Experiment No: 7
Obtain the operational characteristics of the light and temperature
transducer for control applications
Contents
1. Objectives 1
2. Expected outcome of experiment 1
3. Theory 2
4. Equipments required 6
5. Procedure 6
6. Observations 8
7. Results and discussion 9
8. Conclusion 9
1. Objective
A measurement system is comprised of an input device which senses the environment or
surrounding to generate an output, a signal processing block which processes the signal from
input device and an output device which presents the signal to human or machine operator in a
more readable and usable form. A sensor is a device that responds to any change in physical
phenomena or environmental variables like heat, pressure, humidity, movement etc. It is the
heart of a measurement system. It is the first element that comes in contact with environmental
variables to generate an output. The experiment has the following objectives:
Identify the characteristics of light transducers
Identify the characteristics of temperature transducers
Verify the characteristics theoretically.
2. Expected outcomes of the experiment:
Knowledge about light transducers.
Ability to understand the effect of variation of light and temperature on their operations.
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3. Theory:
3.1. Temperature Sensor:
3.1.1. IC temperature sensor (LM 335):
The LM335 series are precision, easily calibrated, integrated circuit temperature sensors.
Operating as a two terminal zener, the breakdown voltage directly proportional to absolute
temperature at 10 mV/° K. With less than 1Ω dynamic impedance, the device operates over a
current range of 0.4 mA to 5 mA. The output voltage so obtained from the IC when multiplied
with 100 gives the measured temperature in Kelvin.
3.1.2. Platinum RTD (Resistance Temperature Detector) Transducer:
Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used
to measure temperature. Many RTD elements consist of a length of fine wire wrapped around a
ceramic or glass core but other constructions are also used. The RTD wire is a pure material,
typically platinum, nickel, or copper. The material has an accurate resistance/temperature
relationship which is used to provide an indication of temperature. As RTD elements are fragile,
they are often housed in protective probes. RTDs, which have higher accuracy and repeatability,
are slowly replacing thermocouples in industrial applications below 600 °C.
The platinum film in the RTD is trimmed with a laser beam to cut a spiral for a resistance of 100
Ω at 0℃. The resistance of the film increases as temperature increases. It has a positive
temperature coefficient. The increase in the resistance is linear, the relationship between
resistance change and temperature rise being 0.385 Ω/℃
RT =R0 +0.385∗T
Where RT is temperature at T ℃ and R0 resistance at 0 ℃.
3.2. Light Transducer:
3.2.1. Photovoltaic cell:
Solar cells, also called photovoltaic (PV) cells by scientists, convert sunlight directly into
electricity. PV gets its name from the process of converting light (photons) to electricity
(voltage), which is called the photovoltaic effect. When a photon is absorbed by a
semiconducting material, it increases the energy of a valence band electron, thrusting it into the
conduction band. This occurs when the energy of incident photons is higher than the band gap
energy. The conducting band electron then produces a current that moves through the
semiconducting material. The amount of current generated by photon excitation in a PV cell at a
given temperature is affected by incident light in two ways:
By the intensity of the incident light.
By the wavelength of the incident rays.
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Figure 1: Cross section of PV Cell
Figure 2: I-V Curve of PV Cell and Associated Electrical Diagram
3.2.2. Phototransistor:
The device differs from the normal transistor in allowing light to fall on to the base region,
focused by lenses. Photo transistors are operated in their active regime, although the base
connection is left open circuit or disconnected because it is not required. The base of the photo
transistor would only be used to bias the transistor so that additional collector current was
flowing and this would mask any current flowing as a result of the photo-action. For operation
the bias conditions are quite simple. The collector of an n-p-n transistor is made positive with
respect to the emitter or negative for a p-n-p transistor.
The light enters the base region of the phototransistor where it causes hole electron pairs to be
generated. This mainly occurs in the reverse biased base-collector junction. The hole-electron
pairs move under the influence of the electric field and provide the base current, causing
electrons to be injected into the emitter.
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Figure 3: Connection for Common emitter phototransistor circuit
Figure 4: Connection for Common emitter phototransistor circuit
3.2.3. Photoconductive cell:
The photoconductive cell is a two terminal semiconductor device whose terminal resistance will
vary (linearly) with the intensity of the incident light. For obvious reasons, it is frequently called
a photoresistive device. The photoconductive materials most frequently used include cadmium
sulphide (CdS) and cadmium selenide (CdSe). Both materials respond rather slowly to changes
in light intensity. The peak spectral response time of CdS units is about 100 ms and 10 ms for
CdSe cells.
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Figure 5: Photoconductive cell symbol and construction
Figure 6: Illumination characteristics of a typical photoconductive cell
3.2.4. PIN photodiode:
PIN photodiode is a kind of photo detector; it can convert optical signals into electrical signals.
The PIN photo diode operates with an applied reverse bias voltage and when the reverse bias is
applied, the space charge region must cover the intrinsic region completely. Electron hole pairs
are generated in the space charge region by photon absorption. The switching speed of frequency
response of photo diode is inversely proportional to the life time. The switching speed can be
enhanced by a small minority carrier lifetime. For the photo detector applications where the
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speed of response is important, the depletion region width should be made as large as possible
for small minority carrier lifetime as a result the switch speed also increases.
Figure 7: Illumination characteristics of a typical photoconductive cell
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4. Equipments required:
Sl. No Unit Quantity
1 Dynalog Transducer and Instrumentation kit 1
2 Digital Multimeter 2
3 Connecting wires As required
5. Procedure:
5.1. Characteristics of an LM335 IC temperature sensor :
Connect the voltmeter to the circuit as show in Fig.8 and switch on the power supply.
Note the output voltage, this (x100) represents the ambient temperature in ° K .
Connect the +12 V supply to the heater input socket and note the voltage reading every
minute until stabilizes.
NOTE: ℃=° K−273
Figure 8: Connection for LM 335 temperature sensor
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5.2. Characteristics of a Platinum RTD transducer :
First it is required to measure the ambient temperature using LM335 and find the
resistance of the platinum RTD. RTD resistance = 100 + 0.385 x ambient temperature (in
℃).
Set the slider of the 10kΩ resistor to midway and connect the circuit as shown in Fig.9
Switch ON the power supply and adjust the slider control of the 10kΩ such that the
voltage drop across the platinum RTD is equivalent to its resistance in mV (e.g. 108mV
for 20℃) using the multimeter. (NOTE : the working current is set to 1mA)
Connect the +12V supply to the heater element input and note the values of voltage
across RTD and the temperature from LM335 IC every 1 minute for 10 minutes.
Figure 9: Connection for platinum RTD
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Characteristics of a photovoltaic cell:
Connect the circuit as shown in Fig. 10 with multimeter connected as ammeter between
PV cell output and ground. Fit an opaque box over the clear plastic enclosure to exclude
all ambient light.
Switch on the power supply and set the 10kΩ wire wound resistor to minimum for zero
output voltage from power amplifier.
Take readings of short circuit output current as lamp voltage increases in 1V steps. Also
connect the multimeter as a voltmeter to measure the open circuit voltage.
Plot the graphs of PV cell short circuit output current and open circuit output voltage
against lamp filament voltage.
Figure 10: Connection for photovoltaic cell
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5.3. Characteristics of a phototransistor :
Connect the circuit as shown in Fig.11 and set the 10kΩ carbon slider to 1 such that the
load resistance is 1kΩ.
Cover the clear plastic enclosure with an opaque box. Switch on the supply and set the
wire wound resistor to minimum zero for zero output voltage from power amplifier.
Take readings of the photo transistor output voltage in steps of 1V.
Plot the graph for phototransistor output voltage against lamp filament voltage.
Figure 11: Connection for phototransistor
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5.4. Characteristics of a photoconductive cell:
Connect the circuit as shown in Fig.12 and set the 10kΩ carbon slider to 3 such that the
resistance is approximately 3kΩ.
Fit an opaque box over the clear plastic enclosure to exclude all ambient light and
measure the output voltage from photoconductive cell.
Switch on the power supply and set the 10kΩ wire wound resistor to minimum for zero
output voltage from power amplifier.
Take readings as lamp voltage is increased in steps of 1V.
Plot the graph of photoconductive cell output voltage against lamp filament voltage.
Figure 12: Connection for photoconductive cell
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5.5. Characteristics of a PIN photodiode:
Offset control of Amplifier #1: with power supply switched on connect amplifier #1
input to 0V and connect output to MC meter + and 0V to mc meter -.
Set fine gain to 1.0 and coarse gain to 10 adjust offset for 0 V output. Again set coarse
gain to 100 and adjust the offset for 0V output.
Switch of the power supply.
Connect the circuit as shown in Fig.13 and fit an opaque box over the clear plastic
enclosure to exclude all ambient light.
Switch on the power supply and set 10 kΩ wire wound resistor to zero
Set coarse gain of amplifier#1 to 10 and fine gain to 1.0.
Take readings of Amplifier #1 output voltage indicated on the digital multimeter as lamp
voltage is increased in the steps on 1V.
Reconnect the circuit for measurement of PIN photodiode by connecting the voltmeter
through a buffer amplifier.
Plot the graphs of PIN photodiode amplifier output voltage and buffer output voltage
against lamp filament voltage.
Figure 13: Connection for PIN photodiode
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6. Observation:
6.1. Characteristics of an LM335 IC temperature sensor :
Time (minutes) 0 1 2 3 4 5 6 7 8 9 10
Voltage of LM335(V)
Temperature ¿)
Temperature ¿)
6.2. Characteristics of a Platinum RTD transducer :
Time (minutes) 0 1 2 3 4 5 6 7 8 9 10
Temperature ¿)
Temperature ¿)
RTD Resistance (Ω)
6.3. Characteristics of a photovoltaic cell :
Lamp voltage (V) 0 1 2 3 4 5 6 7 8 9 10
Short circuit current (μA)
Open circuit voltage (V)
6.4. Characteristics of a phototransistor :
Lamp voltage (V) 0 1 2 3 4 5 6 7 8 9 10
Phototransistor output voltage (V)
6.5. Characteristics of a photoconductive cell:
Lamp voltage (V) 0 1 2 3 4 5 6 7 8 9 10
Photoconductive cell output
voltage (V)
6.6. Characteristics of a PIN photodiode cell:
Lamp voltage (V) 0 1 2 3 4 5 6 7 8 9 10
PIN photodiode current amp
output (V)
PIN photodiode output voltage
(V)
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7. Results and discussion:
Based on your measurements, let us now discuss the results that you obtained. Please provide
responses to following points:
What will be the output voltage of LM 335 at (a) 50℃ (b) -20℃ ?
Platinum RTD has a resistance of 100Ω at 0℃ and 138Ω at 100℃ . Its resistance at 50℃
will be?
A phototransistor is connected to a 10V DC supply via a 2k load resistor. For one level of
ambient illumination the collector current is 2mV. Find the collector voltage?
What is a light dependent resistor?
The lamp used for the experiments of light transducers is an incandescent lamp. What
will be the effect in a white led bulb is used or a red led bulb is used?
8. Conclusion:
Write in your own words the conclusion of performing this experiment. Write about what you
learned from this experiment.
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