1. CAPACITANCE METER Tests capacitors from 1p to 10u, in 6 ranges

2. ACKNOWLEDGEMENT

3. GROUP MEMBERS SYED ALI AHAD (2004-EE-65) RAHIL SHAFI (2004-EE-68) FARHAN AHMED (2004-EE-90) JAWWAD PARACHA (2004-EE-91)

4. BLOCK DIAGRAM OF CAPACITANCE METER

5. PROJECT SCHEMATIC

6. OPERATION OF A SCHEMATIC This circuit reads the value of a capacitor and displays it on an analogue meter. We wanted to use, the 74c14 hex Schmitt Trigger chip in this project. It operates by detecting 2/3 of the rail voltage to change the state of the output. The charging of a capacitor is a non linear function and so we must create a circuit that by-passes the non linear problems and works on a linear arrangement. We have done this by charging the capacitor to a specified voltage level and this level is detected by a gate.

7. OPERATION OF A SCHEMATIC The circuit works on the timing principle. The first oscillator, between pins 1 and 2 has a high time of 100 units and a low of 1 unit. The HIGH is created by the 120k resistor and the capacitor selected by the rotary switch and the low is created by the 2k2 and diode. This gives us our starting point where we can divide a scale into 100 parts. The next section of the circuit charges the test capacitor via a resistor selected by the rotary switch. The requirement of this section is to charge the largest capacitor (in the range) in exactly 100 units of time.

8. OPERATION OF A SCHEMATIC This means a 100p capacitor will take 100 units of time to charge and it will not quite get to the point where the gate detects a HIGH before the output of the timing oscillator goes tow and discharges it, ready for the next cycle. This means the output pin 4 will not go low so that the 3n3 capacitor will remain fully charged. The third gate inverts this result so that output pin 6 remains low and thus the meter gives a full range reading of 10v. This is read as "100" to give a value of 100p. If a 99p capacitor is tested, it will charge in 99 units of time and the output pin 4 of the Schmitt gate will go low for one unit of time and discharge the 3n3 capacitor. This will make output pin 6 of the third Schmitt trigger HIGH for one unit of time and thus the meter will be turned off for one unit of time out of 100.

9. OPERATION OF A SCHEMATIC Since the circuit is operating at a fairly high frequency, the needle of the meter does not have time to swing back to zero during this one unit of time but it does drop a small amount and the result is it sits on 9.9, or "99" on the scale. At the other end of the scale, if a 1p is fitted, the capacitor will charge in one unit of time and output pin 4 will go low for 99 units of time. The inverter between pins 5 and 6 will produce a one-unit pulse to the meter and thus the needle will rise.1 v and this will be read as "1" or 1p.

10. OPERATION OF A SCHEMATIC Now we come to the nulling circuit made up of the 10k pot and 3n3. If no capacitor is fitted to the test terminals, the mark-space ratio oscillator will produce a high for 100 parts, and a low for one part of the time and this will go directly through the inverter between pins 3 and 4. If the nulling components were not present, output pin 6 would be high for 100 parts and low for one part. This means it would put a small "set" of about one part on the meter and cause a unit deflection. In other words the meter would not zero and when measuring the low values in any of the ranges, such as 1p, 1n etc, the needle would add one to the result to get "2" etc.

11. OPERATION OF A SCHEMATIC This may not be important at 100p, but at 2p2 the value will be 3p3 and it would be annoying to have to take 1p off the readings. The 3n3 and 10k trim pot are designed to create a timing circuit that takes one unit of time to charge and thus the first one unit of time is not registered on the output. The other features of the circuit are a voltage stabilized section made up of a zener diode and 100R resistor as well as a LED to indicate the battery voltage is above the10v required to drive the circuit and a test button so that power is only applied

12. GRAPHICAL REPRESENTATION The input of the 74c14 detects the voltage on the capacitor at 70% and up to this point the charging is quite linear.

13. PARTS LIST 1 - 100R 1 - 1k5 1 - 2k2 1 - 10k 1 - 100k 1 - 120k 1 - 1M 1 - 10k mini trim pot 1 - 1n green cap (102) 1 - 3n3 green cap (332) 1 - 10n green cap (103) 1 - 100n monoblock (104) 3 - 1N4148 diodes

14. PARTS LIST 1 - 8v2 zener 1 - 5mm red LED 1 - 74c14 Hex Schmitt trigger IC 1 - 14 pin IC socket 1 - 2 pole 6 position rotary switch 1 - knob to suit 1 - push button 1 - 12v lighter battery 3 - machine pins for test socket 1 - 20cm red hook-up flex 1 - 20cm black hook-up flex 2 - paper clips for multimeter

15. APPLICATIONS/BENEFITS