Flashing Lamp Circuits

Abstract

A flashing lamp circuit for flashing a lamp includes intermittently operable switch means connected in series with a resistor with the lamp to be flashed, voltage sensing means including a transistor for detecting the voltage across the resistor when the lamp is illuminated, said voltage sensing means including a capacitor which is charged in the event that the voltage across the resistor is below a predetermined value when the switch means is closed. A transistor switch including transistors controls the operation of a warning lamp and when the switch means is open the warning lamp will be illuminated. In the event that the capacitor has been charged, the transistor switch will be rendered non-conductive and the warning lamp will remain in an extinguished condition. This invention relates to a flashing lamp circuit and of the kind comprising terminals for connection to the lamp or lamps to be flashed, contact means operable to cause flashing of said lamp or lamps and a resistor in series with said contact means and across which when said contact means is closed, a voltage is developed indicative of the magnitude of the current flow through the lamp or lamps. The object of the invention is to provide such a circuit in a simple and convenient form. According to the invention a flashing lamp circuit of the kind specified includes a warning lamp, a transistor switch operable to cause illumination of said warning lamp when said contact means is open, voltage sensing means for sensing the voltage developed across said resistor, a capacitor associated with said voltage sensing means and which is charged by the voltage sensing means in the event that the voltage across said resistor falls below a predetermined value when said contact means is closed, the voltage across said capacitor acting to prevent said transistor switch operating to cause illumination of the warning lamp when said contact means is opened. Two examples of flashing lamp circuits in accordance with the invention will now be described with reference to the accompanying circuit diagrams.

Description

Referring to FIG. 1 there are provided terminals 10 and 11 for connection respectively to the negative and positive terminals of a source of d.c. supply. The lamps to be flashed are indicated at 12 and comprise one or more lamps connected in parallel with each other and with one terminal thereof connected directly to terminal 10 and the other terminal connected to one terminal of a switch 13. The other terminal of the switch 13 is connected to one end of a resistor 14 the other end of which is connected by way of contact means 15, to terminal 11. A point intermediate the resistor 14 and switch 13 is connected to the emitter terminal of a p-n-p transistor 16 the collector of which is connected by way of a resistor 17, a diode 18 and a resistor 19 to terminal 10. The base of transistor 16 is connected to terminal 10 by way of resistor 20 and to a point intermediate resistor 14 and contact means 15 by way of a resistor 21 and diode 22.

Connected between terminals 10 and 11 is the series combination of a resistor 23 and a zener diode 24 and a point intermediate the resistor 23 and diode 24 is connected to the cathode of a diode 25 the anode of which is connected by way of a resistor 26, to the base of transistor 16.

Also provided is a p-n-p transistor 27 the base of which is connected to terminal 10 by way of resistor 28 and to terminal 11 by way of resistor 29. In addition, the base is connected to the cathode of a diode 30 the anode of which is connected to a point intermediate the diode 18 and resistor 19. Resistor 19 is bridged by a capacitor 31. The emitter of transistor 27 is connected to terminal 10 by way of resistor 32 and to terminal 11 by way of resistor 33. The collector of transistor 27 is connected by way of resistor 34 to the base of an n-p-n transistor 35. The emitter of this transistor is connected to a point intermediate the resistor 14 and contact means 15 and the collector of transistor 35 is connected by way of a warning lamp 36, to terminal 11.

Transistors 27 and 35 constitute a transistor switch for the warning lamp 36 and transistor 27 is biased by the resistors 28, 29 and 32, 33 associated with its base and emitter respectively so that it conducts. With transistor 27 conducting transistor 35 can also conduct. With transistor 35 in a conducting state the warning lamp 36 will be illuminated. When the switch 13 is closed and the switch 15 open, the two transistors 27 and 35 will be rendered conductive by the base drive to transistor 27 by way of resistor 28. When the switch 15 closes, the warning lamp will be extinguished.

Contact means 15 is a switch which may be operated at regular intervals by means of a mechanism or electric circuit which is rendered operative when the switch 13 is closed. With switch 13 closed and the contact means 15 in operation the lamps 12 will be flashed and by virtue of the current flow to the lamps a voltage will be developed across resistor 14 which is proportional to the current flowing through it. If the lamp or lamps are functioning correctly the current flow through the resistor is such that transistor 16 is not turned on. However, in the event that one or more lamps fails, the voltage developed across the resistor will fall to such an extent that transistor 16 is turned on and when this occurs capacitor 31 will be charged a sufficient extent to turn transistor 27 off by way of the diode 30. With transistor 27 in a non-conducting state there will be no base drive for transistor 35 so that when the contact means 15 is opened the warning lamp will not be illuminated thereby an indication will be given that one or more of the lamps 12 has failed. Thus when the lamps 12 are operating correctly the warning lamp will be on when the lamps 12 are off, but if one or more of the lamps 12 has failed, the warning lamp will remain extinguished.

The purpose of the Zener diode 24 is to apply to the base of the transistor 16 by way of the diode 25 and resistor 26 a potential which ensures that the potential at the base of the transistor 16 does not vary linearly with the supply voltage. The arrangement permits matching of the variation of lamp current with supply voltage, so that the circuit operates correctly whatever the voltage of the supply. The diodes 22 and 18 are provided to protect the transistor 16.

Referring to FIG. 2 of the drawings there are provided terminals 37 and 38 for connection respectively to a source of d.c. supply. The lamps to be flashed are indicated at 39, two such lamps being shown connected in parallel with one terminal connected to terminal 37 and the other terminal connected to a switch 40. The other terminal of the switch is connected by way of a resistor 41 to terminal 38 by way of a contact means 42.

A circuit supply line 43 is provided and this is connected to terminal 37 by way of resistor 44, a Zener diode 45 being connected between the line 43 and the terminal 38. Also provided is a p-n-p transistor 46 the emitter of which is connected to a point intermediate the switch 40 and the resistor 41 and the base of which is connected to line 43 by way of resistor 47 and to a point intermediate the resistor 41 and the contact means 42 by way of a resistor 48. The collector of transistor 46 is connected to the anode of a diode 50 by way of resistor 49 and the anode of the diode 50 is connected to line 43 by way of a resistor 49a. In parallel with the resistor 49a is a capacitor 51. Moreover, the base of transistor 46 is connected to terminal 37 by way of resistor 52.

There is also provided an n-p-n transistor 53 having its emitter connected to terminal 38 by way of resistor 54 and its base connected to line 43 by way of resistor 55. The base is also connected by way of diode 56 to a point intermediate diode 50 and resistor 49a. Moreover, the base of transistor 53 is connected by way of a capacitor 57 and a resistor 58 in series to a point intermediate resistor 41 and the contact means 42 and a diode 59 is connected to a point intermediate capacitor 57 and resistor 58 and line 43.

The collector of transistor 53 is connected by way of resistor 60 to the base of an n-p-n transistor 61 the emitter of which is connected to a point intermediate the resistor 41 and the contact means 42. The collector of transistor 61 is connected by way of a warning lamp 62 to terminal 38 and by means of a resistor 63 to the base of transistor 53.

Transistor 53 is biased to the `off` condition so that there is no base drive for transistor 61 and in the absence of resistor 58, when contact means 42 opens the warning lamp 62 will not be illuminated. However, because of the presence of resistor 58 when contact means 42 opens a negative pulse is applied to the base of transistor 53 and this is turned on thereby providing base drive for transistor 61 and causing illumination of the warning lamp. Transistor 53 is maintained in the on condition by reason of the feedback path provided by resistor 63 and the warning lamp will remain illuminated until the contact means 42 closes. Thus the warning lamp 62 will be illuminated when the lamps 39 are extinguished.

If the lamp or lamps 39 are functioning correctly transistor 46 will not be rendered conductive. However, in the event that one or more lamps fails, the voltage developed across resistor 41 will decrease sufficiently to cause conduction of transistor 46 thereby charging the capacitor 51. When the contact means 42 again opens the voltage developed across the capacitor 51 will prevent transistor 53 being turned on so that the warning lamp will remain extinguished to provide an indication of failure of one or more of the lamps 39. When the contact means 42 opens and because of failure of a lamp 39, transistor 53 is not conducting, it is possible for transistor 53 to be rendered conductive by interference appearing at the terminals 37 and 38. This may be minimised by providing a small capacitor indicated at 64, and connected between the base and emitter of transistor 53.

It will be appreciated that in FIG. 1 when the oscillator is turned off by the switch 13 thecontacts 15 are normally closed, and hence the transistor 16 becomes conductive. In other words, the warning lamp is not illuminated on the first flash of normal flasher operation, but is extinguished immediately in the event of a lamp failure. In FIG. 2, because of the initial surge by way of capacitor 57, the warning lamp is illuminated immediately on the first flash of normal flasher operation, but does not extinguish immediately upon lamp failure.

Claims

We claim:

1. A flashing lamp circuit comprising a pair of supply terminals, contact means connected in series with the lamp to be flashed, a resistor connected in series with said lamp and across which when said contact means is closed is developed a voltage indicative of the current flow through said lamp, a warning lamp, a transistor switch including a transistor having its collector emitter path connected in series with said warning lamp across said contact means, said transistor switch being operable to cause illumination of said warning lamp when said contact means is open, voltage sensing means for sensing the voltage developed across said resistor, a capacitor associated with said voltage sensing means, said voltage sensing means establishing a charging path for said capacitor in the event that the voltage across said resistor falls below a predetermined value when the contact means is closed, the voltage across said capacitor acting to prevent conduction of said transistor when said contact means is next opened and thereby preventing illumination of the warning lamp.

2. A circuit as claimed in claim 1 in which said voltage sensing means includes a further transistor having its base connected to a potentiometer chain which ensures that the base potential is a fixed proportion of the voltage across said supply terminals when said switch means is closed, the emitter collector path of said further transistor being connected in series with said resistor and with a further resistor across said supply terminals when said contact means is closed, said capacitor being connected in parallel with said further resistor, so that the capacitor is charged only when said further transistor is rendered conductive.

3. A circuit as claimed in claim 2 including a diode interconnecting the capacitor and said transistor switch.