Patent GB-1472371-A (GB)
Title: Monitoring systems
IPC: H04J1/00
Filed: 1975-02-11
Abstract: 1472371 Physical condition (temperature) monitoring W E BAIR 11 Feb 1975 [26 Feb 1974] 5794/75 Headings G1N(A) and G1N(E) In a physical condition (temperature, strain) monitoring device a plurality of transmitters are attached to a plurality of objects (patients) to be monitored, Fig. 1, (not shown). Each transmitter comprises a transistor multi-vibrator type circuit whose resistance or capacitative components are chosen so that the ratio of the intervals between the switching of the transistor stages is sensitive to the physical condition. This result is an asymmetric waveform. Each transmitter has an output electrode. When the input electrode of a portable receiver is brought into close proximity, the signals from the transmitter concerned are passed via the electrostatic coupling so formed. The receiver then processes the signals and supplies an indication of the physical condition. Fig. 2 shows three types of suitable multivibrator transmitter circuits. In each circuit the physical condition (i.e. temperature) sensitive components are the resistors and capacitors R 1 , C 1 , R 2 , C 2 . Fig. 2 (a) is a basic multi-vibrator circuit producing the aforementioned asymmetric waveform. However the "rise rate" of the waveform is relatively slow and Fig. 2(b), 2(c) are improvements. Fig. 2(b) relies on an extra transistor stage Q 3  with a capacitor diode network, as shown, to produce sharply rising asymmetric pairs of signals, whereas Fig. 2(c) relies on cross-coupled pairs Q 1 , Q 3  and Q 2 , Q 4  of transistors to produce an asymmetric square waveform. The receiver outputs are amplified, Figs. 4 and 8 (not shown) and may be fed to an analogue circuit, Fig. 6, or a digital circuit, Fig. 7. Fig. 6 shows an amplifier with an exponential feedback network and the input to the amplifier A 3  is determined by a switching mechanism Si which alternates the input lines between a source of reference potential and ground potential under the control of the waveform from the transmitter. The time constant of the feedback circuit is long compared with the multi-vibrator components so the resulting amplitude output voltage will oscillate about a mean which is proportional to the desired ratio of time intervals. The output of the amplifier A3 is fed to a meter M. Fig. 7 shows a digital output section of a receiver. Closing switch S 2  causes one shot multivibrator OS 1  to reset counters CN 1  and CN 2  and to set a flip-flop FF2 for enabling gates G 1 , G 2  so that an oscillator OSC can provide pulses as shown to the counters CN 1 , CN 2 . The counting on CN2 is also controlled by the input wave form E<SP>1</SP> from the amplifier stages, Figs. 4 and 8 (not shown) of the receiver. The clock pulses have a frequency larger than the waveform frequency. When the counter CN 1  overflows, the output of the counter CN 1  is coupled to the reset input of the flip-flop FF 2  thereby stopping the counting and initiating the transfer of the value in the counter CN2 to a display buffer DB 1 , and a display DS. The count on the counter at this stage is proportional to the desired ratio of intervals.