GB2056729A - A visual display unit - Google Patents

Abstract

A visual display unit, e.g. for teaching, has a plurality of e.g. lamps connected to be switched on in an order determined by a changeable memory addressed by a counter, and in response to operation of a control switch (10 or 14). The memory may be a ROM, or a RAM loadable from a keyboard. <IMAGE>

Description

SPECIFICATION A visual display unit The present invention relates to visual display units.

One of its aims is to enable an operator to illuminate or otherwise change the brightness of successive different regions of a display area in a pre-programmed order by means of a simple switch. With this aim in view, the present invention is directed to a visual display unit comprising a plurality of lamps or other visual indicators, a memory device having a plurality of cells in which respective data entries can be stored, each data entry being representative of one of the visual indicators, and signal generating means connected to successively address different cells of the memory device in a predetermined sequence, in which the memory device is connected to control operation of the visual indicators whereby that one of the latter which is represented by the data in the cell being addressed at any given instant is switched on, so that the order in which the visual indicators are switched on can be altered by changing the stored data.

One use of such a unit is as a teaching aid.

For this the unit may have a random access memory (RAM) as the memory device, and also a keyboard with program switches respectively associated with the visual indicators. The program switches are connected to feed data into the RAM, so that depression of those switches in a certain order will result in illumination of the indicators in the same order on subsequent operation of the control switch. The order can be altered simply by depressing the program switches in the new desired order.

Another use of the unit is as a communication aid, in which case a further set of lamps or other visual indicators would be connected in the unit to be switched on in the same way as the first set, but the further set would be located remotely from the rest of the unit in a place to which information is to be conveyed.

For example, the further lamp set might be in the kitchen of a house with the rest of the unit in a bedroom.

In a third application of the invention, for environmental control, the display unit includes a distribution box connected to operate in dependence upon which visual indicator or indicators are selected by the user. Outputs from the distribution box lead to a number of different appliances, such as a television, a telephone, an electric fire, a music centre and an electric fan. When one of the visual indicators is left on for longer than a predetermined delay period, the distribution box switches on the associated appliance. The appliance is switched off the next time the associated indicator is switched on for the predetermined period. Some appliances may need an additional indicator lamp to show when they are switched on. In this application of the invention, the memory device may be a read-only memory (ROM).To change the program the whole memory device would be replaced by a different one.

Two examples of a visual display unit in accordance with the present invention are illustrated in the accompanying drawings, in which: Figure 1 is a front elevational view of a first unit; Figure 2 is a rear elevational view of the unit shown in Fig. 1; Figure 3 is a side elevational view of a display box of the unit shown in Fig. 1; Figure 4 shows circuitry incorporated in the display box of the unit shown in Fig. 1; Figures 5 and 6 are timing diagrams of the circuitry shown in Fig. 4 for two respective modes of operation of the circuitry; Figure 7 is a front elevational view of a modified display unit; Figure 8 is a rear elevational view of the unit shown in Fig. 7; Figure 9 is a side elevational view of display boxes of the unit shown in Fig. 7; and Figure 10 shows circuitry incorporated in the upper box shown in Fig. 9.

A display box 2 shown in Figs. 1 to 3 measures 1 1 T" (29 cms) wide, 10" (25 cms) high and 8" (20 cms) deep. It has a translucent face 3 behind sixteen areas 4 of which are respective electric lamps or other visual indicators. These areas are in a 4 X 4 matrix array. A 1 2 volt battery pack 6 is connected to power the lamps, and circuitry within the box 2, via a lead 8. Alternatively, a 1 2 volt mains power unit may be used. Control switches 10 and 1 4 are connected to the display box 2 by means of leads 1 2 and 1 6 respectively. Each control switch comprises a cap which can be depressed to operate a micro-switch.Switch 10 is for turning on successive different lights in a pre-programmed order, and switch 14 for turning them on successively in reverse order.

Fig. 2 shows how the leads 8, 1 2 and 1 6 are connected to the display box 2 by jack plugs 18, 20 and 22 respectively. It also shows a keyboard 24 on the rear of the display box 2 comprising sixteen program switches 26. Each program switch is associated with an individual lamp.

The translucent face 3 of the control box 2 is inclined at 55 to a base of the box, and thus to the horizontal when the box is rested on a horizontal surface, as illustrated in Fig.

3. The box 2 may have two main rear faces, one of which is inclined at 30 to the translucent face 3 and the other at 60 , the box being supportable on a horizontal surface on either one of these rear faces to give alternative slants to the translucent face 3. If de sired, a block with a lower main face and a Vcut in its upper side with one wall of the V at ,30 to the lower main face and the other at 60 may be used to support the main box with its translucent face horizontal.

One mode of operation of the illustrated unit is as a teaching aid. In this mode, a teacher of a handicapped child or children may place a tri-acetate film over the translucent face 3 on which film are drawn, printed or stuck, for example, a number of different figures and symbols, one figure or symbol to each area 4. It will be appreciated that one of a large selection of acetates, having a whole range of different letters, figures, symbols, shapes or pictures on them, may be placed on the face of the display box, depending on what subject the teacher wishes to deal with.

We will assume here that the subject selected is arithmetic, and that numbers and simple arithmetical symbols (such as " +", " " - ", '' . '', " and " perhaps another symbol like a question mark "?") appear on the acetate. The teacher can now program the unit to perform any sum or other calculation for which the appropriate symbols appear on the acetate. Supposing the sum is 4 - 1 = 3.

That program switch of the keyboard 24 is pressed which corresponds-to to''4", then the one for " -", and so on, until the sum is completed. Subsequently, as a user taps the control switch 10, the sum is spelt out by the correct sequence of illumination of the lamps in the display box 2. Tapping the switch 14 will cause the lamps to be illuminated in the reverse order.

When the sum has been repeated, or a few different sums have been repeated, and the child or children have had an opportunity to learn-them, and teacher may re-program the unit to spell out "4 - 1 = ", stopping short of the answer of the sum, and follow this by, for example, the question mark "?" to show that an answer is to be supplied, and then several different numbers among which is the correct answer, like "1", "9", "3" and "7".

Having programmed the unit, the teacher presses the control switch 10 five times, so that the last-symboi to be illustrated is the question mark, and then the child takes over and keeps tapping the control switch until the number is illuminated which he believes to be the correct answer.

Fig. 4 shows the circuitry incorporated in the control box 2 which enables the unit to fundtion in the foregoing manner. The sixteen program switches have respective lines leading to a keyboard encoder 28 which has four binary data outputs A1, B1, C1 and D1. Thus each of the sixteen program switches can be represented by a distinctive combination of signals on these data outputs, which are connected respectively to data inputs 11, 21, 3 and 41 of a random access memory (RAM) 40. The keyboard encoder 28 also has a data available output p connected to an inverter 30 and also to one input of a NOR gate 32. Two further inputs to the NOR gate 32 are fed respectively by inverters 56 and 58 which, in turn, are connected to receive signals from the control switches 10 and 1 4 shown in Figs. 1 and 2.The output r of the NOR gate leads to respective inputs of two mono-stable multivibrators of flip-flops 34 and 36. Both of these are triggered by a falling, trailing edge of an input signal. Flip-flop 34 has a time characteristic 71 of 200 seconds, and flip-flop 36 a time characteristic r2 of 600 seconds.

The Q output s of flip-flop 36 is connected to a control input of a binary up/down counter 38. Lines form four binary data outputs Q1, Q2, Q3 and Q4 of the counter 38 lead to four respective address inputs A2, B2, C2 and D2 of the random access memory (RAM) 40.

The 0 output t of the flip-flop 36 is connected to the input of a third flip-flop 42. This flipflop is also triggered by a falling, trailing edge, and has a time characteristic 3 of approximately 1400 seconds. The Q output u of flip-flop 34 is connected directly to a not memory enable input ME of the RAM 40.

The Q output vof flip-flop 42, and the output c of the inverter 30 lead to respective inputs of 9 second NOR gate 44. The output wfrom this gate is connected to another inverter 46 having an output x connected to a not write enable input (WE) to the RAM 40.

Signals at four binary data outputs 1", 2", 3", and 4" of the RAM 40 are fed to a binary decoder 48 which is connected to operate lamp drivers 50 for successively turning on different lamps of-the display box.

An additional passive display box (not shown) may be incorporated in the unit where communication with someone at a location remote from the main display box is desired.

The additional box would have its own binary decoder 52 connected to operate a further set of lamp drivers 54 of the passive display box.

The operation of the circuitry during programming is illustrated in Fig. 5. At time t1, one of the sixteen program switches is depressed. At time t2, occurring later than t1 by a delay period determined by the debounce capacitor 60, the voltage of the data available output p of the keyboard encoder 28 rises to the "1" level. This causes the voltages of the q and r outputs of the inverter 30 and NOR gate 32 respectively to drop to the "0" level.

The falling edge of the signal at the output triggers flip-flops 34 and 36. The voltages of the Q outputs t and U go to the "1" level, while -the voltage of then output s drops to the "0" level. Thus, at this point in time, u is at "1", so that the ME input prevents the RAM 40 from operating.

At time t3, spaced from t2 by the delay characteristic 1 of the flip-flop 36, s goes to "1" '' and t drops to "O". The rising edge on s puts the counter 38 forward by one step, so that the next cell in the RAM 40 is addressed.

At the same time, the trailing edge on output tflips the flip-flop 42, putting its 0 output v down to the "0" level. Since the output c was already at the "0" level, NOR gate 44 no longer has a "1" signal applied to it, and w therefore jumps to the ''1" level, therefore falling to the "0" level. The write inhibiting signal is therefore removed from the ram 40, and as soon as the Q output vof flip-flop 34 drops to "O" at time t4 (T2 after t2), enabling the RAM 40 to function, the binary data at inputs 11, 21, 31 and 41 is registered in the memory cell being addressed at that instant by the counter 38. Simultaneously, the same binary data is fed through the outputs 1", 2", 3", and 4" to the binary decoder 48. This operates the appropriate lamp driver to turn on the particular lamp in the display box represented by the data.

Another of the sixteen program switches is now pressed, and the process illustrated in Fig. 5 is repeated, but this time the data associated with the program switch now being pressed, and with the corresponding lamp, is fed into the next memory cell in the RAM 40.

When all sixteen program switches have been pressed in a given order, or when only some of the switches have been pressed but all the memory cells have been addressed by pressing some of the program switches more than once, that order is now stored in the RAM 40.

The display unit is thus programmed and ready for a read operation as illustrated in Fig.

6.

Referring to this Figure, at time t5 the control switch 1 0 (shown in Figs. 1 and 2) is tapped so that the output ytherefrom gives a signal which drops to the "0" level, the output ztherefore rising to the "1" level.

Since a "1" level is now present at one of the inputs to the NOR gate 32, the output rdrops to the "0" level, resulting in the same changes in the levels of outputs s, t and u as shown in Fig. 5. However, the output qfrom inverter 30 remains at the "1" level, because there is no "1" signal on p from the keyboard encoder 28. As a result, even when output v gives a "0" signal, the "1" signal on q keeps wand "0" and xat "1". The WE input of RAM 40 therefore inhibits the write operation, and when u falls at time t7 (T2 after t5), only a read operation ensues, the data in the memory cell which is addressed at that time being transmitted to the binary decoder 48 to light the lamp represented by that data.

The next time the control switch is tapped, the process illustrated in Fig. 6 is repeated, but the counter 38 will have been stepped forward to address the next cell in the RAM 40, so that the next lamp in the programmed order is turned on. As this process is continued, only one lamp is turned on at a time.

If the reverse control switch 14 is tapped, the operation in Fig. 6 is brought about, but the next preceding cell of the memory is addressed instead of the next following cell, by virtue of a connection between the switch 14 and an UP/DOWN input to the counter 38.

It will be appreciated that the order in which the lamps are turned on may be readily changed by re-programming the unit.

To save repeated tapping of the control switches, each may be provided with a square-wave pulse generator having a frequency of, say, 1 second. The pulses are emitted for as long as the control switch is depressed, so that the sequential lighting is advanced so long as the switch is depressed, and stopped on release of the switch. Conversely, the switch could be designed to advance the lighting operation until the switch is depressed.

In the environmental application referred to earlier, the display unit described may have a read-only memory in place of the RAM 40.

This would make the keyboard encoder 28, inverter 30, flip-flop 42, NOR gate 44 and inverter 46 redundant, but the other parts of the circuitry are used in this mode. Further circuitry could be incorporated so that a selected sub-routine would temporarily override the normal functioning of the unit's circuitry.

The selection might be made by turning on a lamp, associated with that sub-routine, for a predetermined period. For example, that lamp may illuminate a symbol of a telephone, whereafter, during the sub-routine, the different lamps represent different telephone numbers. When one is left on for a predetermined time, the number is dialled automatically. The sub-routine is ended when the telephone lamp is again lit up.

The display box 2 can be stood on a desk, mounted on a wall, or hung from the ceiling.

It may also be placed in a stand in such a manner that its front 3 lies horizontal, allowing various different objects to be placed in the individual areas 4.

The control switches 10 and 1 4 do not have to take the form illustrated. Any switch may be used which enables a handicapped child to operate the unit. For example, blowpipe switches may be used. A blow-pipe switch is one which is turned on while air is urged into it.

A number of display units could be coupled together to enable a teacher to teach a class of children. It may then be desirable to adapt the units so that only the teacher's can effect programming, whilst the other units can perform read-only functions independently.

The modified unit illustrated in Figs. 7 to 10 has the majority of the features of the unit illustrated in Figs. 1 to 4, and like parts bear the same reference numerals in the two sets of Figures. In addition, the modified unit has an add-on auxiliary display box 59 mounted on the top of the main box 2 by means of a clip 74. Four rubber feet 75 ensure stability of the auxiliary box 60. The latter comprises four display lamps for illuminating four areas 76, a set program switch 61, an indicator 62, a set answer button 64, a store button 65, a thumbwheel or number switch 66 and a four by-four miniature light-emitting diode matrix display 63. The buttons 64 and 65 are provided by respective single-contact push-to make switches.A jack socket 70 is also provided on the auxiliary box 59 into which can be plugged a jack 71 on the end of a lead 72 from an extra child switch 73. Thus the modified unit may be used with up to three child control switches 10, 14 and 73.

The main box 2 and the auxiliary box 60 are interconnected by a seven-line lead 67 extending from the auxiliary box to a plug 68.

The main box 2 is provided with a socket 69 to receive the plug 68. The seven lines com prise a positive and a negative supply rail, four lines respectively connected to the four binary data outputs of the memory 40 shown in Fig. 4, and one line connected to the output rof the gate 32.

Other modifications to the main display box 2 involve an extra socket 20a for receiving a jack-plug 20 or 22 of one of the control switches 10 or 14, and slide switches 21 a and 22a for altering the mode of operation of the unit. The use of these modifications is described in detail later herein.

The modified unit is programmed for operation as follows. Firstly, the main box 2 is plugged into the mains and switched on. One of the sixteen areas of that box lights up as a result, and also the extreme left-hand area 76 of the auxiliary box 59 (viewing the latter from the front). At the rear of the unit, one of the light-emitting diodes of the miniature matrix display 83 comes on. The main box 2 is programmed as already described herein. Program switch 61 is then set in its "on" position, whereupon the programmer indicator lamp 62 lights up. The thumbwheel switch 66 is now turned to position "1", to program the unit to deal with the question allotted to the extreme left-hand area 76 of the box 59.

For example, an acetate might be placed over that area with a transfer showing a man stuck onto the acetate. In that case an acetate would cover the display area of the main box with a number of different words written on the acetate to register with respective areas of the display of the main box 2, these different words including "MAN". The question posed is, which area of the main box lights up the word associated with what is illuminated by the auxiliary box 2. Put another way, the child or pupil is required to indicate when he or she thinks the word lit up by the main box 2 corresponds with what is for the time being illuminated by the auxiliary box 59. To program the unit to tell the child or pupil whether the answer given is correct, set answer button 64 is depressed. Each time it is depressed, the miniature l.e.d. matrix 63 is advanced one step.When the light emitting diode corresponding to the area of the main box 2 which bears the correct answer (i.e. "MAN") is illuminated, store answer button 65 is depressed. This stores the number of the correct main box area in the first cell or location of a random access memory of the auxiliary box, whereupon the first or extreme left-hand auxiliary box area or square 76 (acting as a memory location lamp) lights up or remains lit up to show the answer has been correctly stored. The thumbwheel switch 66 is then turned to its "2" position, and the process is repeated to store the answer to the next question in the random access memory of the auxiliary box.For example, the next acetate square on the auxiliary box 59 may be a picture of a car, in which case the number of the acetate square on the main box 2 which has the word "CAR" on it will be stored in the second memory location of the random access memory of the auxiliary box. The process is repeated for the third and fourth squares of the auxiliary box acetate. After the store answer button 65 has been depressed for storing the answer to the fourth question, the fourth auxiliary-box lamp lights up to show correct storage, and since all four answers are then in the auxiliary box memory, the unit automatically reverts to switching on the first auxiliary-box lamp at the extreme lefthand side viewing the box from the front.

Once programmed, the unit is ready for use. The set programme switch 61 is put back in its "off" or run position. The program indicator lamp 62 goes out as a result. The child or pupil can then advance through the pre-programmed lighting sequence of the display areas on the main box 2. There are two alternative ways for doing this, as foilows: (a) single step control, in which slide switch-22a is placed in its left-hand position and switches 10 and 73 are plugged in to the unit. Each time switch 10 is closed, the next main box matrix area lamp in the lighting sequence is switched on and the previous lamp is switched off. When the child or pupil believes that the correct lamp 4 is on, he or she indicates this by pressing the switch 73.

If a reversing facility is required, switch 14 is also plugged in, so that when the latter is pressed the immediately previous lamp in the lighting sequence comes on.

(b) Automatic scan, in which the slide switch 22a is placed in its right-hand position and a switch, for example the switch 73, is plugged into the socket 20a. With this setting, switching on of the lamps 4 proceeds through the pre-programmed lighting sequence automatically until the plugged-in switch 73 is pressed.

With either method of advance through the lighting sequence, a chime (not shown in Figs. 7 to 9) is connected to be played, after a two second delay in case (b) above, if the pupil selects the correct main box area 4 by depressing the switch 73 at the right time. A buzzer (not shown in Figs. 7 to 9) is also connected to sound instead of the chime if the wrong area is selected. After the buzzer sounds, the lighting sequence of areas in the main box 2 continues with the same lamp of the auxiliary box 59 illuminated. Once the correct answer is given, the next lamp on the auxiliary box 59 is illuminated and the answering procedure is repeated.

The circuitry for the auxiliary box 59 is shown in Fig. 10. It can be divided into three main sections: (a) Memory with associated timing and address logic.

(b) Processing logic.

(c) Display interface.

(a) Memory with associated timing and address logic.

With the thumbwheel or set number switch 66 set at position "1", the inputs to the address counter 80 are set at zero to ensure that the number of the main box area corresponding to the first answer will be stored in the first location of the random access memory 82. The set answer button 64 is then pressed. This causes an input pulse to be fed to a clock input of a counter 84. This is done via a capacitance and resistance circuit (not shown) having a charge time of 2.2 ms, and an integrated circuit (not shown) operating as a Schmitt trigger to ensure contact bounce elimination from the button 64. Each time the button 64 is operated, the counter 84 is advanced. The data outputs of the counter 84 are connected to the memory 82 to provide the data inputs for the latter.Also the output from the Schmitt trigger of the button 64 is connected to the ME memory enable input of the memory 85, which inhibits the memory operation thereof until the Schmitt trigger goes to zero level. The data outputs of the counter 84 are also fed to a decoder 86. This is coupled to a pair of lamp drivers 88 and 90 which illuminate any selected one of the lightemitting diodes of the miniature four-by-four matrix 63. Each light-emitting diode corresponds in position to an area or square on the main teaching aid box 2. Thus any square can be chosen as the answer to a problem. On release of the set answer button following depression thereof, the data in the counter 84 is ready to be loaded into the memory 82.

The switch 61 normally provides an inhibit for the data counter 84 to ensure that the answer cannot be altered when the program is running, and has to be switched to its "on" position to enable the data counter 84.

Store answer button 65 is also provided with a capacitance and resistance circuit (not shown) and an integrated circuit (not shown) acting as a Schmitt trigger to eliminate contact bounce. The button 65 is connected to initiate three further operations when depressed. (i) A positive pulse provides an inhibit to the memory 82 via the diode 92 to the ME input of the memory 82. (ii) It operates a preset enable input 94 to the address counter 80, which causes the code of the setting of the thumbwheel or number switch 66 to be delivered to the four outputs represented by one line 96 of the address counter 80. In this way, the correct location in the memory 82 is addressed for the storage of data. (iii) A delay is initiated via capacitance and resistance circuits (not shown) and Schmitt triggers (not shown) in the memory control logic 98 to produce a negative pulse to WE of the memory 82.With the WE negative, the negative edge of the ME input pulse causes the data present on the four-line input 100 to the memory 82 to be written into the location specified by the address on the outputs 96. The switch 61 is connected via the memory control logic 98 to provide an inhibit to the memory 82. In the run mode the output from the memory control 98 is held at '1' thus inhibiting WE and ensuring that answers cannot be overwritten in the memory 82.

(b) Processing logic The main component of this section is a 4bit comparator 1 02. This compares the complement of the data stored in the memory 82, which appears on a four-line output 104 fed to respective inputs to the comparator 102, with the data on the four-line connection 106 to the main memory 40 of the teaching aid via the plug 68. If the input from the main memory 40 of the teaching aid matches the answer stored in the first location of the memory 82 an equals or correct answer signal is produced on an output 108 from the comparator 1 02.If the inputs do not match, an error signal is produced on an output 110 of the comparator 1 02. These signals are used to produce a chime on a chimer 11 2 via logic circuitry 11 4 for a correct answer, and a buzz on a buzzer 11 6 via logic circuitry 11 8 for an incorrect answer. These signals are initiated in the following manner. A signal on line 1 22 from gate 32 shown in Fig. 4, in the case of automatic scanning, keeps a capacitor in the comparator 102 charged.When the scanning is stored, the capacitor discharges over a period of about 2 seconds after which an inhibit signal on outputs 108, 110 is lifted and the chime 11 2 or buzzer 116 will sound depending upon whether the scanning has been stopped at the correct lamp in the main box 2. If an equals signal is issued from the output 108, circuitry in the comparator 102 is also arranged to produce an output signal on a line 1 24 to the address counter 80. The latter advances switching on the memory 82 to consider the next location in that memory.

Reset circuitry in the comparator 102 then reverts the signal on the output 108 to zero ready for the next scanning through the light ing sequence for the main box- 2.

If, instead, an error signal is issued from the output 110, buzzer 11 6 is operated. A D flip flop in the comparator 102 is connected to be set and reset in such a manner as to ensure that when the unit is indexed to the next question the automatic advancing switch plugged into socket 22a will have to be released and repressed before the chime or buzzer will be allowed to sound again.

For single-step control using the switch 73 plugged into the socket 70 shown in Fig. 8 and switch 10, the line 1 20 is connected to the comparator 102 to by-pass the capacitor which causes the 2 second delay with auto matic operation. Either the buzzer 11 6 or the chime 11 2 will therefore sound immediately on depression of the switch 73.

(c) Display interface.

As already mentioned, the four-line output from the counter 84 is fed to the decoder 86, which is a dual 1 of 4 decoder. This decodes the data signal and feeds it to the lamp drives 88 and 90 constituted by respective transistor arrays. These directly operate the miniature light-emitting diodes forming the four-by-four matrix display 63 which corresponds to the area matrix of the main teaching aid box 2.

The four lamps (130 in Fig. 10) of the auxiliary box 59 are selectively lit up in- accor -dance with output signals from the address counter 80. These are fed to another 1 of 4 decoder 1 26 which in turn operates lamp drivers 1 28 constituted by four transistors.

The latter-operate the four lamps 1 30 in the auxiliary box 59. Thus the location in the memory 82 being addressed by the counter 80 is indicated by the four lamps 1 30 respectively.

When the fourth location has been pro grammed so that both outputs from the ad- dress counter 80 are at the "1" '' level, inver- ters and a Schmitt trigger delay of the counter 80 reset the latter to address the first location after a delay of approximately 1 second, ready for operation by a pupil or child.

Claims (33)

1. A visual display unit comprising a plu rality of lamps or other visual indicators, a memory device having a plurality of cells in which respective data entries-can be stored, each data entry being representative of one of the visual indicators, and signal generating means connected to successively address different cells of the memory device in a. predetermined sequence, in which the memory device-is connected to control operation of the visual indicators whereby that one of the latter which is represented by the data in the cell being addressed at any given instant is switched on, so that the order in which the visual indicators are switched on can be altered by changing the stored data.

2. A unit according to claim 1, in which the memory device comprises a replaceable.

read only memory.

3. A unit according to claim 1, in which the memory device is a random access memory.

4. A unit according-to claim 1 or claim 3, further comprising a keyboard connected to program the memory device.

5. A unit according to claim 4, in which keys of the keyboard are respectively associated with the visual indicators, and the order in which the keys are operated is stored in the memory device thereby to allow the visual indicators to be operated in that order by thecontrol signal generating means.

6. A unit according to any preceding claim, in combination with further lamps or other visual indicators which can be located.

remote from the unit so that information can be conveyed from the unit to that location.

7. A unit according to any preceding claim, in combination with further lamps or other visual indicators and at least one further control signal generating means, separate from those of the said unit, connected to thememory device of that unit.

8. A unit according to any preceding claim, having circuitry connected to operate in accordance with a sub-routine when a particular one of the visual indicators is operated.

9. A unit according to any preceding claim, in which the visual indicators are arranged in a matrix array.

10. A unit according to any preceding claim, in which the visual indicators are-arranged to illuminate respective different areas of a screen.

11. A unit according to claim 10, in which the screen is made of a translucent material, and the visual indicators are positioned behind the screen.

1 2. A unit according to any preceding claim, having a display face which is inclined to a base of the unit

1 3. A unit according to claim 12, in which the angle of inclination is substantially 55".

14. A unit according to any one of claims 1 to 11, having a display face which is inclined at one angle to one rear face of the unit and at a different angle to another, so that the unit may be supported on a horizontal surface with the display face inclined selectively at either one of those angles to the horizontal.

1 5. A unit according to claim 14, in which- those angles are- substantially 30 and 60".

16. A unit according to claim l4orclaim 15, in combination with a support having- a V shaped recess in an upper part, the two sides of the "V" being inclined at the angles mentioned in claim 1 4 to a bottom face of the base, so that the unit may be supported by the base with its display face horizontal.

1 7. A unit according to any preceding claim, in combination with a plurality of triacetate sheets, any selected one of which can be positioned in front of the visual indicators.

1 8. A unit according to any preceding claim, in which the control signal generating means comprises a manually operable switch.

19. A unit according to any preceding claim, in which the control signal generating means comprises a micro-switch.

20. A unit according to any preceding claim, in which the control signal generating means comprises a blow-pipe switch.

21. A unit according to any preceding claim, in which the control signal generating means is capable of automatically generating a train of electrical pulses.

22. A unit according to claim 21 appended to claim 18, in which the pulses commence on manual operation of the switch.

23. A unit according to claim 21 appended to claim 18, in which the pulses cease on manual operation of the switch.

24. A unit according to any preceding claim, comprising a programmable answer memory and a comparator connected to compare data stored in the answer memory with output signals from the said memory device.

25. A unit according to claim 24, comprising a correct answer signal generator connected to generate an indicating signal when the answer memory data and the memory device output signals correspond.

26. A unit according to claim 24 or claim 25 comprising an error signal generator connected to generate an indicating signal when the answer memory data and the memory device output signals are in disagreement.

27. A unit according to claim 25 or claim 26, in which a manually operable switch is provided which is connected to bring about an indicating signal from the correct answer signal generator and/or the error signal generator when operated.

28. A unit according to any preceding claim, further comprising an auxiliary miniature indicator display having lamps or other visual indicators respectively associated with those mentioned in claim 1.

29. A unit according to claim 28, in which the auxiliary miniature indicator display comprises a matrix of light-emitting diodes.

30. A unit according to any preceding claim, comprising further lamps or other visual indicators arranged in an auxiliary display box which can be removably attached to a main display box of the unit.

31. A unit according to any preceding claim, further comprising a distribution box for connecting a plurality of different appliances to the unit.

32. An auxiliary visual display unit comprising a set of lamps or other visual indicators which can be selectively switched on, a memory device having a plurality of cells which respectively correspond to the visual indicators, data entry means connected to the memory device to store, in a selected cell of the memory device, data representative of one of a further set of lamps or other visual indicators of a further unit to which the auxiliary unit is connected when in use, and comparison means connected to the memory device, and also to the further unit when the auxiliary unit is in use, to give a signal in dependance upon whether one of the further set of visual indicators subsequently switched on agrees with the data in a selected cell of the memory device.

33. A teaching aid unit substantially as hereinbefore described with reference to Figs.

1 to 6 or with reference to Figs. 4 and 7 to 10 of the accompanying drawings.