Patent GB-1214981-A (GB)
Title: Control means using light filters
Inventor: HORROCKS JOHN
IPC: G08C23/04
Filed: 1967-05-05
Abstract: 1,214,981. Photoelectric door operating mechanism. J. HORROCKS. 4 July, 1968 [5 May, 1967], No. 21022/67. Heading E2M. [Also in Division G1] Photoelectric control apparatus comprises one or more polarising filters and two or more photoelectric cells which are arranged to produce a control signal only when light polarised in one or more particular directions of polarisation is incident thereon. The aparatus could be used to control movement of a machine tool, a window, lift, crane, &c., but is described with reference to the automatic opening of a garage door or doors. The arrangement of Fig. 2, for example, is mounted adjacent the door to be controlled and comprises a tubular housing 22 containing a protective glass lens or plate 24, a two-way mirror or red filter 26, to reduce the overall light intensity, a polarising filter 28 and two sulphide photoresistors 30, 32 one mounted in front of and one behind, the filter. The cells 30, 32 are connected in the collector-base and emitter-base circuits, respectively, of a switching transistor, (40) Fig. 6 (not shown), mounted on circuit board 34. When ordinary unpolarised light is shone onto the tube 22, the cells 30, 32 are substantially equally illuminated, having no effect on the transistor (40). However, if polarised light of a polarisation plane orientated at 90‹ to that of filter 28 is directed at the device, the cell 32 receives no light and the transistor circuit is unbalanced causing it to conduct and energise a relay resulting in starting of a motor controlling opening of the garage door. Light of this preferred direction of polarisation may be provided from the vehicle to be garaged 'by fitting the vehicle headlamp with a polarised lens of the required characteristics. In a second arrangement, Fig. 3 (not shown), two polarising filters (56), mounted with their polarisation directions at different angles, are positioned in front of two photocells (50, 52) and a further photocell (54) is provided with no polarising filter in front of it. The cells (50, 52) are conected in parallel in the anode circuit of a silicon controlled rectifier and the cell (54) in its cathode circuit, Fig. 7 (not shown). On illumination of the device by ordinary unpolarised light, again no triggering occurs, but light from a headlight having a lens with two polarising sections of different polarisation directions, causes triggering of the SCR since the light reaching cells (50, 52) through the filters is of much lower intensity than that incident on cell (54). The angular settings of filters (56) and the head-lamp lens sections can be adjustable to give unique operation. This selectivity is further improved if three filters and four photocells are used with a headlamp having three polarising sections.  To operate the headlamps, a press button switch on the vehicle is operated, being pressed continuously to keep the transistor of the Fig. 6 circuit energised, but only instantaneously for the SCR circuit. The latter may be switched off by a mechanical switch operated by the motor driving the door. A warning device may be connected to the circuits to be energised when unpolarised light is used to illuminate the photocell 32 or photocells (50, 52). Manual switching of the motor to open the door is possible, e.g. by the vehicle ignition key. Limit switches may provide automatic stopping of the motor when the door is opened or closed.