6a - transducers

8/11/2019 6A - Transducers

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Teacher: Bradley Burnett

Date: 5/02/14

Class: 6A

TRANSDUCERS


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A transduceris a device that converts energy from one

form to another. Examples of transducers are: the

microphone, motors, speakers etc. Further examples of

transducers which will be examined in greater detail areLight dependent resistors (LDR), thermistors

(temperature dependent resistors).

WHAT ARE TRANSDUCERS?


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Transducers can be classified into two (2) maincategories:

Acti ve tr ansducer s (e.g solar cells)

Passi ve tr ansducer s (e.g LDR, thermistor)

Acti ve tr ansducer s are those which generates its ownelectrical voltage during conversion, i.e. it does notrequire any external battery supply to work.

Passi ve tr ansducer s are those which requires external

electrical power like a battery. It cannot generate its ownvoltage or current. It only changes its resistance orcapacitance etc. during conversion.

TRANSDUCERS


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LIGHT DEPENDENT RESISTOR (LDR)


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A light-dependent resis tor (LDR) is a resistor made

from semiconducting material in which electrons are

liberated when light shines on the surface of the

material. Inbr i gh t l i gh t

the resistance of the LDR (RL)therefore decr easesand in total dar kness the resistance

of the LDR increases.

N.B In darkness, the only free electrons are those

shaken free by thermal vibrations of the atoms, so the

resistance of the LDR is very high.

LIGHT-DEPENDENT

RESISTOR(LDR)


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This circuit consists of two or more resistors connected inseries with a supply voltage (V s) and with the outputvoltage (Vout) measured across a particular resistance in thecircuit. The output voltage (Vout) is a fraction of the supply

voltage (Vs). The sumof the voltage drops in the circuit ( V=IR) is equal

to the supply voltage (Kirchhoff s Law).

The current supplied is the samein a ser i es cir cui t (samecurrent flows through all resistances in circuit). This implies

that the output vol tage (Vout) is dir ect l y propor t i onal to theresistance, R, across which it measured (Ohms law).

V= IR, but I = constant

Therefore V R

POTENTIAL DIVIDER CIRCUIT

(RECALL)


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LDR USED IN A POTENTIAL

DIVIDER

Vout= (RLDR/ RLDR+ R) * Vin

Where RLDR= resistance of LDR

R= fixed resistance

Vin= input or supply voltage

Vout= Output voltage


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LDR IN BRIGHT LIGHT

In bright light we know that the resistance of the LDR decreases,so the output voltage is going to be low (Vout= low) as shown in

fig. I. However, if we interchange R and RLDRthe output voltage

becomes high (Vout= high).

* In bright light, RLDR< R


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They are used in street l ights

Smoke detectors

Burglary alarms

PRACTICAL USES OF LDRS


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A light-dependent resis tor (LDR) is connected in series

with a 10kresistor and a 12V d.c. supply.

a) Draw a circuit diagram showing how you would

obtain the voltage across the LDR.b) i) Calculate the potential difference (p.d) across the

LDR (VL) when it is in the dark and has a resistance of

8M, and

ii) when it is in bright light, and its resistance is 500 .

c) Calculate the resistance of the LDR in lighting

conditions which make VL= 4.0V

PRACTICE QUESTION


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A thermistoris a device whose resistancevaries

markedly with temperature. They may have either a

posit iveor negativetemperature coefficient. A negative

temperature coefficient means that the resistance of the

thermistor (RT) increases with an decrease in

temperature. This means that the output p.d will rise as

the temperature falls. This rising p.d could be used to

trigger a frost alarm.

THERMISTOR (TEMP. DEPENDENT

RESISTOR)


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THERMISTOR USED IN A POTENTIAL

DIVIDER

If the temperature of a thermistor decreases, its resistance (RT

)

increases. This implies that the output voltage in a) increases.

However, if we interchange R and RTas in b), the output voltage

would decrease as temperature decreases.

At high temp, RT< R ; At low temp, RT>R

RTR


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TEMP. CHARACTERISTIC CURVE


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A thermistor is connected in series to a 9V d.c source and a

fixed resistance of R= 5k. The thermistor is placed in a

beaker of boiling water. Find the output voltage ( Vout) across

the thermistor if its temperature is now equal to the temp. of

boiling water (100o

C). (Hint: use temp. Characteristic curve)

Ans: Vout= 1.5V

PRACTICE QUESTION


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Can be used to t r i gger f r ost alarms, where if the p.d

rises above a pre-set voltage value then the alarm would

indicate this breach.

May also be used to switch on a heater to maintaintemperature above a given limit.

PRACTICAL USES OF THERMISTORS


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A semiconductor diode that emits light when a small

current passes through it. Like all diodes, they only

conduct in one direction (forward -biased). To avoid

damage, a protective resistor must be placed in series

with an LED to limit the current through it.

LIGHT-EMITTING DIODE(LED)


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The input circuit can work on a low -voltage supply and control

another circuit on a high -voltage (dangerous) supply.

Uses a small current to switch a large current in another

circuit without any direct electrical contact between them.

Relays can be used as level sensors

Used in car ignition circuit to switch on motors.

USES OF RELAY

6a - transducers

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