Patent GB-1425811-A (GB)
Title: Light pen
IPC: G06F3/0354
Filed: 1972-04-27
Abstract: 1425811 Light pens NATIONAL RESEARCH DEVELOPMENT CORP 19 April 1973 [27 April 1972] 19582/72 Heading G1A A light pen comprises an array of at least three photo-electric sensors and control means for comparing the output of each sensor with a reference. As shown, Fig. 3, a light pen co-operating with a computer controlled CRT display comprises a square array of nine light guides each terminating at a photo-detector 3 connected to an amplifier 4 the gain of which can be adjusted to compensate for differences in detector sensitivity. The outputs of the amplifiers 4 are connected to a peak level detection circuit 18 which tracks the highest input and produces a reference voltage which is somewhat below the peak value of the highest input. The outputs of the amplifiers 4 are compared with this reference level by comparators 5 so that an output is produced by at least one of the comparators. The comparator outputs are sampled by bi-stable latches when a bright up pulse is applied to the CRT spot, and the outputs of the latches are applied to a logic circuit 20 which produces UP, DOWN, LEFT and RIGHT signals which are used to move the CRT spot one increment to align it with the central element of the light pen array. A signal is produced on line 36 when the spot is centred. Alternatively the circuit may be arranged to provide an output on line 36 when the spot is anywhere within the field of view of the light pen. The logic is arranged so that if several photo-detectors are energized conflicting outputs, i.e. UP and DOWN, or LEFT and RIGHT, are suppressed. The increment of movement of the spot is selected to be a little smaller than the centre-to-centre spacing of the pick-up ends of the light guides to produce the smallest number of centring steps without oscillation or escape of the spot. If the light pen is rotated about its axis the CRT spot will still be centred but a larger number of incremental movements may be necessary. Correct operation is maintained up to a rotation of 45 degrees, accidental rotation greater than this being unlikely to occur due to the stiffness of the light pen construction. In a modification, Fig. 9 (not shown), the peak level detector is omitted, the comparators being provided with a fixed reference signal greater than the ambient light signal produced by the photodetectors. If the comparators are provided with individually adjustable reference levels these may be used instead of the adjustable gain amplifiers to compensate for detector sensitivity variations. The photo-detectors comprise photo-sensitive FETs acting as source followers and connected to a bi-polar transistor amplifier stage and an emitter follower output stage Fig. 6 (not shown). The peak level detector comprises an input diode gate to select the highest signal charging a capacitor to store the peak level. The capacitor voltage is tracked by a source follower and a compound emitter follower provided with an offset bias circuit arranged so that the output is about half a volt below the maximum input voltage, Fig. 5 (not shown). The required logic functions and the circuit to implement them, Fig. 8 (not shown) are given.