Cool O2 Automatic Sensor Fan

A report on a project performed for ME 106 - Mechatronics San Jose State University

Department of Mechanical and Aerospace Engineering

By: Yan Kin Chan Peter Vuong Andy Yip Saurabh Gupta

May 16, 2006

Table of Contents Summary ............................................................................................................................. 2

Introduction......................................................................................................................... 3

Design Details..................................................................................................................... 4

Results & Recommendations............................................................................................ 11

References......................................................................................................................... 13

Appendices........................................................................................................................ 14

1

Summary An auto on/off fan was built based on a temperature sensor, two motion sensors, a fan,

and a servo. The fan turns on when the ambient temperature reaches a certain degree. The

motion sensors are used to track a person’s movement which then the servo will rotate

the fan toward that movement accordingly. A microcontroller was used to control the

overall operation of this fan. A program has been written and downloaded into the

microcontroller to control the temperature for powering on/off, and to control the degree

of rotation of the servo as it receives signals from the motion sensors.

The temperature sensor and the motion sensors send out a voltage to the microcontroller

which then we regulated so that it would not overload the microcontroller. These voltages

are then used as signals for giving commands to the microcontroller. Then, the

microcontroller, which is also connected to the fan and the servo, sends out voltages

(signals) back to the fan and servo allowing it to power on and off. The fan turns on only

when there is a signal sent to the fan.

From this project, we learned the difficulty in programming the microcontroller, the time

and effort professional engineers spent on producing a quality satisfactory product. When

programming the microcontroller, we encountered many problems such as debugging,

setting the servo to run, letting the motion sensor to run smoothly, and etc. In addition,

we learned more about servos, sensors, and other mechanical interfacing devices.

2

Introduction Mechatronics is one of the fastest growing fields in the engineering industry.

Mechatronics is the combination of the fundamental mechanical engineering knowledge

of mechanical as well as electrical engineering. We can see this great engineering

application almost everywhere and everyday. For example, on washing and drying

machine, there are temperature sensors, timer, and other devices that are all connected

and controlled by a micro-controller that allows users to adjust the time, speed, and even

the temperature at which the machine operates. On HVAC designs, air conditioners have

implemented sensors that detect the temperature of the surrounding which send signals to

a controller that regulate the powering of the machine. Our objective of this project is to

build a device that has at least one sensor and one actuator that is controlled and operated

by a microcontroller.

The main concept of our project is to design a fan which power on itself as the it

detects movement and if the surrounding temperature has reach a certain degree. It also

has two motion sensors that track the person’s movement. The overall design operates

under a microcontroller that controls a servo, a fan, two motion sensors, and one

temperature sensor. The temperature sensor is actually a thermal resistor which changes

its resistance according to the surrounding temperature. The two motions sensors that we

used were the SonaSwitch Ultrasonic sensors. The sensors act as a speaker as well as a

microphone. It sends out signal up to a range of 7 feet, and as the signal bounces back as

it detects an object, the microphone receive that signal and sends out a voltage to the

microcontroller. The Futaba S3003 standard servo is used in conjunction with the motion

sensors to direct the fan towards the moving object. Again, the whole functionality of the

fan is controlled by a microcontroller where a program code has been downloaded into.

3

Design Details The two key components of the design were the temperature sensor and the motion sensor. The

temperature sensor allowed for the fan to turn on automatically when the ambient temperature

reached greater than the threshold temperature. The motion sensor detected motion within a

vicinity of six feet allowing the fan base to rotate and point towards the user. Figure 1 shows the

prototype that was presented. Shown in the figure is a 60 mm fan attached on top of the Futaba

FP-S148 servo. Two Sonaswitch motion sensors were placed in plastic sensor holders which

were fastened to the wooden base. The wooden base housed the breadboard and the Atmega 128

microcontroller. The NTC thermistor which is not shown in the figure was located at the other

end of the wooden base. Detailed schematic diagrams of each of the subsystems are described

below.

Figure 1. Prototype of Cool O2 Automatic Sensor Fan Prototype of the project that was presented during the fair. A 60 mm fan was attached on top of the Futaba FP-S148 servo. Two Sonaswitch motion sensors were placed in plastic sensor holders which were fastened to the wooden base. The wooden base housed the breadboard and the Atmega 128 microcontroller. The thermistor which is not shown in the figure was located at the other end of the wooden base.

4

A temperature sensor circuit was constructed on the breadboard. The key component used in the

construction of the temperature circuit was the NTC thermistor. The thermistor is a temperature

sensitive resistor. A negative temperature coefficient unit was used, that is, the resistance

decreased with an increase in temperature. The datasheet for the NTC thermistor is attached in

Appendix B. The temperature sensor circuit was built such that the temperature and resistance

had a relationship of 1°F = 0.1V. Thus, the circuit output a voltage of 7.5 V at a temperature of

75°F . The NTC thermistor does not have a linear relationship with temperature. However, for

the narrow range of 75°F to 95°F required for this application, the relationship was sufficiently

linear. At room temperature of 70°F, the thermistor had a resistance of 50kΩ. At body

temperature of 98.6°F, the thermistor had a resistance of 36 kΩ. The schematic of the

temperature sensor is shown in Figure 2. The circuit can be divided into two main sections of op-

amp 1 and op-amp 2. The op-amp 1 section contained an inverting amplifier which converted the

variable resistance of the thermistor to a variable voltage. The op-amp 2 section contained an

inverting summing op-amp which provided a voltage offset allowing for the output voltage to be

proportional to temperature. An increase in ambient temperature caused a decrease in resistance

of the thermistor, resulting in an increase in Vout of the circuit.

5

Figure 2. Temperature Sensor Schematic The temperature sensor consisted of a thermistor which changed its resistance as a function of ambient temperature. An increase in temperature caused a decrease in the resistance of the thermistor and subsequently an increase in Vout. A threshold temperature circuit was built to allow the user to set the temperature at which the fan

would automatically turn on. Figure 3 shows the schematic of the threshold temperature circuit.

In this case, the feedback resistor was set at 1KΩ and a trimpot resistor was used as a R1. A

trimpot resistor is a resistor with an adjustable resistance. Using a trimpot for R1 allowed the user

to adjust the threshold temperature to their liking. The gain of an non-inverting op-amp is given

as follows: Vout = Vin[1 + Rf/R1]. Thus, if a user wanted the fan to turn on at a higher

temperature, he or she would simply decrease the trimpot resistance resulting in an increase in

Vout. Alternatively, if the user wanted the fan to turn on at a lower temperature, he or she would

increase the resistance of the trimpot resistance resulting in a decrease in Vout.

6

Figure 3. Threshold Temperature Circuit Schematic A threshold temperature circuit set the temperature at which the fan would automatically turn on. The circuit consisted of a non-inverting amplifier. A trimpot was used as R1 which allowed the user to adjust the threshold temperature. Increasing the resistance of the trimpot will decrease the value of Vout causing a lower threshold temperature. Alternatively, decreasing the resistance of the trimpot will increase the value of Vout causing a higher threshold temperature. The next circuit that was constructed was the comparator circuit. The comparator compared the

output voltage from the temperature sensor and the output voltage from the threshold

temperature circuit and switched its output to indicate the larger value. A standard LF741 op-

amp was used as a comparator. When the output voltage from the temperature sensor circuit was

greater than the output voltage from the threshold temperature circuit, the op-amp output +5 V.

When the output voltage from the temperature sensor circuit became smaller than the output

voltage of the threshold temperature circuit, the op-amp output -5 V. The output of the

comparator was connected to the power supply of the 60 mm fan. A diode was connected

between the output of the op-amp and the fan which allowed the current to flow in only one

direction. Thus, the fan would automatically power on with +5 V when the output voltage of

temperature sensor circuit was higher than the threshold temperature circuit.

7

Figure 4. Comparator Circuit The comparator circuit compared the two output voltages from the temperature sensor and threshold temperature circuit and returned the higher output. The output of the comparator was connected to the power of the fan. When the voltage of the output temperature reached higher than the threshold temperature circuit, the fan automatically powered on with 5V. The prototype also incorporated two SonaSwitch Mini S motion sensors which detected motion

within a vicinity of six feet and rotated the servo motor accordingly. The SonaSwitch motion

sensor consists of a transmitter and a receiver. The transmitter sends out ultrasonic waves which

are either reflected back to the receiver or depart from the sensor range. The datasheet of the

SonaSwitch motion sensor is attached in Appendix B. Figure 5 shows the schematic of the

SonaSwitch Mini S motion sensor [1]. 5 V was supplied to pin 7 and pin 6 was held to ground.

Pin 2, the NPN open collector output was connected to the microcontroller. As the motion sensor

detected motion, a 20mV signal was sent to the microcontroller which made the connected pin

low. Alternatively, when no motion was detected, a 5 V signal was sent to the microcontroller

which made the pin high. A program was downloaded to the Atmega 128 microcontroller

allowing the servo to rotate when a 5 V signal was returned to the microcontroller. The copy of

the programming code is attached in Appendix A.

8

Figure 5. Sonaswitch Mini S Motion Sensor Schematic of the Sonaswitch Mini S motion sensor is shown. Pin 2, the NPN open collector output was connected to the microcontroller. A 5 V signal was returned to the microcontroller when the motion senor did not detect any motion. The sensor returned a 20mV signal to the microcontroller when the sensor detected motion. Pin 6 was held to ground and a 5V power supply was supplied to pin 7. The servo motor served as the base of the fan as the 60 mm fan was attached on top of the servo.

As the motion sensor detected motion, the servo motor and fan rotated to point in the direction of

the user. The Atmega 128 microcontroller sent a stream of pulse-width modulated control signals

in order to rotate the servo. In order to keep the servo from jittering, the 74HC4017 decade

counter was used. The schematic of the 74HC4017 decade counter is shown in Figure 6 [2]. The

yellow wire attached to the servo carried the control signal and was connected to pin 2 on the

decade counter. +5 V was supplied to the red wire on the servo while the black wire was held to

ground.

Figure 6. 74HC4017 Decade Counter The 74HC4017 decade counter was wired between the servo and the microcontroller connection. The signal line of the servo was connected to pin 2. Pin 14 was connected to PB6 (OC1B) and pin 15 was connected to PB2. Pin 13 was held to ground and +5 volts was applied to pin 16.

9

Figure 7 shows the block diagram of the prototype. The comparator compared the two output

voltages from the temperature sensor and threshold temperature circuit and output a voltage to

indicate the higher value. If the voltage form the temperature sensor circuit was greater than the

threshold voltage, the fan would be automatically powered on with +5 volts. The SonaSwitch

Mini S motion sensor was used to for the rotating of the fan base to point towards the user. When

no motion was detected, a 5 volt signal was sent from the SonaSwitch sensor to the

microcontroller which made the pin high. While the pin was kept high, the program downloaded

onto the microcontroller sent a pulse width modulation signal to rotate the servo.

Figure 7 Block Diagram of the Prototype The block diagram of the prototype is shown. The two main components of the project were the temperature and motion sensors. The block diagram shows that the servo motor

10

rotated even when the ambient temperature was below the threshold temperature. Further improvements can be made to the system such that the servo motor only rotates when the ambient temperature is greater than the threshold temperature.

Results & Recommendations The prototype of the Cool O2 Automatic Sensor Fan did perform as planned. The temperature

sensor worked flawlessly. The threshold temperature was set at 95°F in order to show the

operation of the temperature sensor. When the thermistor was touched and a body heat of 98.6°F

was applied, the fan automatically powered on with +5 volts from the output of the comparator

circuit.

The motion sensor design worked as well, but had a few quirks. The original design was that the

servo motor would rotate and the fan would point towards the user as soon as the motion sensor

detected motion. However, the prototype that was produced was setup such that the servo motor

would rotate when the user walked outside of the motion sensor range. When the pin on the

microcontroller went high, the servo would rotate to point the fan towards the user. Ideally, the

servo should have rotated when the pin on the microcontroller went low. Another problem was

rotation of the servo motor. The servo would rotate between the two sensors six times before

turning off for 20 seconds. After the 20 seconds, the servo would function normally for six more

rotations. Due to the lack of time, this problem could not be investigated.

Future improvements can be made to the project. The project was built such that the servo motor

rotated when the motion sensors did not detect any motion. The design could be modified so that

the servo motor rotates as soon as the motion sensors detect motion. If the block diagram is

reviewed, it can be seen that the temperature sensor circuit and motion sensor circuit are not

connected. This means that the servo currently rotates as long as the motion sensors are active

11

even if the ambient temperature is below the threshold voltage. For future work, the output signal

of the comparator should be sent to a pin on the microcontroller which activates the motion

sensor only when the ambient temperature is greater than the threshold voltage. Currently, the

servo rotates between two positions to point the fan towards the user. An improvement that can

be made is to have the servo rotate between three positions. Thus, when both the sensors detect

motion, the servo would point towards the center position. Another simple yet effective

improvement could be to implement an LCD thermometer. The output of the temperature sensor

circuit could be sent to a LCD digital voltage readout and converted to the corresponding

temperature to display the ambient temperature. This would allow the user to read the current

temperature and know when the fan will automatically power on.

12

References 1. EDP Company, “SonaSwitch” (n.d.). Retrieved May 10, 2006 from:

http://www.edpcompany.com/sonaindex.html 2. Furman, B.J., “Interfacing a Servo” (2005). Retrieved May 10, 2006 from:

http://www.engr.sjsu.edu/bjfurman/courses/ME106/ME106pdf/servo-atmel.pdf

13

Appendices Appendix A – Programming Code Shown below is the programming code downloaded onto the microcontroller that allowed the servo motor to rotate depending on the state of the pins. #include <avr/io.h> #include <avr/signal.h> #include <avr/interrupt.h> #include <avrlibdefs.h> #include <avrlibtypes.h> #define RC_Reset PB2 uint16_t RCpulse[10], *iRC; void InitRCout(void); void StartRCout(void); void SetServo(int,int); int main(void) DDRA=0x00;//set input PORTA=0xff;//ultrasonic sensor DDRB=0xff;//set output PORTB=0xff;//servo PORTB=PINA; while(1) switch(PINA) case 0xfe://if it detects the one on left sei();//servo turns left InitRCout(); StartRCout(); SetServo(0,1000); break; case 0xfd: //if it detects the one of right sei();//servo turns right InitRCout(); StartRCout(); SetServo(0,2000); break;

14

return 0; // Servos are connected to pins 1,3,5,7,9 // Dead time on pins 2 (after 1),4 (after 3) ,6 (after5), 8 (after 7), 0 (after 9) // Channel must be 0-4 maps to 1,3,5,7,9 // value is from approx 500 (0.5 ms) to approx 2500(2.5 ms) void SetServo(int channel, int value) int period= 5000; int index= channel*2+1; // Temporarily disable interrupts unsigned char tmp = SREG; cli(); RCpulse[index]= value; index++; if (index == 10) index= 0; RCpulse[index]= period - value; // Reenable interrupts if they were on before SREG = tmp; void InitRCout(void) unsigned char i; // // High Speed Counter settings: CLK/8 normal.5us resolution // TCCR1A = (1<<COM1B1); TCCR1B = (1<<CS11); sbi(DDRB,2); sbi(DDRB,6); for (i= 0; i< 5; i++) SetServo(i, 1500); // Puteveryone at neutral (1.5 ms). void StartRCout(void) unsigned char tmp = SREG; cli(); PORTB |= BV(RC_Reset); // Reset counter (set out0 high) OCR1B = TCNT1; // Capture our start time reference PORTB &= (char)~BV(RC_Reset); SREG = tmp; iRC = &RCpulse[0]; // Point to firstentry TIMSK |= BV(OCIE1B); // Enable Compare on match interrupt

15

SIGNAL(SIG_OUTPUT_COMPARE1B) // PORTC++; if( (iRC == &RCpulse[0]) ) PORTB |= BV(RC_Reset); PORTB &= (char)~BV(RC_Reset); TCCR1A &= (char)~_BV(COM1B0); // Clear on match TCCR1C |= _BV(FOC1B); // Force OCR1B += *iRC++; // Calculate time to next compare. TCCR1A |= _BV(COM1B0); // Set on match if (iRC > &RCpulse[9]) iRC = &RCpulse[0];

16

Appendix B – Datasheets Datasheet of the NTC thermistor:

17

Datasheet of the SonaSwitch Motion Sensor:

18