Unit 3 General-Purpose Transducers
3.1 OBJECTIVES
1. Study principles and applications of gas smoke sensors.
2. Study principles and applications of ethanol sensors.
3. Study principles and applications of magnetic Hall sensors.
3.2 DISCUSSION OF FUNDAMENTALS
3.2.1 Gas / Smoke Sensors
Gas/smoke sensors are commonly used for gas leakage detection; gas concentration measuring, or gas analysis. These sensors can be classified into three types:
contact-burning, semiconductor, and thermal-conductive thermistor.
1. Contact-Burning Gas Sensor
The type of gas sensor is basically composed of a gas-sensitive sensor element
“RD”, and a gas-insensitive compensation element “RC”. The two elements are usually combined with resistors R1 and R2 to form a bridge connection, as shown in Figure 3-1. R1 and R2 are used for the bridge-balance adjustments. When there is no gas in air, balance of bridge is expressed by the equation below.
RD x R2 = RC x R1
Under ideal condition, the bridge output across + and – should be equal to zero which means a 0% concentration of gas.
The change in sensor output is a function of the change in gas concentration due to the increase in sensor resistance by contact-burning.
2. Semiconductor Gas Sensor
Conductivity of semiconductors changes when its surface is exposed to varying levels of gas concentration. This unique characteristics of semiconductors allows it to be used as effective gas sensors.
Semiconductor gas sensors are basically based on the principle of a change in conductivity caused by a change in gas concentration on semiconductor (SnO2, ZnO, or Fe2O3) surfaces. An industrial gas sensor, type TGS 813, shown in Figure 3-2 is widely used in marsh gas and natural gas detections. The sensor element of TGS 813 is made of a SnO2 semiconductor material. The heater coil of 30Ω uses a power of 5V for heating. Top and bottom covers are made of the double-layer stainless steel SUS 316 with 100 meshes. The TGS 813 has some features: high sensitivity, wide detecting range, low noise sensitivity, and short heating time. The pins configuration of TGS 813 is shown in Figure 3-3. Figure 3-4 shows the dimension of TGS 813.
Figure 3-2 Construction of TGS 813
Figure 3-3 Pins configuration of TGS 813
Figure 3-4 Dimension of TGS 813
The operating voltage of TGS 813 is in the range between 5 and 24 V, DC or AC.
The maximum power dissipation is about 15 mW. The circuit of Figure 3-5 is a typical circuit for TGS 813 tests. In circuit, the sensor resistance RS is serially connected to the load resistance RL to form a voltage divider, and then shunted to the operating voltage VC. The VC provides a stable current through the divider and produces a voltage drop across the RL when fresh air absorbed on the sensor surface. If the concentration of gas increases, the output voltage VRL will increase due to the decrease of sensor resistance RS. As a result, the change in output voltage VRL is a function of the change in gas concentration. The value of sensor resistance RS can be calculated by the following equation:
RS = [(VC x RL) / VRL] - RL
Figure 3-5 Typical testing circuit of TGS 813
3. Thermistor Gas Sensor
The thermistor gas sensor is a high-reliability sensor for natural gas or marsh gas detections. In practice, the sensor can detect the gas concentration up to 100 %, whereas its sensitivity is affected by the humidity.
Dimensions in millimeter
3.2.2 Ethanol Sensors
Figure 3-6 shows the internal construction of ethanol sensor TGS 822. The sensor element is made of SnO2 ceramic material. Top and bottom fireproof covers are made of the double-layer stainless steel SUS 316 with 100 meshes. Pins 1 and 3 are connected internally as well as pins 4 and 6.
(a) Cut View (b) Bottom View
Figure 3-6 Internal construction of TGS 822
A basic circuit for TGS 822 tests is shown in Figure 3-7. The VC and VH can be applied by either AC or DC power. The sensor resistance RS is determined by:
RS = [(VC x RL) / VRL] - RL
where VC = Operating voltage VH = Heater voltage RL = Load resistance VRL= Output voltage RS= Sensor resistance
Specifications of TGS 822 are shown in Tables 3-1 to 3-5.
Table 3-1 Absolute Maximum Ratings
Parameters Ratings Remarks
Operating voltage 24V max. AC or DC
Heater voltage VH 5V ± 0.2V AC or DC
Power dissipation Ps 15mV max. Ps = Vc2 . RS / (RS+RL)2 Max. temperature range -30 to +70 ℃ No freeze on sensor Operating temperature range -10 to +40 ℃
Table 3-2 Electrical Characteristics
Parameters Ratings Remarks Sensor resistance 1KΩ to 10KΩ Rs in 300 ppm ethanol/air
Resistance change 0.4 ± 0.1 Rs in 300 ppm ethanol/air Rs in 50 ppm ethanol/air Heater resistance 38Ω± 3Ω At room temperature Heater dissipation 600mW ± 55mW VH = 5V
Table 3-3 Typical Testing Conditions
Items Conditions Air
Clarified air
Temperature: 20 ± 2℃
Relative humidity: 65 ± 5%
Circuit
VC = 10 ± 0.1V VH = 5 ± 0.05V RL = 10.0KΩ± 1%
Time 7 eight-hour days or over
Table 3-4 Mechanical Tests
Items Conditions Vibration test
Frequency: 1000 cpm Vertical amplitude: 4mm Elapsed time: 1 hour Swing test Acceleration: 100G
Table 3-5 Element Materials
Items Materials Sensor element Sn02
Heater coil Gold alloy Diameter : 60μm Lead wire Gold alloy Diameter : 80μm Housing Nylon 66 (UL94AB)
Pin Nickel
Fireproof device Double-layer 100-mesh SUS 316
Weight 2.6 g
Figure 3-7 Typical testing circuit of TGS 822
3.2.3 Magnetic Hall Sensors
Magnetic semiconductor detectors are widely used in industrial applications. The magnetic Hall sensor is a typical element based on the Hall effect. Please refer to the section 2.2.3 for details. Magnetic detectors are normally usable in analog and digital applications. In analog applications, the Hall sensor is used for measuring the absolute magnetic strength or linear output. The Hall sensor is usually used in small magnetic field measuring or non-contact switches.
3.3 DESCRIPTION OF EXPERIMENTAL CIRCUITS
3.3.1 Gas / Smoke Detector
Figure 3-8 shows a gas/smoke detector circuit that can be used for gas leakage, CO, and smoke detections. R2 is used to preset the reference gas concentration level.
The OP AMP U1-d is a level detector for detecting the difference between the preset value at pin 12 and the measured value at pin 13. If the voltage at pin 13 exceeds the voltage at pin 12, U1-d outputs a low potential, the integrator (R5 and C1) does not charge so the output of timer U2 (NE555) is low and the buzzer does not turn on.
If the measured value exceeds the setting value, the output of U1-d goes to a high potential and then the integrator starts to charge. When the charge voltage reach the voltage at J2, the output of U1-c goes to a high potential which triggers a series of output pulses from U2, turning on the buzzer. The buzzer will stay on as long as the gas concentration are higher than the preset level.
Figure 3-8 Gas smoke detector
3.3.2 Ethanol Detector
The circuit of Figure 3-9 is the ethanol detector. This circuit is the same as the gas detector in Figure 3-8.
Figure 3-9 Ethanol detector
3.3.3 Digital Magnetic Detector
Figure 3-10 shows the circuit using a digital Hall IC 3503. The block diagram of IC3503 is shown in Figure 3-11. The internal power regulator Vref provides a constant-current to the Hall element X and a regulated voltage to amplifiers.
Figure 3-10 Digital Hall circuit
Figure 3-11 Block diagram of IC3503
When a magnet moves to the IC 3503, shown in Figure 3-12, the output of Hall element is proportional to the magnetic strength and then is amplified by the differential amplifier. The output of differential is applied to the Schmitt trigger to determine the output state (low or high). The output voltage vs. the distance
Figure 3-12 The sensing method for Hall and magnet
Figure 3-13 Output voltage vs. distance characteristic for IC 3503
3.3.4 Analog Magnetic Detector
Figure 3-14 shows a Hall detector using an analog Hall IC HI410. The output of Hall element is directly proportional to the magnetic field. The analog signal is amplified by the differential amplifier U6.
Figure 3-14 Analog Hall IC
3.4 EQUIPMENT REQUIRED
1 - KL-61001B Trainer 2 - KL-63002A Module 3 - Magnet
4 - Digital Multi-Meter (DMM, Optional Device) 5 - Oscilloscope (Optional Device)
6 - Alcohol (Optional Accessory) 7 - Lighter (Optional Accessory)
3.5 EXPERIMENTS AND RECORDS
3.5.1 Gas / Smoke Detector
1. Place KL-63002A Module on KL-61001B Trainer.
2. Connect the OUT(J4) in GAS / SMOKE SENSOR area to the SIN.IN terminal of BUZZER on KL-61001B.
3. Switch power ON and the display should be ON.
4. Use the meter to measure and record the voltage at J1. VJ1 = ____________ V.
5. Press the lever on the lighter to release the butane but do not ignite; place the discharge valve close to the sensor. Measure and record voltage at pin 12.
Measure the voltage variation at J3 when Vpin12 approaches and exceeds VJ1. 6. Use the scope to measure and observe the waveform at J4.
7. Move the lighter away from the sensor and the oscillating waveform at J4 should remain. Why? ____________________________________________________.
The semiconductor gas sensor needed a short preheating period before test.
Tune R2 to change the sensitivity of detector.
3.5.2 Ethanol Detector
1. Connect the OUT(J8) in ETHANOL SENSOR area to the SIN. IN terminal of BUZZER on KL-61001B.
2. Switch power ON and the display should be ON.
3. Use the meter to measure and record the voltage at J5. VJ5 = _____________ V.
4. Place some alcohol close to the sensor and measure the voltage at pin 3.
Measure and record the voltage variation at J7 when VPIN3 exceeds VJ5.
5. Move the alcohol away from the sensor. Why does the oscillator (J8) continues to oscillate? _____________________________________________________.
The semiconductor ethanol sensor needed a short preheating period before test.
Tune R11 to change the sensitivity of detector.
3.5.3 Digital Magnetic Detector
1. Connect J10 from HALL-EFFECT SENSOR(1) to KL-61001B DCV INPUT + . 2. Switch power ON and the display should be ON.
3. Adjust the R20 to obtain the output to 0V before adding the magnet field.
4. Place the magnet in close proximity to the Hall IC and try to find out which side of the Hall IC does not respond to the presence of the magnetic field.
5. Reverse polarity of the magnet. How does it affect the output?
3.5.4 Analog Magnetic Detector
1. Connect J11 from HALL-EFFECT SENSOR(2) to KL-61001B DCV INPUT +.
2. Switch power ON and the display should be ON.
3. Adjust the R27 to obtain the output to 0V before adding the magnet field.
4. Place the magnet in close proximity to the Hall IC and try to find out which side of the Hall IC does not respond to the presence of the magnetic field.
5. Record the distance between magnet and Hall IC when the output start to
