GB1342248A - Electrical lamp failure indicating circuits - Google Patents

Abstract

1342248 Lamp failure indicators G A KING 22 Sept 1971 [28 Sept 1970 8 Oct 1970 25 Nov 1970 9 Aug 1971] 46019/70 47827/70 55968/70 and 37259/71 Heading G4N A circuit which provides an indication of failure of one or more lamps 33A, 33B comprises a resistor 34A, 34B in series with each lamp, a first normally non-conductive semiconductor switch 40A arranged to be made conductive by the potential at the junction of resistor 34A and lamp 33A when that lamp fails and a second normally non-conductive semiconductor switch 47 arranged to be conductive when the first semiconductor switch 40A becomes conductive so as to operate an indicator 48. The circuit shown is for a motor vehicle having brake lamps 33A, 33B and main and dipped headlamps 38A, 38B. The voltage across each resistor 34A, 34B, 37 is monitored by a respective transistor 40A, 40B, 40C, biased by a diode 41A, 41B, 41C so that the small potential normally developed across resistors 34A, 34B, 37 will not turn on the transistor. Failure of a lamp 33A, 33B, 38A. 38B will turn on the appropriate transistor which is coupled via OR gating diode 43 to the gate of a thyristor 47 so as to turn on that thyristor and operate indicator lamps 48, a hooter, bell or buzzer. An electrolytic cell 52 is provided so as to indicate the time since the failure of the lamp. Cell 52 may be replaced by an accumulator or by a pulse circuit driving a counter. Capacitor 53 prevents unwanted operation due to transients. In Fig. 2 (not shown) transistors 40A, 40B, 40C are replaced by a single transistor having three emitters. In Fig. 1 (not shown) a diode OR gate is provided to feed the monitored voltage across three lamps to the emitter of a single PNP transistor the collector of which feeds an NPN transistor controlling an indicator lamp.