Introduction to Sensors and Motors: Copier Jam Detector

DMS-VEX or DLB-VEX

© 2014 Project Lead The Way, Inc.Digital Electronics

Design Specifications

As the paper passes through a copy machine, three sensors monitor its path. The sensors are switches that are internally wired as Normally Open.

Test fixture constructed with VEX® components.

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Sensor A

SensorB

SensorC

Paper Path

Motor

Design Specifications

When paper makes contact with the switch, the switch outputs a logic one (1). When paper is not present, the switch outputs a logic zero (0).

3Paper = Logic 1 No Paper = Logic 0

Switch Paper

If a jam occurs, the feed motor will stop.

Design Specifications

Under normal operations, paper will pass through the sensors such that adjacent sensors will not simultaneously detect paper. If they detect paper, this indicates that a paper jam has occurred.

Shown are a few examples of both Jam and No Jam conditions.

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No Jam

Jam

Jam

No Jam

Design Specifications

When a paper jam occurs, an LED indicator light will turn on (and/or a buzzer will sound).

The LED indicator will go off as soon as the jam is cleared.

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The buzzer should continue to sound until a Clear button is pressed. This last condition requires that the output controlling the buzzer be latched with a flip-flop.

JAM Signal

CLEAR Signal

Motor Signal

Design Challenges: CMOS vs TTL

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The Cmod-S6 PLD, the Digital Logic Board (FPGA), and VEX limit switches are all designed for CMOS not TTL. This means that the inputs to both these FPGA’s should be kept ideally near 3.3V rather than 5V to protect the hardware and for them to work with VEX limit switches.

Note: some pins on the DLB are not 5V tolerant and require that the voltage be lowered to 3.3V or it will damage the board. Older DLB’s may have a 3.3V option.

The new Digital Protoboard also has a 3.3 V option when using Vext. -Pictured above

Regardless what development board you are using we will assume that the power rail are providing 5V so that we may learn about voltage dividers.

Design Strategy:

Voltage Dividers

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One way to reduce the 5V provided to the 3.3V desired is to use a voltage divider. A voltage divider is a linear circuit that produces an output voltage which is a fraction of the input voltage. (See schematic below.)

V.VVout

VinRR

RVout

23535330180

330

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2

R1

R2

Design Strategy:

Pull Down Resistors

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Another way to help distinguish between a high or low signal at the inputs is to use pull down resistors. Pull down resistors hold the logic signal near zero when no other active device is connected.

Design Strategy:

Switches

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The red wire on the switch is not connect inside. The switch is wired as a SPDT using only the black and white wires.

Design Challenges: Motors

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• Motors and other devices often require more than the 5V used in TTL logic to power them.

• In this case, the VEX 2-Wire Motor 393 requires 6V to operate.

• It is often wise to ensure that the power being supplied to the PLD (or other controller) is NOT the same power supply being used to drive the motor.

Design Strategy:

H-Bridge Drivers

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Aside from keeping the motor voltage source separate from the logic voltage source, in future designs we will want motors to move in both directions.

An H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction.

Wiring the SN754410

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Complete Block Diagram

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Challenge: Copier Jam Detector

Design Specifications: • The copier jams only when 2 adjacent inputs are triggered.• The MOTOR output should remain on until the copier jams. • The LED should remain on and the MOTOR off until the

CLEAR pushbutton is pressed. This will require the pushbutton be latched with a flip-flop.

• The design should include pull down resistors. • The design should include a voltage divider to change the

signal voltage from 5V to 3.3V. • The design should include a SN754410 Quadruple Half-H

Driver or L298 Full Bridge Driver and a 6V external voltage source to drive the motor.

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Challenge: Copier Jam Detector

Challenge: • DMS-VEX• myDAQ supplying 5V to

the Digital Protoboard• SN754410 Quadruple

Half-H Driver• (4) AA batteries supplying

6V to the Motor

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