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ECE 2B Lab #5

Timers and Oscillators
This lab continues our exploration of basic transistor switching circuits to include timing circuits and oscillators. We will explore some simple multivibrator circuits using discrete transistors, comparators, and finally circuits that use the popular 555 timer chip. We will use the latter to create a simple fan speed controller using a variableduty-cycle driver.

Timers and Oscillators
Pre-lab Preparation Before Coming to the Lab Parts List Duty Cycle In-Lab Procedure 5.1 Discrete Multivibrator Circuits Manual One-Shot Timer Bistable-Multivibrator or “Flip-Flop” Another Monostable-Multivibrator or “One-Shot” BJT-based Astable Multivibrator 5.2 Relaxation Oscillator with Variable Duty Cycle Schmitt Trigger Relaxation Oscillator CircuitVariable Duty-Cycle Oscillator 5.3 The 555 Timer IC Astable Multivibrator Linear Ramp Generator Fan/Motor Speed Control

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© Bob York 2007


Timers and Oscillators

Pre-lab Preparation
Before Coming to the Lab Read through the lab experiment to familiarize yourself with the components and assembly sequence. Before coming to the lab, eachgroup should obtain a parts kit from the ECE Shop. Parts List The ECE2 lab is stocked with resistors so do not be alarmed if you kits does not include the resistors listed below. Some of these parts may also have been provided in an earlier kit.

Laboratory #5

Timers and Oscillators
Qty 1 4 1 2 2 4 4 4 2 8 4 2 3 6 2 3 8 4 2 Description 2N7000 NMOS 2N3904 NPN BJT 2N3906 PNP BJT LM393 ComparatorLM555CM LED, Red diffused, 5mm (T1 3/4) 20mA 0.1uF capacitor (low-volt. ceramic) 10uF capacitor (electrolytic, 25V, radial) 100uF capacitor (electrolytic, 25V, radial) 330-Ohm 1/4 Watt resistor 1-KOhm 1/4 Watt resistor 2.2-KOhm 1/4 Watt resistor 4.7-KOhm 1/4 Watt resistor 10-KOhm 1/4 Watt resistor 22-KOhm 1/4 Watt resistor 33-KOhm 1/4 Watt resistor 100-KOhm 1/4 Watt resistor PC Mount Tactilepush-buitton switch (PB1) 10k trimpot

© Bob York 2007


Background information
Suggested background reading: ■ Hands-on Electronics, Chapter 9 ■ ■ Phillips Application Note AN170, “NE555 and NE556 Applications” (posted on course web site).

Duty Cycle In this lab we will work with rectangular pulse waveforms, for which the term “duty cycle” comesup frequently. For such waveforms, the duty cycle is defined as the ratio of pulse duration to pulse period. For example, consider the waveform shown in Figure 5-1:

Figure 5-1 – Rectangular waveform for duty cycle definition

The pulse duration is τ ; this is how long the pulse remains high (amplitude=1 in the figure).The pulse period is T ; this is the duration of one complete cycle, and isjust the inverse of the frequency in Hz ( f = 1/ T ). So the duty cycle in this case would be

(5.1) T Clearly 0 ≤ D ≤ 1 , which we usually express as a percentage. So, for example, a 50% duty cycle is one where D = 0.5 , or τ = 0.5T .




© Bob York 2007


Timers and Oscillators

In-Lab Procedure 5.1 Discrete Multivibrator Circuits
A multivibrator is a circuit used toimplement a variety of simple two-state systems such as oscillators, timers and flip-flops. There are three types of multivibrator circuit: ■ Astable, in which the circuit is not stable in either state, and continuously oscillates from one state to the other. ■ Monostable, in which one of the states is stable, but the other is not. The circuit will flip into the unstable state for a determinedperiod, but will eventually return to the stable state. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot. A common application is in eliminating switch bounce. ■ Bistable, in which the circuit will remain in either state indefinitely. The circuit can be flipped from one state to the other by an...
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