Describe a reflective optical switch and give a use for one.

A reflective optical switch has an infrared LED and a phototransistor side by side, facing the same way. The sensor detects light bouncing off a nearby surface, so a line-following robot can sense a dark line.

What is an encoded disc?

An encoded disc has concentric tracks of clear and opaque segments, each track read by its own optical sensor. Every angular position produces a unique binary code.

How many positions can a three-track encoded disc distinguish, and what angle does each cover?

Three tracks give 2³ = 8 positions. Each position covers 360° ÷ 8 = 45°.

CCEA A-Level Technology and Design 8900

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Microelectronic Control SystemsOption A

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Microelectronic Control Systems

Optical switches, encoded discs and Schmitt triggers

3 things to know
A2 1

Overview

Phototransistors, slotted and reflective optical switches, encoded discs and Gray code, binary coded decimal, position sensors and Schmitt triggers with hysteresis.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

How does a phototransistor work?

Key term

Compare a phototransistor with an LDR as a light sensor.

Key term

How does a slotted optical switch measure the speed of a shaft?

Questions from this subtopic

Each one with the model answer underneath.

1
How does a phototransistor work?
Model answer
Light falling on the base region produces the base current, so many phototransistors have no base lead. The collector current rises with the light level.
2
Compare a phototransistor with an LDR as a light sensor.
Model answer
A phototransistor responds within microseconds, far faster than an LDR. An LDR is slower but simple and cheap, suiting slowly changing daylight.
3
How does a slotted optical switch measure the speed of a shaft?
Model answer
A slotted optical switch has an infrared LED facing a phototransistor across a gap. A slotted disc on the shaft interrupts the beam, and the pulses per second give the speed.
4
Describe a reflective optical switch and give a use for one.
Model answer
A reflective optical switch has an infrared LED and a phototransistor side by side, facing the same way. The sensor detects light bouncing off a nearby surface, so a line-following robot can sense a dark line.
5
What is an encoded disc?
Model answer
An encoded disc has concentric tracks of clear and opaque segments, each track read by its own optical sensor. Every angular position produces a unique binary code.
6
How many positions can a three-track encoded disc distinguish, and what angle does each cover?
Model answer
Three tracks give 2³ = 8 positions. Each position covers 360° ÷ 8 = 45°.

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