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UNIVERSITY OF MASSACHUSETTS DARTMOUTH COLLEGE OF ENGINEERING EGR 101 INTRODUCTION TO ENGINEERING THROUGH APPLIED SCIENCE I 555 INTEGRATED CIRCUIT TIMER THE 555 TIMER USED AS AN ASTABLE MULTIVIBRATOR The outline and pin configuration for a 555 Integrated Circuit Timer is shown in Figure 1. At this point, it is not necessary to discuss the internal devices in any detail. Figure 1. A typical 555 IC Timer. In your project, the 555 timer is used in a configuration known as an Astable Multivibrator, where an output voltage is delivered as a rectangular wave. The pulse and space width can be controlled by properly selecting two time constants. In this configuration, the 555 timer is connected as shown in Figure 2. The time constant that determines the pulse width depends on resistors RA, RB, and capacitor C. The pulse width is the time it takes for the capacitor C to charge via resistors RA and RB from 1/3VCC to 2/3VCC, as shown in Figure 3. While this charging is taking place, the output voltage will be equal to VCC. Figure 2. The capacitor C1 charges via RA and RB. Capacitor C1 “charging” VCC Figure 3. The waveforms of the capacitor and output voltages during charging. 2 Inside the 555 timer there is a component known as a transistor, which is labeled Q1. Think of the transistor as an ON/OFF switch, which is done electronically. When the capacitor voltage reaches 2/3VCC, the transistor Q1 will be switched ON, creating a short-circuit path to ground from pin #7 causing C1 to discharge towards 0 Volts via resistor RB, as shown in Figure 4, and the output voltage to fall to 0 Volt. When the capacitor voltage reaches 1/3VCC, as shown in Figure 5, the internal transistor Q1 is switched OFF, causing the capacitor to charge towards 2/3VCC again. This “cycle” repeats over and over again. Figure 4. The capacitor C1 discharges via resistor RB. 3 Capacitor C1 “discharging” 0V Figure 5. The waveforms of the capacitor and output voltages during discharging. MATHEMATICAL DESCRIPTIONS OF THE OUTPUT WAVEFORM Pulse Width The mathematical expression for the capacitor voltage on the “charging” (pulse width) portion of the output waveform is given by t vC (t ) VCC 1 e Solving for the pulse width (PW) yields PW ln 2 RA RB C1 . Space Width On the “discharging” (space width) portion of the output waveform, the capacitor voltage can be expressed as vC (t ) VCC e t The space width (SW) turns out to be SW ln 2 RBC1 4 Frequency The frequency of the resulting rectangular output voltage (in Hz) is determined by f0 1 RA 2RB C1 Period The period (in seconds) of the output voltage is the sum of the PW and SW, and can also be determined as the reciprocal of the frequency. T PW SW ln 2 RA 2RB C1 Duty Cycle We can define a quantity that relates the output voltage’s “On-time” (the PW) to the total period (T). This “Duty Cycle” is expressed as a percentage and is given by R RB %DutyCycle A 100% RA 2 RB 5 THE 555 TIMER CONFIGURATIONS USED IN THE PROJECTS The 555 timer can be simulated using MultiSim, as shown in Figure 6. The frequency of oscillation and/or duty cycle of the oscillator can be changed by a combination of resistance and capacitance. XSC1 G T VCC 9V A B RA 10k RB 68k 4 7 6 2 5 C1 0.01uF 8 U1 VCC RST OUT 3 DIS THR TRI CON GND LM555CN 1 LOAD 8 Figure 6. The basic 555 Timer Astable Multivibrator using MultiSim. There are two modifications that can be made to this basic configuration which allow independent control of the timing periods or frequency control by means of an externally applied voltage. Those circuits are discussed next. 6 THE 555 TIMER WITH INDEPENDENT CONTROL OF THE TIMING PERIODS The frequency and duty cycle are mathematically related since each is a function of both RA and RB. If we could “remove” resistor RB from the charging portion of the output voltage, making its value equal to 0, the pulse width would be given by PW ln 2 RAC1 . The space width would still be given by SW ln 2 RBC1 , In this case, the timing period of the output becomes independently adjustable, allowing us to adjust the frequency and duty cycle individually. This can be accomplished by connecting a diode across resistor RB, as shown in Figure 7, in such a direction that it acts as a short circuit on the charging cycle (eliminating resistor RB from the circuit) and an open circuit on the discharging cycle (leaving resistor RB in the circuit). XSC1 G T VCC 9V 65% D1 A B RA Key = A 100k RB 68k 1N4007GP C1 0.47uF 4 7 6 2 5 8 U1 VCC RST OUT 3 DIS THR TRI CON GND LM555CN 1 LOAD 8 Figure 7. A 555 Timer with independent control of the timing periods. 7 THE 555 TIMER AS A VOLTAGE-CONTROLLED OSCILLATOR (VCO) Up until now, pin 5 of the 555 Timer IC has been left open. This pin is labeled CONTROL (CON for short) and can be used to change the frequency of a 555 timer configured as an oscillator by connecting it to a variable voltage supply. In the circuit shown in Figure 8, a potentiometer is connected end-to-end across the 9-V battery, and the potentiometer arm is connected to pin 5 of the 555 Timer. XSC1 G T VCC 9V A B RA 68k RB 10k Frequency C1 0.01uF 4 7 6 2 5 8 U1 VCC RST OUT 3 DIS THR TRI CON GND LM555CN 1 LOAD 8 Key = A 100k 50% Figure 8. A 555 Timer connected as a Voltage-Controlled-Oscillator (VCO). The next example illustrates how to simulate and calculate in MultiSim the frequency and duty cycle for a basic astable multivibrator. 8 MULTISIM EXAMPLE A typical 555 Timer configured as an Astable Multivibrator is shown in Figure 9. Let’s check the expected values of the frequency and duty cycle. f 1 RA 2RB C1 %DutyCycle 1.44 10 (2)(68)103 (0.01106 F ) R A RB R A 2 RB 100% 10 K 68 K 10 K (2)(68 K ) 986 Hz 100% 53.42% The Oscilloscope data shows that the period of the output is 1.020ms, so the frequency is f 1 T 1 1.02 103 s 980 Hz Figure 9a. Measuring the period of a 555 Timer configured as a 1-kHz oscillator. 9 Checking the duty cycle, Figure 9b. Checking the duty cycle of the 1-kHz 555 Astable Multivibrator. %DutyCycle 535.71 106 s 1.02 103 s 100% 52.51% 10