Patent GB-1189321-A (GB)
Title: Improvements in or relating to Audible Warning Circuits
Inventor: FELLERMAN BERNARD FREDERICK
IPC: G01P1/10
Filed: 1967-01-11
Abstract: 1,189,321. Transistor oscillating circuits. SMITHS INDUSTRIES Ltd. 21 March, 1968 [11 Jan., 1967], No. 1598/67. Heading H3T. [Also in Divisions G1 and G4] An audible warning circuit comprises a transistor TR2, Fig. 1, having its emitter electrode connected to a point of reference potential on a potential divider VR2 connected across the D.C. supply terminals 50, 51 of the circuit, and its base electrode connected to receive an input voltage dependent upon a condition to be detected, a feedback transformer 55 coupling the collector and base circuits of the transistor to form a blocking oscillator which oscillates when the input voltage reaches a predetermined level relative to the reference voltage, and a diaphragm 57 associated with the transformer to generate audible signals when the oscillator oscillates. In an application of the invention such a circuit forms part of a speed warning device for a motor vehiclewhich will give an audible warning when a preset speed limit is exceeded. A contactor (Figs. 3, 4a, 4b, not shown; see Specification 1,189,322, in Division H1) serves to make and break the connection between terminals 52, 53 at a frequency determined by the speed of the vehicle, being rotated by the speedometer cable. When the contactor is closed transistor T1 is conductive, but when it breaks the circuit a positive-going voltage step is applied to the base of transistor T1 rendering non-conductive until capacitor C1 again charges up over resistors R3, R1 sufficiently to render it again conductive. The output at the collector of transistor TR1 comprises a series of pulses of constant width which are integrated by resistor R5 and capacitor C2 to produce a smoothed D.C. voltage, the magnitude of which is proportional to the rate at which pulses are received, i.e. the speed of the vehicle, and the width of the pulses as determined by setting variable resistor VR1. This D.C. voltage is applied to the base of transistors TR2, the base circuit of which comprises resistors R7, R8 and a temperature compensating thermistor TH1; the transistor TR2 is normally biased non-conductive by an emitter bias applied from potentiometer VR2. When the base bias reaches a sufficient value, transistor TR2 becomes conductive and blocking oscillations occur, audible pulses being emitted by diaphragm 57; base current in transistor TR2, however, tends to discharge capacitor C2, lowering the potential applied to the base so that oscillation ceases. In this way the circuit gives rise to audible pusels the mark-space ratio of which increases as the speed of the vehicle is increased above the pre-set limit determined by potentiometer VR2, and so increases the rate at which capacitor C2 is charged. The arrangement is such as to balance out the effect of changes in supply voltage. In a second embodiment the contactor is connected across contacts 66, 67, Fig. 2, which when connected together cause capacitor C6 to be charged via diode D1 to a potential determined by setting potentiometer VR3. When the contacts are open capacitor C6 discharges via resistor R14, part of potentiometer VR3, resistor R13 and the emitter-collector path of transistor TR4 into capacitor C7. Thus capacitor C7 receives a definite increment of charge for each cycle of operation of the contacts, producing at the junction of resistors R15, R19 a voltage in accordance with the rate at which the contacts 66, 67 are operated, i.e. the speed of the vehicle. This voltage is applied to the base of transistor TR3, blocking oscillator which operates in the manner described with reference to Fig. 1. Capacitor C5 reduces signal noise caused by irregularities in the make and break contacts while silicon diode D2 and resistor R19 provide compensation for changes in temperature. A device for setting the speed at which the audible warning comes into operation is also described in the Specification (see Division G1).