Patent GB-1421981-A (GB)
Title: Electronic digital read-out circuit for a clinical thermometer
IPC: H03M1/00
Filed: 1973-01-18
Abstract: 1421981 Measuring temperature electrically; measuring resistance DIATEK Inc 18 Jan 1973 [19 Jan 1972] 2650/73 Headings G1N and G1U An electronic temperature measuring circuit for use with a temperature sensor 11, preferably a thermistor connected to a plug and jack 15, 17 incorporating a bridge circuit for the thermistor, the voltage from the bridge feeding an A-D converter 27, 29, 31, 33 in which capacitor Cl is charged by the bridge output and discharged, the total charge-discharge period being fixed say one second, through a given resistance to a given voltage so the discharge time constitutes a measure of temperature and is measured by counting pulses from clock 49 into counter 37 for transmission to display 39; the thermometer including a prediction facility with circuits 105, 107 sensing positive or negative temperature differentials of greater than zero but less than a given value and controlling the switch 31 through which the capacitor Cl discharges so as to lengthen the discharge time and thereby increase the count in counter 37 to correspond to the temperature which would be sensed by the thermistor if it were allowed to reach the temperature of its surroundsings. A-D converter The thermistor voltage is integrated on capacitor Cl until time t 2  when flip-flop 33 is triggered by the overflow signal from counter 37 appearing on line 47 and which initiates discharge of Cl through switch 31. A signal appears every second on line 47 since the counter has four decodes and clock 49 runs at 10 kilohertz. Detector 29 senses when voltage on C1 has reached V A1 (mm) e.g. t3 and resets the control flip-flop 33 which produces a transfer signal, Fig. 2(b), (not shown) on line 43 to cause latch 51 to store the count in counter 37. The count retained in latch 51 is sampled by a multiplexing circuit 56, which is controlled by clock 57 running at e.g. 60 hertz, and passed to a display 39 having light emitting diodes or other indicators. Details of the A/D converter are given in Fig. 5 (not shown). Prediction circuits Amplifier 101 receives the analog voltage from the thermistor bridge and provides an output voltage at 103 driving parallel positive sensing and negative sensing circuits 105, 107 respectively which detemine when the rate of change of the thermistor unit falls below a given value. As illustrated in Fig. 6(a) the temperature rise curve of a thermistor initially at a lower temperature than that to be measured rises to its equilibrium level 109 at about time T 2 . But since the temperture rise characteristics of the thermistor are predictable, it is possible to estimate at time T 1 , when the slope of the curve first falls below a given value, what the ultimate temperature to be reached at T 2  will be. Similarly the negative sensing circuit 107 monitors the slope of the temperature change of a thermistor that is initially higher in temperature than a body to be measured as shown in Fig. 6(b). The voltage at 103 is sampled once during each charge and discharge cycle of the capacitor C1 under the control of the inverse transfer signal 45 from the control flip-flop 33. If the change in the voltage at 103 during the charging cycle of Cl is greater than a predetermined value set into the circuits 105, 107 a reset pulse is developed on line 121 from the appropriate circuit depending on whether the temperature is increasing or decreasing. If the charge is less than the predetermined value no reset pulse is developed in the line 121. Pulse generator 123 clocks the counter 127 in response to circuit 105 so that one clock pulse occurs for each charge and discharge cycle. Output 131 of the counter changes state after two clock pulses unless it has been reset by a pulse on line 121. Thus the counter output changes state after two clock pulses denoting to cycles during which either the positive or negative sensing circuits have noted a voltage change that is less than the predetermined slope at which a temperature measurement is taken. When the output changes state, a blanking/transfer inhibit circuit 133 changes its output level to remove the blanking from the display circuit 39 so that it displays. In this so-called "temperature mode", selected by mode control switch 137, the temperature displayed is a single temperature for every temperature measurement and which is that predicted or forecast prior to the thermistor actually reaching the temperature of the body to be measured. The temperature added on to that actually measured prior to the equilibrium point being reached is accomplished by switching transistor Q, Fig. 5 (not shown), off so that capacitor Cl will take longer to discharge and thus more pulses will be stored in the latch circuit 51, whereby a higher reading will be displayed than is actually sensed at the time T 1 .