OPTICAL SENSORS

13 JULY 2004Chung Hsieh

Basics Operation

An Light Emitting Diode (LED) sends out light which bounces back from the target. Exciting the BASE of the phototransistor, causing a signal.

Benefits of Optical Sensor

• Quick response• Low noise consideration• Signal already in a digital format for

processing

Drawbacks of Optical

• High current drain• Resolution dependent on number of

physical sensors• Large number of sensors needed• Large number of components needed• Small operating range

Uses of Photo Optical Sensors

As track sensors

As linear speed encoder

As OptoCoupler

How to Read Data Sheet

Refer to FairChild QRB1134 PDF

How To Bias The LED• Use Resistor Divider and make sure only

less then 1.5 volts drop across the LED

V15Vdc

DbreakD1

R1

1k

DbreakD5

DbreakD3

DbreakD2

V25Vdc

DbreakD4

One Resistor and Diode Pair. Multiple Diodes

Advantages? Advantages?

Disadvantages? Disadvantages?

How to Bias Photo Transistor

• Since the BASE of the transistor is a photosensor, the types of configuration are limited.– Common Emitter– Common Collector (Emitter Follower)– Darlington Pair

Common Emitter

QbreakN

Q1

R1

1k Vout

V15Vdc

An inverting amplifier.Absent of signal at the BASE,Vout equals to Vcc (5volts).When Signal is applied Vout is dependent on R1.

Common Collector

V15Vdc

R1

1k

VoutQbreakN

Q2

None inverting amplifier. Gain of approximately 1.

Vout equals zero when no signal at BASE.Vout equals Vbase , when signal at BASE.

Darlington Pair

Vout

QbreakN

Q4QbreakN

Q3

V15Vdc

R1

1k

Inverting Amplifier, similar to common emitter. But higher gain then common emitter.Normally would have higher input impedance, but in this case it is meaningless.

Break

Back in 10 Minutes.

Any questions?

Signal Amplification

• Do we need amplification?– Yes, if using common emitter or common

collector, the signal must be amplified.• What type of amplification?

– Op Amp?– Additional Transistor? – Level Shifter?

Level Shifter

• Use in digital circuits to change one logic level to another.– Ie. Make a 3.3v logic 1 into a 5v logic 1.

Level Shifters

• Easily change the logic high voltage• Acts as a filter for any low amplitude noise• Easier to work with then Operation

Amplifiers

Filters

• Are filters needed?– Yes and No depending on the applications– As track sensor that will go thru level shifters,

and is far away from the motor (large level of noise) the Optical Sensor does not need additional hardware filters.

– If using as speed encoder near the motor, then additional filters will be needed, as the amplitude of noise will be very high.

What Type of Filter

• Depends on application again– Around the wheel encoders, a low pass filter

with cut off frequency of highest wheel rpm.– IF use around track sensor, another low pass,

but cut off frequency would be much lower, how much lower?… any ideas?

End

• Any questions?

– Homework and pdf for this class posted atwww.ee.ucla.edu/natcar