Patent GB-1093608-A (GB)
Title: Improvements in or relating to microphones
Inventor: BOOTH ROBERT
IPC: H04R3/00
Filed: 1966-06-28
Abstract: 1,093,608. Microphones. STANDARD TELEPHONES & CABLES Ltd. June 28, 1966, No. 28875/66. Headings H3T and H4J. [Also in Division G5] A microphone handset comprising a transducer unit 1 (Fig. 1), in a housing includes a pair of resistively interconnected electric terminals 4 and 5 acting as a switch and responsive to the placing of a finger or thumb on to a predetermined surface region 3 to significantly alter the resistance between the two terminals 4, 5. The handset is for use with a dictation machine having relays for record (R), playback (P) and backspace (B) and preferably three regions are provided. In a second form of handset, Fig. 2 (not shown), the three regions have four vertical conductive strips (8), the first and third strips being interconnected and the strips in each region being so dimensioned that the placing thereon of a finger or thumb significantly alters the resistance between the interconnected pairs of strips. Local circuit.-Each stage (Fig. 3), comprises a low noise transistor T1 having its collector connected to the base of a Darlington connection T2, T3. Relays S1 are situated on the emitter circuit of the final transistor T3 of a different stage. In operation, the transistors are normally " off." Placing a finger or thumb across the appropriate touch contact lowers the contact resistance, causing the transistor T1 to switch on and thereby, through the Darlington pair T2, T3, energize the relay S1. The final emitters of the first and second stages are interconnected by a diode D1 so that, while touch contact P is bridged, both the first and second stages are switched to energize two relays S1. Handset using light source.-A handset (Fig. 4) contains a transducer unit 10 and has a light bulb 12 or gallium arsenide infra-red emitter (not shown), the light being focused by a plastics lens on to silicon or other photooonductive cells R, P, B. Shields 14, 15 for the light source 12 and photo-cells are provided, together with individual light guides 16. With the light source energized the photocells are in a low resistance condition. Placing a finger or thumb in a guide 16 blocks the light from the respective cell, whose resistance is thereby increased. Local circuit.-Two transistors T4, T5 (Fig. 6) in a Darlington connection are located in the handle 11. Relays S2 on the recording machine effect the appropriate function and are each situated in the collector circuit of a final transistor T5. The base bias of each transistor T4 includes a photo-cell. A relay S2 is energized by cutting off the light from the appropriate photo-cell, increasing the resistance of the cell so that the associated Darlington pair T4, T5 is switched on to energize the relay. Two relays are energized by one cell P by a coupling 17 between the emitter of the final transistor T5 of the first stage with the final transistor of the second stage. If the bulb 12 holding the relays " off " fails, a trip circuit comprising a transistor T6 deriving its condition from a resistance R2 in series with the bulb 12 provides a return path for the relay driving stages. In the case of bulb failure, the transistor T6 will switch off removing the return path from the driving stages. A diode D2 in the emitter circuit of transistor T6 provides reverse bias, switching the remaining transistors normally off. This provides temperature stability.