GB2172120A - Graticule illuminator circuit for night-sight - Google Patents

Abstract

Modern night-sights commonly employ image intensifers that require batteries delivering a mere 2 or 3 volts. The available voltage may not be sufficient to drive the sight's graticule illuminator (usually an LED). The invention overcomes this difficulty by using as the power supply to the LED (10) a switched mode dc/dc converter fed from the intensifier's own battery power source (12). <IMAGE>

Description

SPECIFICATION Illuminator circuits This invention concerns illuminator circuits, and relates in particular to such circuits for use in night-sights employed in conjunction with image intensifiers.

It is now common practice for many sorts of imaging systems to be used with intensifiers. Typically, an image intensifier receives as its input very weak light (as might ordinarily be available on a dark night in the absence of any form of artificial illumination of the viewed scene), converts this into an electronic image the intensity of which can be electronically increased many times, and then converts the electron image back into a visible image that is, of course, now very much brighter than the original viewed scene. One use for such an imaging system is as a night-sight for a weapons system such as a rifle.

Early forms of image intensifier-now known as the "first generation" devices-were bulky and cumbersome, and generally required a 6 volt input supply. Modern devices-the so-called second and third generation devices-are smaller and operate from a lower voltage source. Indeed, a modern second- or third-generation intensifier may run off a small battery delivering from 2.7 to 3.5 volts (depending on type and age), with an end-of-life EMF of say as little as 2.0 volts.

Examples of such batteries for use in tropical and temperate climates are the conventional manganese alkaline or mercury cells (two of these in series provide a suitable voltage), while an example for use in colder (arctic) climates is the more expensive and less common lithium thionyl chloride cell (one of these provides sufficient voltage).

Night-sights commonly include an actual sight-a target graticule-often in the form of cross-hairs, that indicate precisely where the sight (and the weapon it is mounted on) is pointing. In an ordinary sight, for use in daylight, the graticule is visible silhouetted against the background, but in a night-sight, where the background is necessarily rather dark (even after image intensification), this is not acceptable, and therefore it is common practice to illuminate the graticule itself. This can be done very easily simply by shining a light onto the sight/cross-hairs, and in second- and third-generation intensified night-sights it is possible to use the intensifier s battery power source to drive the illuminator, which latter is very conveniently one or more Light Emitting Diode (LED).

Most LEDS require a certain minimum voltage-at least 2.2 volts-before they will emit light and a higher voltage-up to 2.4 volts, say is desirable for full control. It is regrettably the case that a tired battery, at the end of its operating life, may well produce a voltage of lower than 2.2 volts-which is still capable of operating the intensifier but is incapable of driving the LED into luminescence. The present invention seeks to overcome this difficulty in a simple but hitherto undisclosed fashion. Specifically, the invention proposes that the power supply to the LED (or other) illuminator in an intensified night-sight be a switched mode dc/dc converter fed from the intensifier's own battery power source.

In one aspect, therefore, the invention provides, for use in a battery-powered intensified night-sight having an illuminatable target graticule, a graticule illumination circuit arrangement including one or more illuminator powered from a switched mode dc/dc converter drivable by the night-sight's intensifier's battery.

The inventive circuitry is for use in a batterypowered intensified night-sight having an illuminatable target graticule. This night-sight may be of any type, intensified in any way (thought the inventive circuitry is primarily for use with sights having second or third generation intensifiers requiring low voltage power sources), and no more need be said about it here.

The graticule-illumination circuitry of the invention includes one or more illuminator. The illuminator may be a hot-filament bulb, but is very preferably an LED. A typical LED is that minature device available from Litronix/Siemens under the designation YL-56. Naturally, there may be one or more illuminator, depending upon how much light is required and upon the capabilities of the switched mode dc/dc converter to drive them. Three LEDS should be quite suitable.

The main feature of the invention is the use of a switched mode dc/dc converter. Such converters are not new the high voltage spark required for a motor car s ignition is usually generated from the low voltage car battery using the rectified output of a simple converter - and in general involve merely a coil across which the battery voltage is supplied via two parallel pathways, one being the load (in this case tHe illuminator) and the other being a switched conductor having a resistance that is low compared to that of the load. The principle of operation is simple.

With the switch closed so "shorting out" the load current passes through the coil building up a magnetic field in which energy is stored.

As the field builds up, so the "internal" potential across the coil is directed to resist this, and opposes that of the battery. If the switch is abruptly opened, the field collapses and the stored energy is released -but the potential across the coil is now such as to resist the field collapse (the actual potential depends primarily on the rate of collapse of the field-the faster, the higher), so that it now adds to that of the battery.Thus, powered by a 2.0 volt battery the converter could momentarily output well over 6 volts (its power capacity de pending upon the size in turns of the coil)-which is more than enough to drive an LED with a forward voltage of 2.2 volts: Clearly, if the arrangement is such that the converter can output its higher dc voltage both for a significant time ( > 10 microseconds) to drive the illuminator into luminescence, and sufficiently often (every 50 milliseconds) that because of persistence of vision the pulses of light seem to the human eye/brain to be continuous, then there will be attained a satisfactory illumination of the target graticule.In practice high switching rates, of up to 100 kHz,are preferable in order to enable the size of the coil to be kept small, and these rates are easily achievable using as the switch a transistor (or other similar semiconductor device) fed by a control signal produced by an oscillator (another semiconductor arrangment).

Though the simple combination of coil and switch described above can provide an acceptable arrangement, it is preferable to be able to exert somewhat more control over the converter. It is therefore desirable to arrange for some feedback from the coil (conveniently by providing a linked secondary coil, so making the combination a transformer, the output of which can be fed to a switch-control network).

It will be apparent that, using in accordance with the invention a simplified switched mode dc/dc converter to convert upwardly the available battery voltage, a night-sight having a weak battery will nevertheless still give an acceptable level of graticule illumination, and the invention naturally extends to such a nightsight using the inventive illuminator circuit arrangement.

Various embodiments of the invention are now described though by way of illustration only with reference to the accompanying Drawings in which: Figure 1 is a simple circuit diagram explaining the principle of the illuminator circuit arrangement of the invention; and Figure 2 is a diagram of a more complex and realistic circuit for the same purpose.

The simple circuit of Figure 1 illustrates the basic illumination circuit arrangement of the invention. It comprises a chain of three light emitting diodes (LEDs; as 10) in series with an inductance coil (11) and powered by a battery (12). Across the LEDs (in parallel therewith) is a transistor (13) that can become conducting or non-conducting dependent upon the control signal fed to its base from a "source" (14: the origins of the signal are not shown). The combination of inductance 11 and switching transistor 13 is the switched mode dc/dc converter.

The series of three LEDs 10 requires in toto about 6.6 volts before they will switch on.

Since this is more than the battery voltage, the LEDs can be left permanently connected across the battery 12 via the inductance 11 without passing a significant current.

Considering the circuit, it is clear that while the transistor 13 is "on" the current flowing through the inductance 11 increases linearly, while the potential across the LEDs is low (typically < ;0.5V), and they pass no current.

When the transistor is switched "off" the EMF across the inductance 11 reverses and adds onto the battery voltage (this conveniently reduces the voltage step-up ratio reqired from the inductance) driving the LED chain into conduction. The output voltage of the converter is automatically limited by the LEDs themselves to the required 6.6 volts. The current passed by the LEDs can be controlled by a negative feedback system of the type shown in Figure 2.

A schematic circuit of an LED current controller is shown in Figure 2. Base drive for the switching transistor 13 is provided via a base current limiting resistor (20) by a secondary winding (21) on the inductance 11 (now converted to a transformer). Bias is controlled an amplifier (22) via a buffer transistor (23) in emitter-follower mode. The control reference signal (from source 14) is compared with the feedback signal provided by a differential amplifier (24) with inputs connected across a current-sensing resistor (25) supplying the chain of LEDs. Since the LED current is pulsed it requires peak detection and smoothing. however, the appropriate diodes and capacitors between the sensing resistor 25 and the inputs of the amplifier 24, as well as the resistors to provide differential operation, have been omitted from Figure 2 for clarity.

Claims (6)

1. For use in a battery powered intensified nightsight having an illuminatable target graticule, a graticule illumination circuit arrangement including one or more illuminator powered from a switched mode dc/dc converter drivable by the night-sight's intensifier's battery.

2. A circuit arrangement as claimed in Claim 1, wherein the or each illuminator is a Light Emitting Diode (LED).

3. A circuit arrangement as claimed in either of the preceding Claims, wherein the switched mode dc/dc converter operates at a switching rate of up to 100 kHz, and there is used, as the switch therein, a transistor fed by a control signal produced by an oscillator.

4. A circuit arrangement as claimed in any of the preceding Claims, wherein there is supplied some feedback from the coil by providing a linked secondary coil, so making the combination a transformer, the output of which can be fed to a switch-control network.

5. A graticule illumination circuit arrangement as claimed in any of the preceding Claims and substantially as described hereinbefore.

6. A night-sight using a graticule illumination circuit arrangement as claimed in any of the preceding Claims.