Download LED Dimmer

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
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.
http://www.williamson-labs.com/480_555.htm
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).
Monostable Multivibrator
• 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.
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
Light Emitting Diode (LED)
• A nonlinear component that ‘looks’ like a
battery when it is on and an open when it is
off.
Discrete LEDs
Anode vs. Cathode
• Switch your DMM to the diode symbol.
• Place the red probe into the V-W plug and the
black probe into the COM plug.
• Place your probes across the diode.
– If the result is a very small number, then your red
probe is contacting the anode and the black probe is
contacting the cathode of the diode.
– If the result is an overload (overflow) condition, then
the red probe is contacting the cathode and the black
probe is contacting the anode of the diode.
Simulating a LED in PSpice
No LED part in the student PSpice so we use a
series combination of parts.
– Dbreak (diode breakout part)
• Allows current to flow when the voltage on the anode
is 0.7V higher than the voltage on the cathode.
– Vdc
• Set to the difference in the voltage needed on the
anode to turn the LED on
– Our red and green LEDs need between 2-2.5V.
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.
• Replace Rf on the summing amplifier if the output voltage is
not close of a 0V-5V square wave.
– The circuit usually works even if the max. of the output voltage is
3.5V instead of 5V.
• 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 one of the OpEL
GTA for assistance.
Pre-Lab
• This is a longer lab so the answers for the
questions in the Analysis Section are:
RLED = 2.5V/10mA = 250 W.
f = 1/[2ln(3)(910 W)(1mF)] = 500Hz
Vopamp2= -(1k W /3.6k W) Vsqwave -(1k W /3.6k W)(-9V
Vopamp2= -0.278 Vsqwave + 2.5V
• You must enter the results from the PSpice
simulations described in the experimental
procedure (and on the next slide).
Pre-Lab Report
1. The plot generated from the transient analysis of the circuit in Figure 1 using:
– a 10 kΩ resistor instead of the trim pot for Ra
– V2 is a sinusoidal input voltage source with amplitude of 2V with a 2V offset
voltage and a frequency of 60Hz.
– Vpulse should have a magnitude of 5V and a period of 200us with a 50% duty
cycle.
– V2 and V3, are displayed as well as the output voltage. The final time for the
transient analysis should be 50 ms.
2. The plot generated from the transient analysis of the circuit in Figure 1 where the
value of Ra is changed to 1 kΩ.
3. The plot generated from the transient analysis of the circuit in Figure 1 where the
value of Ra is changed to 100 Ω.
4. The plot generated from the transient analysis of the circuit in Figure 2 where the
value of Ra is changed to 1 kΩ where voltage probes are placed atthe output pin of
both operational amplifiers. The final time for the transient analysis should be 40
ms.
Post-Validation Report
• See the description of items to be include in
the Instructions for LED Dimmer