SP.764, Practical Electronics Dr. James A. Bales Lecture 8: Final Project – Fan Controller
Topics:
1) Final Project Introduction 2) Background Information Final Project Introduction:
-
The final project will evolve from the 555 timer circuit. -
The project will consist of building a fan controller.
-
Involves giving the system a reference signal from a thermistor. (Therefore, one is expected to know how to read thermistor values.)
-
Use the 555 timer circuit as a one-shot to catch any input signal and produce
a fixed-width pulse output signal.
-
The thermistor driving a 555 timer will give a variable frequency signal.
Background information:
-
PWM -
How motors work. o
Constant current
o
Variable voltage
o
Behave like a LP filters
o
Require lots of current
Voltage
1V
2V
3V
Speed
2500 rpm
3500 rpm
4500 rpm
Current
0.5A
0.5A
0.5A
Transistor
multiplies
current
IN
+
555
PMW
Fan motor
needs high
current
Controlling the fan:
-
Measure temperature
o
-
Thermometer
Measure revolutions per minute (RPM)
o
Photo-transistor
Displaying temperature values:
+5V
(
V 25D C
)
+5V
37 D C
27 D C
Connect the input of a series of op-amps in such a way that there is a common
voltage (VTHERM) reaching the inputs of all op amps. Choose resistor values to create
reference voltages that represent different temperature values. Since the 555 is
better at sinking current rather than sourcing it, connect LEDs as shown above.
SP.764, Practical Electronics
Dr. James A. Bales
Lecture 8 Page 2 of 3 +5V
R1
8
4
7
R2
3
555
6
Out
2
1
2/3 1/3 5
C1
555PIN −2
VTHERM
+5V
555 PIN − 2
VTHERM
t
VOUT
t
Since VTHERM is a variable voltage, use a comparator to control the average voltage
delivered to the motor. If VTHERM increases (implying higher temperature), the output
of the comparator will have a higher duty cycle and the motor will spin faster. If
VTHERM decreases, the duty cycle of the output decreases and the motor spins slower.
SP.764, Practical Electronics
Dr. James A. Bales
Lecture 8 Page 3 of 3 MIT OpenCourseWare
http://ocw.mit.edu
EC.S06 / EC.S11 Practical Electronics
Fall 2004
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