GB2122366A

GB2122366A - Automatic liquid-crystal light shutter

Info

Publication number: GB2122366A
Application number: GB08216252A
Authority: GB
Inventors: Hsieh Tong-Shen
Original assignee: TONG SHEN HSIEH
Current assignee: TONG SHEN HSIEH
Priority date: 1982-06-03
Filing date: 1982-06-03
Publication date: 1984-01-11
Also published as: GB2122366B

Abstract

A liquid-crystal light shutter includes two photocells one behind hole 13b responds to direct light while the other behind hole 13a and black painted central portion 14a responds to light from the side. An electrical circuit comprising a NAND gate integrated circuit forms an oscillator, and the outputs of the oscillator are gated to liquid crystal layers in dependence upon the output of the photocells whereby the shutter automatically blocks light when strong direct light falls on it. <IMAGE>

Description

SPECIFICATION Automtic liquid-crystal light shutter This invention relates to an automatic liquid-crystal light shutter. It is especially useful when applied to a welder's helmet for the purpose of shutting off strong light but passing weak light, but it is also useable in the shutter of a camera, for space glasses, automobile rear view mirrors, etc.

According to the invention, there is provided an automatic liquid-crystal shutter comprising a liquid crystal for selectively passing or blocking light, an oscillator, a first photocell responsive only to direct light falling on the shutter, a second photocell responsive only to side light falling on the shutter, and gate means responsive to the outputs of the photocells to gate the output of the oscillator to the liquid crystal to cause the liquid crystal to block light when strong direct light falls on the shutter.

In its preferred form the light shutter comprises a NAND gate integrated circuit and an RC oscillator operating in cooperation with two photocells able to change their internal resistance on receiving light.

The change of the internal resistance of these two photocells makes the two output of the NAND gates of different potential or the same potential which results in the liquid crystal passing or blocking light.

Thus this automatic liquid-crystal light shutter can automatically pass weak light but can shut off strong light.

An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a front view of a shutter according to the invention applied to a welder's helmet; Figure 2 is a section through the upper part of the apparatus of Figure 1; Figure 3 is a section on the line A-A of Figure 1; Figure 4 is a complete circuit diagram of the apparatus; Figure 5 is a circuit diagram of the oscillator of the apparatus; Figure 6 is a pin diagram of the NAND gate integrated circuit 11 of Figure 4; Figure 7 is an equivalent circuit of the two photocells of the apparatus.

The automatic liquid-crystal light shutter shown in the drawings comprises a liquid crystal lightcontrolling portion and an electrial control circuit therefor. Referring to Figures 1, 2 and 3, all the components of the shutter are mounted on a base 16. In the case of use in a welder's helmet, the base must be placed at the level of the worker's eyes. The light controlling portion is shown in section in Figure 3: it comprises a light-polarising layer 1, a glass sheet 2 with a metal film 2a, a liquid crystal 3, a glass sheeet 4 with a metal film 4a and light-polarising layer 5 which together make up a first part of the light controlling portion. A similar second part of the light controlling portion comprises a light-polarising layer, glass 7 with a metal film 7a, liquid crystal 8, and glass 9 with a metal film 9a and light-polarising layer 10.The metal films 2a, 4a and 7a, 9a of liquid crystals 3 and 8 are connected with the electrical circuit; as Figure 4 shows, the metal films are connected to output leads 3 and 4 of NAND gate integrated circuit 11. When there is a potential difference between output leads 3 and 4, liquid crystals 3 and 8 allow light to pass through them; when there is no potential differene between output leads 3 and 4, liquid crystals 3 and 8 prevent light from passing through them. The principle of polarising light in liquid crystals is well known and does not need to be explained here.

Two photocells El and E2 are mounted in the upper half of the apparatus. Photocell E2 is mounted so as to receive only direct light and photocell El is mounted so as to receive only side light. As shown in Figures 1 and 2, both photocells El and E2 are soldered onto a printed circuit board 15. In front of the board 15 is an opaque board 13 with a large hole 13a in front of photocell El and a small hole 13b in front of photocell E2. A piece of glass 14 is mounted behind board 13 and part of the glass 14a in front of photocell El is painted black so that direct light cannot reach photocell El. On the other hand, photocell E2 can only receive direct light as there is a hole 15a in printed circuit board 15 facing hole 13b.

Figure 4shows the electrial control circuitforthe liquid crystal shutter. It comprises a NAND gate integrated circuit 11 and oscillator 12, the photocells El and E2 of 0.5 volts, variable resistance VR, coupling resistance R3, two mercury cells E3 of three volts and switch S. The NAND gate integrated circuit 11 is a type D401 C or the like and comprises four NAND gates. Two gates are used in oscillator 12 and the other two gate the outputs of the oscillator to the liquid crystals under the control of photocells El and E2. Specifically, when strong direct light is received the outputs are blocked so that the shutter does not pass light.

Oscillator 12 is an RC oscillator shown fully in Figure 5. It has two outputs which are supplied to pins 1 and 6 of the integrated circuit 11 and these outputs are of opposite phase, that is to say, when the input at lead 1 is high or 1 that at lead 6 is low or 0. Resistances R1 and R2 are 1 Mohm while capacitors C1 and C2 are 0.005 microfarads. The oscillator periodT= R1C1 = R2C2,andfrequencyf= lIT.

The variable resistance VR can be changed from 500 kohm to 2 Mohm. The coupling resistance R3 is around 330 kohm. The mercury cells E3 are the power source of the circuit and switch S opens or closes the circuit. The photocells can be considered to be variable sources VE1 and VE2 and variable resistances VR1 and VR2 combined in series as shown in Figure 7. Their internal resistance can change from 1 to 10 Mohms. The strength of light they receive changes their internal resistance and accordingly input leads 2 and 5 may become high or low, thereby controlling the light passing or blacking condition of the liquid crystals.

Now the operation of the shutter will be explained.

When switch S is turned on and photocell El does not receive strong side light and photocell E2 does not receive strong direct light, a potential difference arises between leads 3 and 4 which will enable the liquid crystals to pass light. The voltage at point A will be no less than 1 .SV after the voltage drop through VR and R3 and so the input at leads 2 and 5 will be high. Thus, for example, if the input at lead 1 is high the output at lead 3 will be low, the input at lead 6 will be low and consequently the output at lead 4 will be high. Thus, when the shutter does not receive any strong light from any direction it enables light to pass.

If a strong side light shines towards the shutter but not a strong direct light, the internal resistance of photocell El changes dropping from 10 Mohm to 1 Mohm and the voltage at the point A will become larger than 1.5 V; this condition gives rise to the potential between leads 3 and 4 in the manner just mentioned above so that the light shutter again passes light.

If a strong direct light shines towards a shutter, the internal resistance of photocell E2 changes and this change operates to make the voltage at point A less than 1.5 V; then the input to leads 2 and 5 becomes low so that the output at both leads 3 and 4 becomes high. Thus, there is no potential difference between leads 3 and 4 and so the liquid crystals block light.

Claims

1. An automatic liquid-crystal shutter comprising a liquid crystal for selectively passing or blocking light, an oscillator, a first photocell responsive only to direct light falling on the shutter, a second photocell responsive only to side light falling on the shutter, and gate means responsive to the outputs of the photocells to gate the output of the oscillator to the liquid crystal to cause the liquid crystal to block light when strong direct light falls on the shutter.

2. An automatic shutter as claimed in claim 1 wherein the oscillator is an RC oscillator providing pulses of reversed phase and wherein the gate means comprises a pair of NAND gates arranged to gate the respective osillator outputs to respective sides of the liquid crystal.

3. An automatic liquid crystal light shutter comprising a NAND gate integrated circuit, an RC oscillator, mercury cells as a voltage source, and two photocells, one of which only changes its internal resistance when receiving strong side light and the other of which only changes its internal resistance when receiving strong direct light; the RC oscillator supplying pulses of opposite phase to two gates of the NAND gate integrated circuit, the other inputs of thse gates being connected together and to the mercury cells via a variable resistance and the two photocells so as to be of high potential when the shutter does not receive a strong direct light so that these gates provide different potentials to the liquid crystal which then passes light; and when direct light stronger than the side light shines towards the photocell that can only receive direct light and makes the photocell change its internal resistance, the said other inputs become low so that the outputs to the liquid crystal become the same potential which in turn makes the liquid crystal block light.

4. An automatic liquid-crystal light shutter sub stantial ly as hereinbefore described with reference to the accompanying drawings.