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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

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

PWM Voltage on Control pin Output voltage of the 555 Timer

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:

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
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.

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