555 TIMER APPLICATIONS AND VOLTAGE REGULATORS OBJECTIVE The purpose of the experiment is to design and experimentally verify astable and monostable multivibrators using 555 timers; to design variable voltage regulator using regulator IC 317. EQUIPMENT REQUIRED 555 timer, LM317 Resistors (designed values), ¼ W Potentiometer, ¼ W Capacitors (designed values) 0-30 V, 1A dc dual regulated power supply Trigger pulse generator 30 MHz Oscilloscope Digital Multimeter Breadboard THEORY The 555 timer IC is an integrated circuit used in a variety of timer, pulse generation, and oscillator applications. 555 timers can be used to provide time delays, as an oscillator, and as a flip-flop element. A monostable multivibrator or a one-shot multivibrator, is a pulse generator circuit in which the duration of the pulse is determined by the R-C network, connected externally to the 555 timer. Here, one state of output is stable while the other is quasi-stable (unstable). An astable multivibrator, often called a free-running multivibrator, is a rectangular-wave generating circuit. Unlike the monostable multivibrator, astable circuit does not require any external trigger to change the state of the output, hence the name free-running. The LM317 is an adjustable 3 terminal positive voltage regulator capable of supplying in excess of 1.5A over an output voltage range of 1.2 V to 37 V. It employs internal current limiting, thermal shutdown and safe area compensation FURTHER READING 1. Ramakand A. Gayakwad, Op-amps and linear integrated circuits, PHI learning, 2009. 2. R.M.Marston, Op-amp Circuits Manual, Newnes, 1989. 3. Robert Diffenderfer, Electronic Devices: Systems & Applications, Cengage Learning, 2005 84
CIRCUIT DIAGRAM Figure 1. Astable multivibrator Figure 2. Monostable multivibrator Figure 3. Voltage regulator using LM317 85
DESIGN Astable multivibrator using 555 timer T = 0.69 (RA + RB)C Frequency f = 1.45 / (RA + RB)C Duty cycle D = RB/(RA + RB) Monostable multivibrator using 555 timer T = 1.1 RC Adjustable output voltage regulator Vo = Vref (1 +R2/R1) + IADJR2 From data sheet I ADJ = 50µA Take R 1 = 240Ω, Ci = 0.1 µf, Co = 1 µf, V ref = 1.2 V PRACTICE PROCEDURE Astable multivibrator using 555 timer 1. Construct the circuit as per the diagram shown in Figure1. 2. Observe the output waveform at pin 3 and waveform across the capacitor at pin 2 on the oscilloscope. 3. Note down the frequency and duty cycle. Monostable multivibrator using 555 timer 1. Construct the circuit as per the diagram shown in Figure2. 2. Apply the external trigger and note down the output voltage. Adjustable output voltage regulator 1. Construct the circuit as per the diagram shown in Figure3. 2. Keeping the input voltage constant at 15 V, vary the adjustable resistor R2 and observe the variable regulated output voltage. 3. Keeping the adjustable resistor R2 constant, vary the input voltage and observe the regulated output. 86
PRELAB 1. Design and simulate astable multivibrator using 555 timer. Observe the output waveform and waveform across the charging capacitor. Note down the time period, duty cycle and frequency. Compare the parameters with the designed values. 2. Design and simulate monostable multivibrator using 555 timer. Apply a momentary external trigger and observe the output waveform. Note down the time period and frequency. Compare the parameters with the designed values.. 87
Exp. No.: Date: OBJECTIVE 555 TIMER APPLICATIONS AND VOLTAGE REGULATORS OBSERVATION Astable Multivibrator using 555 timer Design Circuit Diagram Waveforms 88
Inference Monostable Multivibrator using 555 timer Design Circuit Diagram 89
Waveforms Inference Adjustable output voltage regulator Design 90
Circuit Diagram Table 1: Resistor R 2 Input voltage V in volts With V in constant Output voltage, Vo Volts With V in constant Inference 91
UNDERSTANDING & LEARNING 92
RESULTS AND CONCLUSION Prepared by: Name: Reg. No.: Actual Date of Experiment:. ASSESSMENT Date of Performance:.. Report Submission Date: Submission Delay:... Student Task Max. Marks Graded Marks Pre-lab Preparation 20 Inference 10 Results & Discussion 10 Signature Post-lab / Viva-voce 10 Total 50 93