GB2178531A - Improvements in light sensitive electronic switches - Google Patents

Abstract

The combination of a light sensitive electronic switch, in the form of a photo-transistor (4), with a solid state laser device (2), the laser chip (8) of which is capable of producing a narrow beam of a coherent light. The laser device (2) and the photo-transistor (4) are positioned on either side of the chopper disc (12) provided with equispaced apertures (14). The combination can be used in the electronic ignition system of an internal combustion engine, the photo-transistor forming the first stage of an inverse switching transistor chain. <IMAGE>

Description

SPECIFICATION Improvements in light sensitive electronic switches The present invention relates to improvements in light sensitive electronic switches.

One type of light sensitive electronic switch has been used in our dpto-electronic ignition systems. In our British Patent Specification 1,330,453, we have disclosed the use of an infra-red gallium arsenide lamp and photo-transistor. These elements are housed in respective "C" shaped housings which each also contain a convex lens. As shown in Fig. 5 of this Specification, the beams of infra-red radiation from the lamp are intended to cross at substantially the same point at which the edge of the chopper disc having equi-spaced blades or apertures is located.

In actual practice, it was found impossible to reduce the size of the infra-red beam at the crossing point by focusing because of the size of the radiating chip which did not constitute a point source of light.

As a result, this meant that the photo-transistor would switch off (become non-conductive) when the beam was partially covered by a blade of the chopper disc. If the light from the lamp was reduced or obscured by dirt the switching point would be earlier due to the beam width and the need to cover less of the beam before switching took place.

Whilst this was not particularly important, as pointed out in Specification No. 1,330,453, a aforementioned problem arises if the lenses become partially covered in oil or dirt since this can alter the ignition timing.

In view of the difficulty over not being able to focus a beam emitted by a non-point source, i.e. a square semiconductor chip, it follows that the beam width of a gallium arsenide solid-state lamp can never be smaller than the area of the emitting chip because all parts of the square surface of a typical chip are emitting radiation.

It is therefore an object of the present invention to overcome the above referred to disadvantages.

According to the present invention there is provided in combination a light sensitive electronic switch and a solid state narrow beam laser, arranged on respective sides of a chopper disc provided with alternate opaque and translucent regions around a peripheral zone, to enable the laser beam to be alternately blocked from reaching and transmitted to the electronic switch.

Preferably the light sensitive electronic switch is a photo-transistor. The photo-transistor preferably forms the first stage of an inverse switching trigger in which alternate stages of transistors switch in inverse relation to one another. The solid state laser may be a diode laser.

The present invention will. now be described in greater detail by way of example with reference to the accompanying drawings, wherein the sole figure shows a preferred form of the combined light sensitive electronic switch and solid state narrow beam laser designed to be used with an opto-electronic ignition system for internal combustion engines for obtaining an accurate count of crank-sh'aft rotation.

Referring to the drawing, the opto-electronic ignition system includes a lovQ powered solid state narrow beam laser 2 and a light sensitive electronic switch in the form of phototransistor 4. Any suitable type of narrow beam laser can be used and this includes a diode laser which consists of a p-n junction diode based on gallium arsenide. A narrow laser beam of coherent ligth 6 is emitted from a laser chip 8 of the solid state laser 2 and is directed towards a light receiving chip 10 forming the base electrode of the photo-transistor 4. Interposed between the laser 2 and the photo-transistor 4 is a chopper disc 12 driven from the cam-shaft of the internal combustion engine, the disc 12 being positioned such that a peripheral zone 14 of the disc cuts the laser beam 6.The peripheral zone 14 is effectively provided with alternate equishaped opaque and translucent sections right around its entirety, so that the laser beam is alternately transmitted to the photo-transistor, and blocked from reaching the light receiving chip 10 of the photo-transistor 4.

The translucent sections of the peripheral zone 14 of the chopper disc 12 may consist of slots or apertures cut in the disc which otherwise presents an opaque surface to the laser beam. Alternatively the disc can be made of a translucent material, e.g. Perspex (Registered Trade Mark) and opaque material in the form of equi-spaced radially aligned strips secured around the peripheral zone 14.

The above described combined light sensitive electronic switch and solid state narrow beam laser can be formed as a single unit for mounting in the distributor of the internal combustion engine. It is designed to operate with a fast inverse switching transistor circuit such as that disclosed in British Patent Specification No. 1,497,346 in which two stages of inverse switching Darlington pairs follow the photo-transistor. At least part of the circuit including the photo-transistor 4 may be monolithically formed on a single chip as disclosed in British Patent Specification No. 1,526,584.

The above described system enables the use of very small multiple chopper blades capable of discriminating single engine degrees.

Although it is preferred that the laser should emit a beam of monochromatic coherent light is the visible region of the electro-magnetic spectrum, it is possible to use a laser which emits a beam of coherent radiation in the infra-red region of the electro-magnetic spec trum provided a photo-transistor capable of receiving infra-red radiation is used as the light sensitive electronic switch.

Whilst the above embodiment has been described in connection with opto-electronic ignitions system, it will be appreciated that it is not limited thereto. For example the combined light sensitive electronic switch and solid state narrow beam laser can be used in connection with position transducers or shaft encoders.

Claims (8)

1. The combination of a light sensitive electronic switch and a solid state narrow beam laser, arranged on respective sides of a chopper disc provided with alternate opaque and translucent regions around a peripheral zone to enable the laser beam to be alternately blocked from reaching and transmitted to the electronic switch.

2. The combination according to claim 1, wherein the light sensitive switch is a phototransistor.

3. The combination according to claim 2, wherein the photo-transistor forms the first stage of an inverse switching trigger in which alternate stages of transistors or Darlington pairs switch in inverse relation to one another.

4. The combination according to any one of the preceding claims, wherein the solid state laser is a diode laser.

5. The combination according to any one of the preceding claims, wherein said chopper disc is opaque and is provided with equispaced slots or apertures around the peripheral zone.

6. The combination according to any one of the preceding claims 1 to 4, wherein said chopper disc is made of translucent material and opaque material in the form of equispaced radially aligned strips are secured around the peripheral zone.

7. The combination of a light sensitive electronic switch and a solid state narrow beam laser, constructed substantially as herein described with reference to and as illustrated in the accompanying drawing.

8. An electronic ignition system for an internal combustion engine incorporating the combination according to any one of the preceding claims.