GB2169161A - Direction indicator system for a vehicle and a trailer - Google Patents

Abstract

A trailer has direction indicators 18 and 19 which operate synchronously with direction indicators 16 and 17 of a vehicle to which the trailer is attached. Operation of a pilot indicator 20 is controlled by a transistor 23 of a sensor unit 13 in accordance with the voltage drop generated across a monitor resistance 22, such that an L.E.D. 30 emits light whenever the trailer indicators 18, 19 are energised. Fluctuations in this voltage drop are smoothed out by a smoothing circuit formed by a capacitor 28 and a resistor 24, and a diode 29 is connected in parallel with the monitor resistance 22 to reduce the initial voltage peak across the latter. A series resistor 31 integrated with the L.E.D. 30 protects the latter from damage in the event that full battery voltage is applied across the indicator 20, and a further series resistor 32 protects the transistor 23 in the event of the output terminal 21 of the sensor 13 being shorted to earth. Where a sensitive L.E.D. 30 is used, a resistor may be connected in parallel with the indicator 20 to prevent it glowing due to leakage current through the transistor 23 when only the vehicle lamps 16, 17 are connected. <IMAGE>

Description

SPECIFICATION Direction indicator system for a vehicle and trailer This invention relates to a direction indicator system for a vehicle and a trailer.

When a trailer is attached to a vehicle, it is necessary that direction indicators on the trailer should flash synchronously with direction indicators on the vehicle. Also, a pilot indicator must be provided in the vehicle to indicate to the driver that the trailer's direction indicators are flashing properly. If the pilot indicator fails to operate, then the driver will know either that there is a fault in the trailer's direction indicators or that he/she has forgotten to connect these indicators to the vehicle electrics.

Typically, a direction indicator system for this purpose comprises a flasher unit operable intermittently to energize the direction indicators of the vehicle (and also those of the trailer when the latter is attached to the vehicle), and a sensor unit which operates the pilot indicator in synchronism with the trailer's direction indicators, the sensor unit including a transistor whose main conduction path is connected in series between the vehicle power supply and the pilot indicator. In conventional systems, the pilot indicator takes the form of an incandescent lamp which is connected to the aforesaid transistor without an intervening resistance.

Consequently, if the lamp should ever become short-circuited, then maximum current will flow through the main conduction path of the transistor, thereby causing the transistor to blow.

In so-called "front-end" systems, the flasher unit is designed to maintain a constant flash rate irrespective of the number of direction indicators, to which it is connected. It is therefore possible to connect the trailer's direction indicators directly in parallel with the direction indicators of the vehicle.

In this case, the sensor unit also includes a monitor resistance connected in series between the flasher unit and the direction indicators, and the aforesaid transistor controls the energisation of the pilot indicator in accordance with the voltage drop developed across the monitor resistance. That is to say, the transistor is turned on to energise the pilot indicator each time the flasher unit supplies a current pulse to the trailer's direction indicators.

In theory, it should be possibleto gauge the sensitivity of the sensor unit so that it operates the pilot indicator only when an increased loading is placed on the flasher unit by the presence of .he trailer's direction indicators. In practice, however, this is very difficult to achieve because the peak inrush current with only the vehicle's direction indicators connected is about equal to the steadystate current with the trailer's direction indicators connected also. The net result of this is that the pilot indicator is caused to operate even if the trailer's direction indicators are not connected to the vehicle electrics.

It is an object of the present invention to overcome these particular problems.

According to one aspect of the present invention, there is provided a direction indicator system for a vehicle and a trailer, comprising a flasher unit operable to energize intermittently direction indicators of the vehicle, and a sensor unit which detects when direction indicators of the trailer are being operated synchronously with the vehicle's direction indicators and which operates a pilot indicator in response to such detection, the pilot indicator being composed of a solid-state light-emitting element having a first resistance connected electrically in series therewith, the first resistance being disposed externally of the sensor unit, a second resistance being connected electrically in series with the first resistance and being disposed internally of the sensor unit. Preferably, the light-emitting element and the first resistance together form an integrated unit.

According to a second aspect of the present invention, a direction indicator system for a vehicle and a trailer comprises a flasher unit which supplies current pulses to direction indicators of the vehicle and also to direction indicators of the trailer when the latter is attached to the vehicle, a sensor unit including a monitor resistance connected electrically in series between the flasher unit and the direction indicators, the sensor unit also including a switching element which controls the supply of current to a pilot indicator in accordance with the voltage drop developed across the monitor resistance such that the pilot indicator is operated when the trailer's direction indicators are energized, the switching element being connected to the monitor resistance by way of a smooth circuit which smooths out fluctuations in said voltage drop.

Preferably, a diode is connected electrically in parallel with the monitor resistance to reduce the initial voltage peak across the latter.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure lisa circuit diagram of a direction indicator system according to the invention; Figure 2 is a diagram showing a voltage waveform across a monitor resistance in a conventional system of this type; Figure 3 is a diagram showing a voltage waveform which would appear between the base and emitter of a transistor in the system of Figure 1 if a diode were not connected in parallel with the monitor resistance; Figure 4 is a diagram showing a voltage waveform across the monitor resistance with the diode connected; and Figure 5 is a diagram showing a voltage waveform between the base and emitter of the transistor with the diode connected in parallel with the monitor resistance.

In the circuit shown in Figure 1, reference numeral 10 denotes a 12 volt positive feed derived from a battery of the vehicle in which the direction indicator system is provided. Connected to the positive feed 10 is a conventional flasher unit 11 of the type whose flashing rate is load-independent, an output ofthe flasher unit 11 being connected to an input terminal 12 of a sensor unit 13. An outputterminal 14 of the sensor unit 13 is connected to a conventional direction indicator switch 15 which can be operated selectively to energise either left-hand direction indicator lamps 16 or right-hand direction indicator lamps l7ofthevehicle.

When a trailer is attached to the vehicle, its direction indicator lamps 18 and 19 are connected in parallel with the vehicle's direction indicator lamps 16 and 17 respectively so as to flash synchronously therewith. A pilot indicator 20 is connected two a second outputterminal 21 ofthe sensor unit 13, and is disposed so as to be visible to a driver ofthe vehicle. As will be apparentfrom the ensuing description, the pilot indicator 20 flashes in synchronism with the trailer's direction indicator lamps 18 and 19 to indicate to the driverthatthese lamps are functioning properly.

The sensor unit 13 is composed of a monitor resistance 22 connected between the inputterminal 12 and the outputterminal 14, and a transistor 23 whose collector-emitter path is connected in series with the pilot indicator 20 and whose base is connected to the resistance 22 by way of a resistor 24.

The conduction state ofthe transistor 23 and hence the energisation condition ofthe pilot indicator 20 is thus dependent upon the voltage drop across the resistance 22.

In conventional systems ofthis type, the sensor unit 13 contains no other significant components above and beyond those already described, and Figure 2 shows the voltage waveform which is developed across the monitor resistance 22 in such a system. Upon initial energisation, the direction indicator lamps are cold and hence the current flowing through them is relatively large: however, as the lamps warm up, the resistance oftheir incandescentfilaments increases and the current falis off. This gives rise to an initial peak 25 in the voltage waveform. During subsequent flashes, a similarvoltage peak26 also occurs but is of a smaller magnitude than before due to the lamp filaments still being relativelywarm from the previous flash.After the initial peak 26 in each flash,the voltage levels off to a steady state value as indicated at 27.

The currentflowing through the monitor resistance 22 will of course be increased when the trailer's direction indication lamps are connected, due to the effective resistance ofthe parallelconnected lamps being reduced. This meansthatthe voltage drop across the resistance 22 will be greater than when the trailer's direction indicator lamps are not connected: this can be seen to advantage in Figure 2, wherein the solid lines representthe voltage waveform with thetrailer's lamps connected and the broken lines represent the voltage waveform with the trailer's lamps disconnected. In theory, therefore, it should be possible to arrangeforthe transistor 23 to trigger only on the highervoitage waveform, so thatthe pilot indicator 20 is not energized when thetrailer's direction indicator lamps are disconnected.In practice, however, this is virtually impossible to achieve because the peak voltage 26 with onlythe vehicle's direction indicator lamps operating is greaterthan the steady state voltage 27 with the trailer's direction indicator lamps connected.

In orderto overcome this particular problem, the present invention provides a smooth circuit between the monitor resistance 22 and the transistor 23to smooth outthe initial voltage peaks 25, the smoothing circuit being composed of the aforesaid resistor 24 and a capacitor 28 connected between the base and the emitter ofthe transistor.Figure 3 shows the voltage waveform appearing across the capacitor 28, from which itwill be seen that (afterthefirstflash) there is a substantial difference betweenthevoltage levels with and withoutthe trailer's direction indicator lamps connected. lttherefore becomes a simple matterto arrange forthe transistor 23 to trigger only on the highervoltage waveform, so that the pilotindicator20 does notoperatewhenthe trailer's direction indicator lamps are not connected.

In the case where only the vehicle direction indicators are operating, the voltage appearing across the capacitor 28 during the first flash will exceed the minimum value Vmin required to allow currentto flow th rough the main conduction path of thetransistor23, and hence lightwill be emitted from the pilot indicator 20 for this period. Although not adversely affecting the operation ofthe direction indicatorsystem,foraesthetic reasons it is desirable to avoid this pulse of light from the indicator since it may create an unwanted distraction forthe driver of the vehicle. To this end, a diode 29 is connected electrically in parallel with the monitor resistance 22 to reduce the initial voltage peak across the latter.

Figure 4 shows the voltage waveform appearing across the monitor resistance 22 with the diode 29 present, and by comparison with Figure 2 itwill be seen thatthe initial voltage peak 25 is very substantially reduced and thatthe subsequent voltage peaks 26 are reduced also by a lesser amount. FigureS shows the voltage waveform appearing across the capacitor28,from which it can be appreciated that, when only the vehicle direction indicators are operating, the voltage across the capacitor never exceeds the minimum value Vmjn required to allow current to flowthrough the main conduction path of the transistor 23: hence, the pilot indicator20will not emit lightatall underthese conditions, not even during the firstflash.

Referring backto Figure 1 ,the pilot indicator 20 is composed of a solid-state light-emitting element 30 (preferably an L.E.D.) connected in series with a resistor 31, the two components 30 and 31 together forming an integrated unit. The use of a solid-state element 30 in place of an incandescent lamp has the advantagethatthe element 30 is inherently more reliable and has a much greater lifethan such a lamp.

Afurther resistor 32 is incorporated within the sensor unit 13, being connected in series between the collector ofthetransistor 23 and the outputterminal 21. The resistors 31 and 32 togetherform a conventional series resistanceforthe light-emitting element 30, butwhich is split into two parts. The advantage ofthis splitting is that the resistor 31 will protectthe element 30 from damage in the eventthat the full battery voltage is applied across the pilot indicator 20, whilethe resistor 32 will preventthe transistor 23 from blowing in the eventthatthe outputterminal 21 ofthe sensor unit 13 becomes shorted to earth.

The principle of splitting the series resistance for the light-emitting element 29 is applicable not only to "front end" systems as described above but also to "rear end" systems wherein energisation of the trailer's direction indicator lamps is controlled by transistors whose bases are connected to the vehicle's direction indicator direction lamps, so that each transistor becomes conductive when the respective direction indicator lamps of the vehicle are energised by the flasher unit.

If the component chosen as the light-emitting element 30 is particularly sensitive, then leakage current from the transistor 23 can cause the element 30 to emit a faint glow when the vehicle's direction indicators are energized and the trailer is not connected. This problem becomes particularly acute when the vehicle includes side repeaters (i.e.

additional direction indicators provided on its sides), since then the collector current of the transistor 23 is increased. This situation can however be avoided simply by connecting a resistor (not shown) in parallel with the series connections of the element 30 and the resistor 31,so that the voltage developed across the element 30 will not be high enough to cause the element 30 to function under the conditions described above. The value of the resistance will of course be chosen so that it is not low enough significantly to affect the brightness of the element 30 during normal operation.

Claims (10)

1. A direction indicator system for a vehicle and a trailer, comprising a flasher unit which supplies current pulses to direction indicators of the vehicle and also to direction indicators of the trailer when the latter is attached to the vehicle, a sensor unit including a monitor resistance connected electrically in series between the flasher unit and the direction indicators, the sensor unit also including a switching element which controls the supply of current to a pilot indicator in accordance with the voltage drop developed across the monitor resistance such that the pilot indicator is operated when the trailer's direction indicators are energized, the switching element being connected to the monitor resistance by way of a smoothing circuit which smooths out fluctuations in said voltage drop.

2. A direction indicator system as claimed in claim 1, wherein the switching element is a transistor, and the smoothing circuit comprises a resistor connected between the base of the transistor and the monitor resistance, and a capacitor connected between the base and the emitter of the transistor.

3. A direction indicator system as claimed in claim 1 or 2, wherein a diode is connected electrically in parallel with the monitor resistance to reduce the initial voltage peak across the latter.

4. A direction indicator system as claimed in claim 1,2 or 3, wherein the pilot indicator is composed of a solid-state light-emitting element having a first resistance connected electrically in series therewith, the first resistance being disposed externally of the sensor unit, a second resistance being connected electrically in series with the first resistance and being disposed internally of the sensor unit.

5. A direction indicator system as claimed in claim 4, wherein the light-emitting element and the first resistance together form an integrated unit.

6. A direction indicator system as claimed in claim 4 or 5, wherein a resistor is connected electrically in parallel with the series connection of the lightemitting element and said first resistance.

7. A direction indicator system for a vehicle and a trailer, comprising a flasher unit operable to energise intermittently direction indicators of the vehicle, and a sensor unit which detects when direction indicators of the trailer are being operated synchronously with the vehicle's direction indicators and which operates a pilot indicator in response to such detection, the pilot indicator being composed of a solid-state light emitting element having a first resistance connected electrically in series therewith, the first resistance being disposed externally of the sensor unit, a second resistance being connected electrically in series with the first resistance and being disposed internally of the sensor unit.

8. A direction indicator system as claimed in claim 7, wherein the light emitting element and the first resistance together form an integrated unit.

9. A direction indicator system as claimed in claim 7 or 8, wherein a resistor is connected electrically in parallel with the series connection of the lightemitting element and said first resistance.

10. A direction indicator system for a vehicle and a trailer, substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.