AMRI-3100

Optical Sensor for Ultra-slim Scrollwheel with 5-way Switch

Design Guide

Introduction

Avago Technologies has been the industry leading supplier

of optical motion or navigation sensor used in optical

mouse. With its core technology in motion encoders,

Avago started out designing and selling classical optical

encoders and has progressed to designing and manu-

on designing an ultra-thin optical scrollwheel integrated

with 5-way switch in conjunction with Avago’s AMRI-3100

optical sensor.

Features for Scrollwheel

• Scrolling feature integrated with four directional

switches and a center push button

• Z-height profile of 2mm and above

• Typical 18.5 mm diameter

• Two-channel Quadrature Output (ChA & ChB)

• 16 Cycles Per Revolution (CPR)

Output (with optional component)

• Single 2.4V to 3.3V supply

• Simple Power Down feature (with optional component)

• Mobile devices

• Digital cameras and camcorders

• Entertainment consoles

• Handheld GPS or navigation devices

• Portable audio and video players

• Photo printers

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Operating Principle of Scrollwheel

AMRI-3100 combines an emitter and a detector in a single

surface mount package. When used with a codewheel

rotates, an alternating pattern of light beams and shadows

which corresponds to the reflective and non-reflective

pattern of the code wheel falls onto the photodiodes to

generate the internal signals (A, compliment of A, B and

compliment of B). These signals are fed into the process-

ing circuitry part of the sensor to produce the final outputs

in channel A (ChA) and channel B (ChB).

Block Diagram of Scrollwheel

Figure 3. Block diagram of Scrollwheel

Figure 4. Matrix Configuration for Switches

It consists of the current limiting resistor (R1) and two

LEDs that diffuse light through the illumination ring.

The Sensor block is the most important block in terms of

the actual operation of converting motion (in this case dial

rotation) into quadrature output signals. The main com-

ponents are emitter, specular code wheel, optical encoder

or detector and the digital signal processing circuitry as

described in the previous section.

The Switches block consists of all the 5 tactile switches

for the 4-way directional navigation and the center push

is the matrix configuration, can be used. For matrix con-

figuration, only 5 I/O pins are required as shown in Figure

4. Therefore, for scrollwheel with 5-way switches and il-

lumination, minimally there will be 11 pin outs (using the

matrix configuration for switches).

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Power Down Control block is an optional feature to

minimize current consumption to a few microampere

from a typical operating current of 7-10mA (without illu-

mination) or 30mA (with illumination). This feature is es-

pecially useful during stand-by mode when the device is

the output level at CHA and CHB from a default 3.3V to

1.8V to provide TTL/CMOS logic compatible single ended

output. The voltage level translator digital switch such

as ADG3242 can be used to provide such function to the

scrollwheel if necessary.

For scrollwheel with only the scroll function and 4-way

switch and a center button, minimally, 9 microcontroller

(MCU) ports required (VCC, GND, CHA, CHB and 5 ports for

switches - based on matrix configuration).

Placement of Critical Optical Components

(refer to document number AV02-1468EN). The sensor

can be rotated at the dial rotation axis to suite the overall

design as shown with the rectangle dotted lines in Figure 6.

Do not rotate the sensor itself. Misplacement of the sensor

will result in bad scrollwheel performance (indicated

through its output signals) and unit mal-function, thus

positioning of AMRI-3100 sensor must be according to

Figure 5 and Figure 6.

Figure 5. Mounting Outline

Figure 6. Sensor Placement Options on PCB

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Mechanical Structure of Scrollwheel

The mechanical structure of the scrollwheel can be divided

into two portions, the top assembly and the bottom

The bottom assembly that consist mainly the PCB or FPC

and metal dome sheet attached onto it can be done by

PCB manufacturer. The AMRI-3100 sensor is layout and

mounted onto the PCB or FPC by the PCB reflow process.

Figure 13. Cross Section of Bottom Assembly

Figure 14. Cross Section of AMRI-3100 and Scrollwheel

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Bracket whereby it provides a base for the Wheel to slide

on. UHMW (Ultra High Molecular Weight) sheet is added

to provide smoother rotation. UHMW is a hi-tech, self-

lubricating anti-friction plastic that makes material and

surfaces slide effortlessly.

of the 4-way switch. The Wheel Bracket and Rubber Pad

play an important role here (refer Figure 17). A protruding

structure of the bracket labeled as Stopper is recommend-

ed at the location adjacent to AMRI-3100 as to prevent the

CW from being scratched by excessive force when the

Z – gap between CW and top plane of AMRI-3100

Figure 15. Position of Stopper on Wheel Bracket

specification published in the datasheet of the sensor.

If the gap is too large, some potential issues could be

missing pulse or/and out of specification in the phase on

the quadrature output signals.

Figure 17 shows the cross section across the metal dome

at B-B instead of the sensor. The Rubber Pad that holds up

the Wheel Bracket to guide rotation is resting on the metal

dome. Four posts on the Rubber Pad are designed to sit

on top of the four metal domes for clicking. When the

metal dome switch is clicked or pressed, the metal dome

A-A (Bracket hold down by Wheel Deco with Screw, 4x)

Figure 17. Cross Section across B-B in Figure 14

B-B (Bracket hold up by metal dome, 4x)

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its light receiving surface is covered with dust.

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friction of the scrollwheel. From Figure 22, friction at point

2 can be reduced by adding a UHMW sheet and a Teflon

based material for the bracket. Rotating Wheel must not

be rubbing against Wheel Deco and Center Button.

For better grip, following recommendations can be imple-

mented:

1. SF coating paint on Wheel or Dial (Example of paint

2. Mechanical protruded rib

Do not use circular spin lines chrome finishing for the

Contact area should be minimized to reduce friction.

This is accomplished by designing the edge of the Wheel

Bracket slanted inward from the contact area instead of a

straight edge (refer to black dotted line at Gap A).

Besides adhering to the minimum size of the critical gaps,

there are other factors that are also important to provide

smooth rotation such as to ensure no rubbing of parts

against each other and having all possible sliding contact

surfaces polished to eliminate scratchy feel during

rotation. To ensure no rubbing between Wheel and Wheel

The wobbliness of the Wheel or Dial varies with the applied

force to rotate the Dial hence varying the torque created

to rotate it. When the Wheel wobbles, it will impact the

smoothness of rotation. Therefore it is recommended that

the Wheel Bracket is designed to hold on to the Wheel

firmly at the holding point that is further away from the

axis of rotation as illustrated in Figure 23. This feature of

the bracket will be able to consistently hold the Wheel

firmly and minimize wobbling during rotation.

Avago, Avago Technologies, and the A logo are trademarks of Avago Technologies in the United States and other countries.

Data subject to change. Copyright © 2005-2008 Avago Technologies. All rights reserved.

AV02-1491EN - September 10, 2008

Appendix A: Bill of Material (BOM) for Scrollwheel

Below is the BOM list for a scrollwheel.

1 Wheel Deco

Plastic Housing Manufacturers

(Should be of Precision Plastic

Molders with capability up to

0.05mm tolerance)

1 Customized part from plastic parts supplier.

parts supplier. Material: PC, PC+ABS or SUS 304.

5 Center Button 1 Customized part from plastic parts supplier.

Material: PC or PC+ABS

6 Codewheel Precision Silk Screen Printing

and Cutting Supplier

(Eg. C16S-A935-1 from

Nichibei Parts Sdn Bhd.)

1 Thickness is 0.23mm that is inclusive of stiffener

7 UHMW sheet

(optional)

Polymer Manufacturer

(Eg. 0470-000034 from

Chiyoda Integre Co.)

1 ID Ø14.4mm, OD Ø16.6mm, thickness 0.12mm.

For other dimension, please contact supplier.

Optional component, it provides smoother rotation.

iii) Chip LED 2 0603 SMT package; top firing

iv) Resistor – 1kOhm 1 0201 SMT package; 1% tolerance; to be connected

to pin 5 (VC1) of AMRI-3100 (refer to datasheet)

v) Resistor – 10Ohm

(optional)

2 0201 SMT package; 10% tolerance; current limiting

resistor for Chip LED – only require if designed with

illumination

vi) Load Switch

(optional)

Eg. MIC94060 from Micrel For power down control

vii) Voltage level

translator switch

Eg. ADG3242 from Analog

Devices

As level shifter to convert output signals of sensor

of 3.3V to 1.8V if necessary