the emitter of the transistor and also to a variable tapping of a potentiometer 7 which is connected across the battery and the tapping is adjusted so that normally the transistor is biased so that it does not oscillate. When the photo-electric cell is illuminated the cell produces a voltage and its resistance decreases so that the operating point of the transistor is moved to the negative resistance portion of its characteristic and hence causes it to go into oscillation. An amplifying transistor 8 is connected to the secondary of the transformer 4 and its output is applied to the operating winding 9 of a relay which is adapted to close an energizing circuit for an aural or visual indicator when the transistor 2 is in oscillation. In a modification the photo-electric cell 1 may be replaced by an ordinary resistor and the resistor 3 in the base circuit of the transistor 2 is replaced by a thermistor so that the detector becomes sensitive to heat variations. The photo-electric cell can also be replaced by a body of semi-conductor material having a high Hall coefficient or with a conventional variable resistance arranged to be varied automatically in accordance with variations of a characteristic of the phenomena. When a photo-electric cell is used the detector can be arranged to count a series of objects moving past a predetermined point by using the reflection of light from a fixed source which is reflected by the moving objects. Each object will then produce a pulse and the pulses can be counted by a suitable circuit in the output of the detector. In the arrangement shown in Fig., 3 a free-running multivibrator is used which includes a pair of PNP junction transistors 12, 13 which are energized by a constant voltage source 14 across which a potentiometer 11 is connected. The potentiometer is mounted in a sealed pressure-sensitive capsule (not shown) which can be mounted on an object such as a trawl whose depth is to be measured, the deformations of the capsule from variations in pressure causing a variation in the tapping point of the potentiometer. The tapping point of the potentiometer is connected to the base electrodes of the transistors 12 and 13 through resistors 15 and 16 and variations of the tapping point change the frequency of the multivibrator. The constants of the circuit are such that the voltage at the collector electrode of the transistor 13 is of substantially square waveform and this is applied to a suitable frequency meter (not shown). Thus by measuring the frequency of the oscillations the depth may be determined.