Patent GB-1326002-A (GB)
Title: Speed measuring apparatus for a vehicle
IPC: G01P3/48
Filed: 1970-07-17
Abstract: 1326002 Transistor pulse shaping and frequency discriminating circuits SMITHS INDUSTRIES Ltd 17 July 1970 [1 Aug 1969] 38599/69 Headings H3A and H3T [Also in Division G4J A speed measuring apparatus derives input pulses at a rate dependent on the speed of a vehicle and passes them to a monostable circuit to produce control pulses for a constant current source which produces current pulses of a predetermined magnitude. An input signal, e.g. derived from a vehicle gear-box, whose frequency is to be measured is squared in a Schmitt circuit 20 and applied to a monostable circuit 40 which produces constant width pulses at the input signal frequency. A first output from the monostable is applied to a switching circuit 60 which applies constant current pulses to a meter, the average current through the meter indicating the frequency and hence the vehicle speed. A second output from the monostable is integrated, and the integrated voltage applied to a threshold circuit 80 which operates an indicator at a predetermined speed. A third output controls an odometer comprising a counter circuit 100, 120. In overdrive, an output from unit 140 changes the width of the pulses from the monostable, and also the radix of the counter. Squarer circuit, Fig. 3 (not shown), comprises a Schmitt trigger with its input shunted by a transistor (24) strapped as a diode, a further transistor (23) being connected with its ernittercollector path in series between the shunt transis - tor and the trigger. In a modification, Fig. 3a (not shown), the further transistor is connected in cascade with the trigger.  Monostable circuit, Fig. 4, comprises a pair of cross coupled transistors 41, 57 the coupling from the collector of 41 to the base of 57 including a temperature compensation circuit 44, 47. The relaxation time is initially set by adjusting resistor 50 and its value is switched by changing the bias applied at the junction of resistors 50, 52. In a modification, Fig. 4a (not shown), a transistor (58) is provided between  resistor 46 and point A to provide a constant  current charging path for capacitor 49.  Meter drive circuit, Fig. 5.-Output pulses from the monostable are applied to gate a constant current source 61, 62, 69 which applies constant current pulses to a meter (not shown) connected to terminal 70. Temperature drift correction is provided by long-tail pair 64, 65, the base of transistor 64 being connected to a stabilized power supply, Fig. 2 (not shown). Diode strapped transistor 71 clamps terminal 70 to a reference potential to protect the meter in the case of overload. Speed detector circuit, Fig. 6 (not shown).- Pulses from the monostable are applied via a potential divider chain comprising diodestrapped transistors (81, 83, 84) to an integrator (not shown), the output from which is connected to one input of a long tail pair 89, 90 which acts as a threshold switch. Temperature compensation is provided by resistors (95, 96) providing emitter and collector bias for transistor 90. Odometer drive circuit. Fig. 11, comprises a bi-stable circuit having a double pair of cross coupled transistors which are switched by pulses applied to input transistor 165. Outputs to drive a stepping motor are obtained via transistors 174, 175. During periods when 174, 175 are off, their base potentials float and allow the transistors to attain avalanche conditions under peak collector-emitter voltage conditions due to the inductive load of the stepping motor. Divider circuit, Fig. 7 (not shown), comprises four bi-stables similar to that of Fig. 11, the first and third including a transistor (106) shunting the input transistor (105) to inhibit some of the input pulses when a control signal is applied from the gate circuit, Fig. 8 (not shown).