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İzmir University of Economics EEE 332 Digital Electronics Lab EXPERIMENT 8 The 555 IC Timer A. Background The 555 Integrated Circuit (IC) Timer is very popular IC since it is first introduced in 1972. It can be used to construct several multivibrator circuits easily. The basic block diagram of 555 IC Timer is given in Fig. 8.1. VCC 555 TIMER R1 Comparator 1 Threshold Control R R1 Q FF S Output Q' VTL Trigger Comparator 2 R1 R2 5K Discharge Q1 Ground (a) Basic Block Diagram (b) Pin Diagram Fig. 8.1. The 555 Timer IC 8-1 A.1. Astable Multivibrator When two resistors and one capacitor is connected externally as shown in Fig. 8.2, the 555 Timer IC forms an astable multivibrator. Fig. 8.2. Astable Multivibrator 555 IC The output waveform is shown in F. 8.3. At steady state, the capacitor voltage swings between VCC/3 and 2VCC/3. The charging period starts with the voltage VCC/3 and tends to go to VCC, with a time constant of = (RA + RB)C. However it is stopped when vC = 2 VCC/3. The discharging period starts with the voltage 2VCC/3 and tends to go to 0, with a time constant of = RB C. TC vO vC TD VCC VCC/3 VSS VCC/3 VSS 0 0 t Fig. 8.3. Waveforms in Astable Multivibrator 555 IC The charging period is determined as TC = (ln2)(RA + RB)C = 0.693(RA + RB)C The discharging period is then TD = (ln2)RBC = 0.693RBC The period of oscillation is then T = TC + TD = 0.693(RA + 2RB)C and the frequency f is f = 1/T The duty cycle is defined as Duty cycle = (TC/T) x 100 % 8-2 A.2. Monostable Multivibrator Connecting one resistor and one capacitor externally, the 555 Timer IC forms a monostable multivibrator as shown in Fig. 8.4. Fig. 8.4. Monostable Multivibrator 555 IC Normally the trigger input is connected VCC. Then the capacitor is discharged to 0 and the output voltage is 0. T vO vC Tr VCC VCC/3 VSS VCC/3 t Fig. 8.5. Voltage waveforms in the 555 IC Monostable Multivibrator When the trigger input is connected 0 momentarily, the capacitor starts charging towards VCC with a time constant of = RAC (Fig. 8.5). The output is switched to VCC then. The charging is stopped when vC voltage reaches 2VCC/3. The output pulse duration is determined as T = (ln3)RAC = 1.1RAC Recovery time TR is very small. 8-3 B. Preliminary Work 1. Consider the astable multivibrator circuit given in Fig. 8.6 with C = 100 nF and VCC = 5 V. Fig. 8.6. Astable Multivibrator 555 IC Determine the values of the resistors RA and RB to set f = 1 kHz and duty cycle = 60 % RA = ……………….... RB = …………………… Duty cycle = ………. % 2. Now consider the monostable circuit of Fig. 8.7. Assume RA = 100 k. Fig. 8.7. Monostable Multivibrator 555 IC Determine C to get a monopulse of approximately 5 sec. C = …………………….. 8-4 C. Experimental Work 1. Construct the astable multivibrator circuit given in Fig. 8.8 with C = 100 nF, VCC = 5 V, and the values of RA = ……………. and RB = ……………… as calculated in the Preliminary Work. (a) Observe and plot vC and vO. Fig. 8.8. Astable Multivibrator 555 IC Channel 1 scale: ………… ……./div Channel 2 scale: ………… ……./div Time division: ………… ……./div Vp-p = …………… T = ………………. f = ……………….. Fig. 8.9. Waveforms in 555 IC Astable Multivibrator (b) Measure the frequency f and duty cycle. f = ……………………….. kHz Duty cycle = ……………. % 8-5 2. Construct the monostable multivibrator circuit given in Fig. 8.10 with R A = ……………. as calculated in the Preliminary Work. Fig. 8.10. 555 IC Monostable Multivibrator Measure the monopulse width. T = .................. sec Component List: Exp C(1) 555 Timer RA= …….(calculated) Exp C(2) 555 Timer RA= …….(calculated) RB=…(calculated) 100 nF 8-6