GB2153069A - Light pen - Google Patents

Abstract

A light pen for use with a home or personal computer comprises an elongate housing 12 having a collimator 16 projecting from its tip, for contacting the screen of a monitor or television connected to the computer. The collimator directs light from the screen onto a photodiode 26 within the housing, to produce a first output signal in response to this light. Additionally, the collimator is movable longitudinally of the housing against the bias of a spring 32 and is connected to a switch, so that pressing the light pen against the screen causes the collimator to operate the swich to produce a second output signal. <IMAGE>

Description

SPECIFICATION Photoelectric apparatus suitable for use as a light pen This invention relates to photoelectric apparatus suitable for use as a light pen, that is apparatus which can be used in conjunction with a computer and a visual display unit (VDU) such as a cathode ray tube in the electronic creation of pictures, patterns, characters or other computer-generated graphics on the VDU.

Known light pens for use with a computer and a VDU typically comprise a photoelectric device adapted to be positioned close to the VDU and connected via a cable to an amplifier in the computer. Typically they also include a separately operable switch, for example a thumb-operated switch, for sending a control signal to the computer, to instruct it to modify the display on the VDU in some predetermined manner. These known light pens suffer from a poor signal-to-noise ratio, and are ergonomically unsatisfactory by virtue of the separately operable switch. It is an object of the present invention to provide photoelectric apparatus which is suitable for use as a light pen and which in its preferred embodiments alleviates one or both of the drawbacks of the known light pens.

According to the present invention, there is provided photoelectric apparatus suitable for use as a light pen, the apparatus comprising: an elongate housing open at one end; a collimator projecting from the open end of the housing, and extending longitudinally into the housing from said open end; photoelectric means disposed inside the housing to receive light via the collimator, for producing a first output signal in response to the light received; and selectively operate switch means for producing a second output signal; wherein the collimator is movable longitudinally of the housing, and is coupled to the switch means to operate it upon movement of the collimator into the housing.

Thus if the projecting end, or tip, of the collimator is placed in contact with the screen of a cathode ray tube or other VDU, the apparatus produces a first output signal representative of the amount of light or illumination present at the point of contact. Assuming that the screen is that of a raster-scanned cathode ray tube which is illuminated to some extent all over, this first output signal is indicative of the position of the tip of the collimator of the apparatus on the screen. If the apparatus is then pressed against the screen, this operates the switch means, so causing it to produce the second output signal.This second output signal can be used in the creation or modification of pictures, patterns, characters or other graphics to be displayed on the VDU, typically by creating lines following successive points of contact of the tip of the collimator as the apparatus is moved across the screen-hence the name "light pen".

In a preferred embodiment of the invention, the photoelectric means comprises a photoelectric device, such as a photodiode, and an amplifier connected to receive and amplify the output of the photoelectric device to produce said first output signal.

The inclusion of the amplifier in the apparatus considerably enhances its signal-to-noise ratio.

Advantageously, the photoelectric means forms part of an assembly which is mechanically coupled to the collimator, to move therewith.

Preferably, the assembly including the photoelectric means is movable longitudinally into the housing with the collimator against the action of a spring, and includes a control terminal arranged to contact a spring-loaded terminal mounted in the housing as the assembly is moved into the housing, said terminals forming part of the switch means. In this case, the switch means preferably further comprises a semiconductor switching circuit which forms part of the same assembly as, and shares the same power supply as, the photoelectric means, and which is arranged to be operated by the contact of said terminals.

Conveniently, the photoelectric means and the semiconductor switching circuit are together implemented as a single hybrid circuit.

The invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a somewhat schematic sectional view of a light pen in accordance with the present invention; and Figure 2 is a circuit diagram of a hybrid circuit forming part of the light pen of Fig. 1.

The light pen shown in Fig. 1 is indicated generally at 10, and is primarily intended for use with a Commodore VIC 20, a Commodore 64 or an Atari personal computer, connected to a colour monitor or a colour television.

However, use with other computers and VDUs is possible, with little or no modification of the pen 10.

The light pen 10 comprises a hollow cylindrical metal housing or body 12, eg of steel, open at one end 14. Projecting from the open end 14, and coaxially aligned with the body 12, is an elongate, cylindrical, plastics collimator 16, which extends longitudinally back into the body 1 2. The end of the collimator 1 6 disposed within the body 1 2 is an interference fit within a cylindrical moulded nose cone 18, which is movable axially within the body 1 2 as will hereinafter become apparent.

The nose cone 1 8 has an axially extending bore 20 aligned with and forming a continuation of an axially extending bore 22 in the collimator 16, the two bores 20, 22 together performing a collimating function.

Mounted in a recess 24 in the end of the nose cone 1 8 remote from the collimator 16, so as to receive light via the aligned bores 22, is a photodiode 26 forming part of a hybrid circuit 28. The hybrid circuit 28 mechanically interconnects the nose cone 1 8 with a further cylindrical moulding 30, which is also movable axially of the housing: the collimator 16, the nose cone 18, the hybrid circuit 28 and the moulding 30 thus form a single assembly, which is slidable axially of the housing against the action of a spring 32. The spring 32 is entrapped between the moulding 30 and a moulded closure member 34, which fits into and closes the other end of the body 1 2.

The hybrid circuit 28 includes an amplifier for amplifying the output signal produced by the photodiode 26, and a semiconductor switching circuit, as will be described hereinafter with reference to Fig. 2. To this end, the cylindrical moulding 30 is effectively an electrical connector, and includes a number of connector spills or pins 36 extending therethrough. The pins 36 are connected at their leftmost ends (as viewed in Fig. 1) to power supply inputs of the hybrid circuit 28, to the output of the amplifier therein, and to an input of the aforementioned semiconductor switching circuit. At their rightmost ends (as viewed in Fig. 1), all the pins 36, except the one connected to the input of the semiconductor switching circuit, are connected to respective wires of a multi-wire output cable 38.

These wires are long enough to cater for the aforementioned axial movement of the moulding 30, while the end of the cable 38 is effectively clamped to the moulded closure member 34 by a clamping ring 40.

The pin connected to the input of the aforementioned semiconductor switching circuit is indicated at 36a, and is bent over to form one terminal 42 of a switch 44, whose other terminal 46 is spring-loaded and soldered into a metal clip 47 which fits over the leftmost end of the closure member 34. The terminal 46 is connected to one of the wires of the cable 38, as will become apparent hereinafter.

The cable 38 is a coiled, extensible, cable, and has a connector 48 suitable for connection to the joystick port of the aforementioned Commodore 64 computer.

The circuit diagram of the hybrid circuit 28 is shown in Fig. 2. As can be seen from Fig.

2, the photodiode 26 is coupled to the base of a PNP transistor TR1, which is effectively connected in the grounded emitter configuration to form a first amplifying stage of the aforementioned amplifier. The collector of the transistor TR 1 is AC-coupled to the base of an NPN transistor TR2, which is again connected in the ground emitter configuration and forms a second amplifying stage of the aforementioned amplifier. Finally, the collector of the transistor TR2 is AC-coupled to two further amplifying stages comprising two NPN transistors TR3 and TR4 connected in cascade, again both arranged in the grounded emitter configuration. The transistors TR3 and TR4 complete the aforementioned amplifier, the collector of the transistor TR4 constituting the output 50 of this amplifier.The transistor TR4 produces at the output 50 a TTL-compatible logic 1 or 0 output signal in dependence upon whether the photodiode 26 is illuminated or not.

The aforementioned semiconductor switching circuit in the hybrid circuit 28 is based upon an NPN transistor TR5, which is again connected in the grounded emitter configuration and whose collector constitutes the output 52 of the switching circuit. The transistor TR5 is normally biassed into its conductive state, so that it produces at the output 52 a TTL-compatible logic 0 output signal. However, the base of the transistor TR5 is connected to the aforementioned pin 36a bearing the contact 42 of the switch 44. The other terminal 46 of the switch 44 is grounded (ie connected to the 0 volt supply rail). Thus contact between the terminals 42, 44 renders the transistor TR5 non-conductive, causing it to produce at the output 52 a TTL-compatible logic 1 output signal.

It will be appreciated that the amplifier based on the transistors TR1 to TR4 and the semiconductor switching circuit based on the transistor TR5 share the same 5 volt power supply.

In use, the light pen 10 is first plugged into the computer, and then the tip of the collimator 1 6 is simply placed on the screen of a colour monitor or colour television whose display is being controlled by the computer: the computer has of course already had a suitable graphic-creating program loaded into it. Each time the raster scan of the monitor or television moves the electron beam over the position on the screen where the tip of the collimator 1 6 of the light pen 10 has been placed, the photodiode 26 is illuminated via the collimator, causing the transistor TR4 to produce a logic 1 output pulse whose timing tells the computer the position of the tip of the pen on the screen. If the user then presses the light pen 10 against the screen, the assembly comprising the collimator 16, the nose cone 18, the hydrid circuit 28 and the moulding 30 moves back into the housing, so closing the switch 44. This tells the computer to create a spot of a predetermined colour at the position where the pen is being held. If the light pen 10 is then moved, while still held pressed against the screen, it appears to draw a line of the predetermined colour on the screen.

It will be appreciated that the light pen 10 is particularly easy to use: all the user has to do to "write" or "draw" on the screen of the monitor or television is to press the pen against the screen while moving it, in a manner exactly analogous to normal writing or drawing. The mounting of the hybrid circuit 28 containing the amplifier based on thus transistors TR 1 to TR4 in the pen 10, along with the photodiode 26, provides the pen with an excellent signal-to-noise ratio, the avoiding spurious triggering of the computer.

The screening provided by the metal body 1 2 further enhances the signal-to-noise ratio of the pen.

Several modifications can be made to the described embodiment of the invention. For example, the hybrid circuit 28 can take forms other than that specifically shown in Fig. 2. In particular, one or both of the relationships between the respective logic outputs and the illumination of the photodiode 26 and the closure of the switch 44 can be reversed if desired (eg logic 0 at the output 52 can be arranged to represent closure of the switch 44). Further, the circuit 28 need not be movable with the collimator 1 6 and nose cone 18: instead it can be fixedly mounted in the body 12, in which case, the bores 20, 22 can if desired be provided with a coaxially disposed, spring-loaded, telescoping extension tube, which extends the collimating action of the bores 20, 22 to the photodiode 26.

Claims (9)

1. Photoelectric apparatus suitable for use as a light pen, the apparatus comprising: an elongate housing open at one end; a collimator projecting from the open end of the housing, and extending longitudinally into the housing from said open end; photoelectric means disposed inside the housing to receive light via the collimator, for producing a first output signal in response to the light received; and selectively operable switch means for producing a second output signal; wherein the collimator is movable longitudinally of the housing, and is coupled to the switch means to operate it upon movement of the collimator into the housing.

2. Apparatus as claimed in claim 1, wherein the photoelectric means comprises a photoelectric device and an amplifier connected to receive and amplify the output of the photelectric device to produce said first output signal.

3. Apparatus as claimed in claim 2, wherein the photoelectric device comprises a photodiode.

4. Apparatus as claimed in any preceding claim, wherein the photoelectric means forms part of an assembly which is mechanically coupled to the collimator, to move therewith.

5. Apparatus as claimed in claim 4, wherein the assembly including the photoelectric means is movable longitudinally into the housing with the collimator against the action of a spring, and includes a control terminal arranged to contact a spring-loaded terminal mounted in the housing as the assembly is moved into the housing, said terminals forming part of the switch means.

6. Apparatus as claimed in claim 5, wherein the switch means further comprises a semiconductor switching circuit which forms part of the same assembly as, and shares the same power supply as, the photoelectric means, and which is arranged to be operated by the contact of said terminals.

7. Apparatus as claimed in claim 6, wherein the photoelectric means and the semiconductor switching circuit are together implemented as a single hybrid circuit.

8. A light pen substantially as herein described with reference to Fig. 1 of the accompanying drawings.

9. A light pen substantially as herein described with reference to Figs. 1 and 2 of the accompanying drawings.