Transcript Automatic Holiday Light Display

Pulse Width Modulation (PWM)
LED Dimmer Circuit
Using a 555 Timer Chip
Goals of Experiment
• Demonstrate the operation of a simple PWM
circuit that can be used to adjust the intensity of
a green LED by varying the value of one resistor.
• Demonstrate how a square wave generator can
be built using an operational amplifier.
• Demonstrate how a summing amplifier can be
used to add a dc offset to a time-varying signal.
• Describe how to test complex circuits.
Pulse Width Modulation
An energy efficient technique that is used
widely in industrial and consumer electronics
to vary the power dissipated by a load.
– Electric motors
– Electric stove elements
– Light dimmers
PWM Circuit with 555 Timer
How a 555 Timer Works
• Astable Multivibrator Mode
http://www.williamson-labs.com/480_555.htm
Voltage Comparator
• Op Amp circuit without a feedback
component.
– Output voltage changes to force the negative
input voltage to equal the positive input voltage.
• Maximum value of the output voltage, Vo, is V+ if the
negative input voltage, v1, is less than the positive input
voltage, v2.
• Minimum value of the output voltage, Vo, is V- if the
negative input voltage, v1, is greater than the positive
input voltage, v2.
Transistor
• As you will learn in ECE 2204, a transistor can
be designed to act like a switch.
– When a positive voltage is applied to the base of
the transistor (B), the transistor acts like there is a
very small resistor is between the collector (C) and
the emitter (E).
– When ground is applied to the base of the
transistor (B), the transistor acts like there is a an
open circuit between the collector (C) and the
emitter (E).
Pulse Width Modulation
• The frequency of the output from the 555
timer is constant. It is determined by the RC
time constant of the external components, Ra
and C1, and the frequency of the voltage
source tied to the Trigger input pin.
• The width of the 5V pulse within a cycle is
determined by the amplitude of the signal on
the Control pin of the 555 Timer chip.
How a 555 Timer Works
http://www.williamson-labs.com/480_555.htm
PWM
Voltage on
Control pin
Output voltage
of the 555 Timer
http://www.hobbyprojects.com/general_theory/pulse_modulation.html
Square Wave Generator:
Relaxation Oscillator
The duty cycle of the
output will be 50%
when R1 = R2.
The frequency of the
square wave output is
given by:
1
f 
2 ln( 3)  R  C
The delay in obtaining the square wave output is a result of
- the initial condition of the capacitor (IC = 0V),
- the amount of electronic noise in the circuit,
- the degree to which your op amp is nonideal, and
- the RC time constant of the charging circuit for the capacitor.
Square Wave with Offset Voltage
Vo
0-5V
square
wave
Amplitude Reduction and Voltage
Offset
PSpice Simulation using Vpulse
PSpice Simulation using Digclock
Experimental Procedure
• First, construct and test the square wave generator.
• Second, construct and test the summing amplifier
circuit.
– Only after verifying that the two circuits operate
correctly, connect them together.
• Third, construct the pulse width modulator circuit
using the 555 timer.
– If the circuit doesn’t work properly, rework the circuit as a
monostable vibrator and connect the Velleman function
generator (500 Hz square wave with a peak-to-peak
voltage of 5V and an offset voltage of 2.5V) to the trigger.
Debugging
• Find reasonably points within circuit to make
measurements.
• Start at the beginning of the circuit:
– Compare measured voltages with expected
voltages obtained either through circuit analysis
or simulation.
• This means that you should do your calculations and
simulations before you construct the circuit.
– If they match reasonably well, move on to the
next measurement point.
– If they don’t, do the simple things first:
• Verify that power is on and connected at the right
places (particularly V+ and V- on op amps).
• Check component values.
• Run through wiring to make sure that the wires are
going where they are supposed to and nothing is
accidently touching.
• Make sure that your oscilloscope probes are connected
to the correct points in the circuit.
• Only if you are confident that you have everything
above right, consider replacing the op amp or 555 timer
chip.
– If the circuit still doesn’t function properly, see Joe, the OpEL
GTA, for assistance.