GB2080965A

GB2080965A - Drive apparatus for a slide projector magazine

Info

Publication number: GB2080965A
Application number: GB8022390A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-07-09
Filing date: 1980-07-09
Publication date: 1982-02-10

Abstract

A magazine drive for a slide projector in which a drive gear is attached to the magazine and a DC motor attached to the projector base, the motor having gears engaging the drive gear and being controlled by a control circuit including means for detecting the passage of index markings on the drive gear as the magazine is driven by the motor. The control circuit also includes a motor braking circuit which provides rapid braking of the motor and operates by the method of applying increasing pulses of reverse voltage to the motor. The drive allows any slides in the magazine to be selected for projection.

Description

SPECIFICATION Drive apparatus for a slide projector magazine The present invention relates to a drive apparatus particularly for a slide projector magazine.

In certain slide projectors slides are carried in a slide magazine which may be moved automatically past a slide gate, the slides being transferred successively into and out of the slide gate from the magazine as it passes. Such an apparatus is intended to display the slides from the magazine in a particular sequence and when a slide is needed out of sequence the magazine must normally be moved by hand to the required position. Some projectors are known which provide for automatic selection of slides out of sequence but such projectors are expensive and complex.

According to one aspect of the present invention there is provided a magazine drive for a slide projection apparatus, comprising a drive gear engageable with the slide magazine and carrying index markings spaced at positions corresponding to slide receiving locations in the slide magazine, sensor means positionable on the slide projection apparatus adjacent the drive gear and capable of detecting the index markings as the slide magazine moves, motor drive means positionable on the slide projection apparatus and drivably engageable with the drive gear to move the slide magazine, and a position control circuit controlling the motor drive means and including a counter connected to the sensor means which counts the index markings and so determines the position of the slide magazine relative to an initial position.

This magazine drive enables a conventional slide projector to be converted for automatic selection of slides out of sequence in a simple and economical manner.

When driving a slide magazine rapidly to a new position it is necessary that the motor be braked in a controlled manner so that the magazine can stop accurately with the required slide opposite the slide gate. This can be difficult where the weight of slides loaded in the magazine varies considerably.

According to another aspect of the present invention there is provided a controlled braking circuit for a DC motor comprising a motor speed sensor capable of producing a signal dependent on the speed of the motor and a source of alternating voltage having a rectangular waveform of variable mark/space ratio connectable to the motor, the mark space ratio being determined by the signal from the speed sensor so that when operated by a braking signal a reverse voltage is applied to the motor with an equal forward voltage being applied for periods which increase in length as the speed decreases until the waveform becomes a square wave as the motor stops.

A drive apparatus for a slide projector magazine constructed in accordance with the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a plan view of.a slide projector incorporating the drive apparatus; Figure 2 is a side view of the slide projector in Figure 1; Figure 3 is a block diagram of a position control circuit for the drive apparatus; and Figure 4 is a more detailed block diagram of part of the circuit in Figure 3; Figure 5 is a waveform diagram of the signal applied to the motor in Figure 4; Figure 6 is a circuit diagrarri corresponding to the block diagram in Figure 4.

Referring to Figures 1 and 2 a slide projector consists of a base unit 1 containing the opticai part of the projector including a slide gate and a mechanical arrangement to move a slide into and out of the gate from a circular slide magazine 2 located on the top of the projector base unit, when the magazine is positioned with the appropriate slide above a slide receiving slot in the base unit.

The magazine holds up to approximately 80 slides which may be made of a variety of materials ranging from light cardboard to metal. Each slide location in the magazine is numbered and the position of the slide receiving slot is marked on the base unit so that the magazine can be positioned with the first required slide at the slot position.

Around the slide magazine is a drive gear ring or collar 3 having on its outer circumference gear teeth 4. On the top surface of the drive gear ring 3 are a plurality of radial grooves 5 equally spaced around the ring, one groove being positioned opposite each slide location in the magazine. The grooves are coloured white with the remainder of the ring surface being coloured matt black. Grub screws 6 are provided to engage the outside of the magazine and hold the ring in position.

A casing 7 removably secured to the top of the base unit 1 contains a light source comprising one or more infra-red emitting LEDs and a light detector sensitive to infra-red positioned above the drive gear ring as it passes through the casing, a DC motor and its gearing meshing with the teeth on the drive gear ring, and a position control circuit.

The position control circuit is shown in Figure 3 and comprises a shift register 8 having positions equal to the number of slide locations in the slide magazine and a memory 9 having an equal number of memory locations. A comparison device 10 is connected between the memory 9 and shift register 8 so that when the number in the shift register equals that held in the memory an output signal is produced from the device 10 to operate a motor braking circuit 11. The input of the shift register 8 is connected to the light detector 12 via a pulse shaping device and the memory 9 is programmed by a controller 13.

In operation, a number corresponding to the position of the required slide in the magazine is put into the memory 9 by the controller 13. If this number does not equal that in the shift register 8 then the motor is operated by a signal from the comparison device 10, which rotates the magazine by means of the gear teeth on the ring 3. As the magazine rotates the white grooves pass under the infra-red light source and detector producing a pulse each time the magazine advances by one slide location. These pulses are counted by the shift register until the number equals the number in the memory when the comparison device produces a signal which causes the motor braking circuit to stop the motor.

Thus provided the magazine is initially positioned with its first slide location at the slot position it will rotate automatically to any other required position. Where the projector is normally a sequential type in which the magazine can only be rotated by one location at a time, the casing 7 can be arranged to disengage the normal drive when it is secured to the base unit. The grooves are wide enough to give a pulse sufficient to operate the shift register reliably. They may be repiaced by white dots or by some other type of index markings which are remotely detectable such as for example magnetic strips detected magnetically.

The motor braking circuit 11 is shown in more detail in Figures 4 and 6 and comprises a motor speed sensor 14 and a rectangular wave generator 1 5. The output of the rectangular wave generator 1 5 is connected to the DC motor 1 6 via an amplifier 1 7 and comprises a rectangular alternating voltage wave of variable mark/space ratio as shown in Figure 5. Two control input signals are applied to the generator 1 5 one being the signal from the comparison device 10 and the other being a voltage from the motor speed sensor which increases as the speed of the motor increases. This motor speed sensor can be a voltage generator driven by the drive shaft of the motor or any other means of producing a voltage dependent on the motor speed.

When the signal from the comparison device is that produced when the numbers in the shift register 8 and memory 9 are not equal then this causes a constant voltage of +V to be applied to the motor 16. This operates the motor and the slide magazine rotates.

When the signal from the comparison device is that produced when the numbers in the shift register 8 and memory 9 are equal then the generator 1 5 operates. The mark/space ratio of the output of the generator 1 5 depends on the voltage from the motor speed sensor. When this voltage is zero, i.e. the motor has stopped, then the mark/space ratio is 1/1 and a square wave is produced but as the voltage increases so the mark/space ratio moves towards shorter positive voltage periods. Thus since initially the motor is running at high speed no positive voltage is applied and a constant reverse voltage of-V is applied which slows the motor down rapidly as it opposes the momentum of the rotating magazine.

As the motor speed drops positive voltage pulses start to appear of longer and longer duration thus gradually reducing the reverse torque of the motor until when the motor stops the wave form is a square wave having no effective DC component and the motor remains at rest, provided the frequency of the square wave ishigh enough.

This circuit gives a rapid braking action by applying reverse bias to a DC motor which is automatically reduced during the braking action.

Thus it can cope with a wide range of loads of varying inertia such as wouid be experienced with different numbers of slides made from different materials in the magazine.

The braking circuit is also applicable to other situations where a controlled rapid braking action is required.

In a preferred arrangement aå sharn in Figure 6 the generator 1 5 is based on two 555 timer integrated circuits 1 8 one determining the frequency of the waveform and the other the mark/space ratio, and the input signal from the comparison device changes from +V to 0 volts when the motor is to be stopped. An output differential amplifier 19 has one input connected through a diode to the input of the braking circuit and the other input connected to the output of the two 555 timers so that when the'braking signal of 0 volts is applied the output of the differential amplifier is determined by the 555 timers, but when the motor running signal of +V volts is applied the output is determined bv this signal, operation of the 555 timers being Inhibited thereby.

Claims

1. A magazine drive for a snide projection apparatus comprising a drive gear-engegeabie with the slide magazine and carrying index markings spaced at positions corresponding to slide receiving locations in the slide-magazine, sensor means positionable on the slide projector apparatus adjacent the drive gear årid capable of detecting the index markings'as the slide magazine moves, motor drive rneans positionable on the slide projection apparatus and drivably engageable with the drive gear to move the slide magazine, and a position control circuit controlling the motor drive means and including a counter connected to the sensor means which counts the index markings and so determines the position of the slide magazine relative to an initial position.

2. A magazine drive according to claim 1, wherein the position control circuit includes a memory capable of holding a number corresponding to a required position of the slide magazine and comparison means between the memory and the counter which produces a stop signal for the motor drive means when the number held in the memory is equal to that in the counter.

3. A magazine drive according to claim 1 or claim 2 wherein the index markings are regions of greater light reflectivity than the adjacent parts of the drive gear and the sensor means is sensitive to light reflected from these regions.

4. A magazine drive according to any preceding claim, wherein the drive gear has projections on its outer surface and the motor drive means includes a gear meshable therewith.

5. A magazine drive according to any preceding claim wherein the drive gear is a ring capable of fitting snugly around a circular slide magazine.

6. A magazine drive substantially as herein described with reference to and as illustrated by the accompanying drawings.

7. A controlled braking circuit for a DC motor comprising a motor speed sensor capable of producing a signal dependent on the speed of the motor and a source of alternating voltage having a rectangular wave form of variable mark/space ratio connectable to the motor the mark space ratio being determined by the signal from the speed sensor so that when operated by a braking signal a reverse voltage is applied to the motor with an equal forward voltage being applied for periods which increase in length as the speed decreases until the waveform becomes a square wave as the motor stops.

8. A controlled braking circuit according to claim 7, wherein when a signal other than the braking signal is applied to the circuit operation of the source of alternating voltage is inhibited thereby and an output voltage dependent on the input signal is applied to the motor.

9. A controlled braking circuit substantially as herein described with reference to and as illustrated by Figures 4 to 6 of the accompanying drawings.

10. A method of braking a DC motor comprising applying a reverse voltage to the motor with an equal forward voltage being applied for periods which increase in length as the speed of the motor decreases, the periods of forward and reverse voltage becoming equal when the motor stops.

11. A method of braking a DC motor substantially as herein described with reference to and as illustrated by Figures 4 to 6 of the accompanying drawings.