Expt 4

Capacitance Measurement Using 555 Timer EXPT: 4 Objective: This experiment is intended to measure the unknown capacitance using mono-stable(one-shot) mode of 555 timer.

Theory: When the 555 timer is operated as a mono-stable (one-shot) mode the output voltage is low until a negative going trigger pulse is applied to the input. The output switches to high as soon as it is triggered by a negative pulse. The time the output is high is determined by the RC network externally connected to the IC. At the end of the timing interval. The output returns to the low state again. Thus a mono-stable 555 timer produces a positive plus at the output those width is given by t=1.1RC seconds. If the one-shot timer is triggered by negative pulse train(series of pulses) of frequency f. the average dc output of the timer will be Vout=Vcctf=1.1VccfRC (1) Now the voltage output is first measured for a standard capacitor of capacitance Cs=So that V1=1.1VccfRCs (2) Then the unknown capacitor of capacitance Cx is connected instead of the standard capacitor and the voltage is measured again when: V2 =1.1 Vcc fRCx (3) From the equation (2) and (3) the unknown capacitance is determined by Cx =Cx (V2 /V1) (4)

Apparatus: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Timer 555 Transistor C828 Ragistor(1K-2Pc., 10K-2Pc.) Capacitors (0.01 uf, 0.1 uf, 0.22 uf, 0.33 uf, 0.47 uf, 0.68 uf, 1 uf,) DC power supply Bread board Wires and leads Oscilloscope Function Generator

Circuit Diagram:

Data: Cs

V1

V2

Cx

Questions: 1. Show wave shapes at different points. 2. Measure the capacitance of an unknown capacitor five different standard capacitors 3. Take the average and compare the value with the value marked on it.