LED Dimmer

8/11/2019 LED Dimmer

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Pulse Width Modulation (PWM)

LED Dimmer Circuit

Using a 555 Timer Chip


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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.


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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


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PWM Circuit with 555 Timer


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How a 555 Timer Works

Astable Multivibrator Mode

http://www.williamson-labs.com/480_555.htm
http://www.williamson-labs.com/480_555.htmhttp://www.williamson-labs.com/480_555.htmhttp://www.williamson-labs.com/480_555.htmhttp://www.williamson-labs.com/480_555.htm


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http://www.williamson-labs.com/480_555.htm


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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.


8/29http://www.williamson-labs.com/480_555.htm


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Transistor

As you will learn in ECE 2204, a transistor canbe 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 avery small resistor is between the collector (C) andthe emitter (E).

When ground is applied to the base of the

transistor (B), the transistor acts like there is a anopen circuit between the collector (C) and theemitter (E).


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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 thefrequency of the voltage source tied to the Triggerinput pin.

The width of the 5V pulse within a cycle is determined

by the amplitude of the signal on the Control pin of the555 Timer chip.


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PWM

Voltage on

Control pin

Output voltage

of the 555 Timer

http://www.hobbyprojects.com/general_theory/pulse_modulation.html
http://www.hobbyprojects.com/general_theory/pulse_modulation.htmlhttp://www.hobbyprojects.com/general_theory/pulse_modulation.html


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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:

CRf

)3ln(2

1


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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.


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Square Wave with Offset Voltage

Vo

0-5V

square

wave


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Amplitude Reduction and Voltage

Offset


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PSpice Simulation using Vpulse


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PSpice Simulation using Digclock


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Light Emitting Diode (LED)

A nonlinear component that looks like a

battery when it is on and an open when it is

off.


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Discrete LEDs


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Anode vs. Cathode

Switch your DMM to the diode symbol.

Place the red probe into the V-W plug and theblack 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 iscontacting the cathode of the diode.

If the result is an overload (overflow) condition, thenthe red probe is contacting the cathode and the blackprobe is contacting the anode of the diode.


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Simulating a LED in PSpice

No LED part in the student PSpice so we use aseries 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.


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Experimental Procedure

First, construct and test the square wave generator.

Second, construct and test the summing amplifiercircuit.

Only after verifying that the two circuits operatecorrectly, connect them together.

Third, construct the pulse width modulator circuitusing the 555 timer.

If the circuit doesnt work properly, rework the circuit as a

monostable vibrator and connect the Velleman functiongenerator (500 Hz square wave with a peak-to-peakvoltage of 5V and an offset voltage of 2.5V) to the trigger.


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Debugging

Find reasonably points within circuit to makemeasurements.

Start at the beginning of the circuit:

Compare measured voltages with expectedvoltages obtained either through circuit analysisor 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.


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If they dont, 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 goingwhere they are supposed to and nothing is accidentlytouching.

Make sure that your oscilloscope probes are connected tothe correct points in the circuit.

Replace Rf on the summing amplifier if the output voltage isnot 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 doesnt function properly, see one of the OpEL

GTA for assistance.


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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).


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Pre-Lab Report

1. The plot generated from the transient analysis of the circuit in Figure 1 using:

a 10 k 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 k.

3. The plot generated from the transient analysis of the circuit in Figure 1 where thevalue of Ra is changed to 100 .

4. The plot generated from the transient analysis of the circuit in Figure 2 where the

value of Ra is changed to 1 k where voltage probes are placed atthe output pin of

both operational amplifiers. The final time for the transient analysis should be 40

ms.


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Post-Validation Report

See the description of items to be include in

the Instructions for LED Dimmer

LED Dimmer

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Transcript of LED Dimmer