Introduction

construction-1

Construction-2

Construction-3

Calibration

As a reference you may wish to open up a new window to view the circuit while on this page .

Calibration

Optional Ranges . By adding the 1.uF capacitor in parallel to the .1uF capacitor we obtain a x10 expansion of range #3 and # 4 . I did not want to use a six position switch when a simple SPST switch can do the work only when very large capacitors need to be tested . On the other hand some may object to having both ranges 3 & 4 expanded when only expansion of the last range is needed .
If this is the case , removed the jumper between pin # b-4 and # b-5 and install an additional .1 uF Capacitor from # b-5 to pin # 12 of IC-4049 thus the range # 4 will not be expanded . You can mount the additional capacitor directly on the switch lug .
I was very pleased to be able to measure capacitance as small as 3 pico with nary a flicker of the display and I hope you can get the same results.

Before you start you calibration ensure that the supply voltage is exactly the one you will be using for the system operation then proceed.

Calibration is easy and can be accomplished two ways;

1 - If you have a frequency meter adjust RV1 to RV3 for each range to the approximate following frequency ( reading from pin # 12 of IC-4049 ) . I say " approximate " because IC's like other components are not all exactly the same . Of the three MC 4049 I used for testing I found a range of 5 kHz difference , so it is not exactly a matter of timing but rather matching two oscillators.

Picofarad	85 kHz
.001	8.5 kHz
.1	850 Hz

1 - If a frequency meter is not available the best result will be obtained by measuring known value capacitors that will read in the lower three digits range as follow:

RANGE	TEST VALUE	DISPLAY
Pico	100 pico	100
.001	.01	100
.1	1.uF	100
10. uF	100 uF	100

Final Notes

For simplicity I used two short lengths( 4 ") # 22 stranded lead wires with a small hook type connector at each end . With the test leads open ( no test capacitor connected ) the first digit display should read zero but a small residual reading of 1 or 2 may be seen and this residual should be added to capacitance reading below 100 pico.These conditions are normal and are the product of the very high internal oscillation with open leads when in the Picofarad range .The next two ranges should read zero on the first digit and all zero on the Microfarad range .

Range 4 and 5 do not need to be calibrated, but if you wish to do so , add and additional RV to switch # a-4 and remove the jumper between pins #a-3 and # a-4 , the other end connects to pin # 9 of IC4049 .

The total recommended RV's resistance is 7K ohms , a combination of one resistance named " R " on the layout is used in series with the RV's so that any RV of a smaller value will make it easier to calibrate the required frequency . If " R " is not used merely install a jumper across the PC connection points .

The actual total resistance setting for RV's on my prototypes was measured as 6K ohms so a combination of 5 K for " R " is series with 2K Rv's would make for easier calibration . The other approach is to use 10 turns trimmers of 10 K ohms .

It is greatly satisfying to be able to reach a set goal developed from an idea and I am well pleased with the outcome , now the owner of a Digital Capacitance Meter that I can trust and rivals professional units . Have fun and tell me how yours turns out and I will be glad to answer any of your queries.

Where

For reference only , the following products descriptions were found listed in the Digikey catalog , prices are in Can-$ .

Rotary Switch PC lugs

3-poles,4-positions ,single deck

Cat # EG 1955

(Can. ) $ 7.92

Found one exactly the same" Lorlin-UK" from Intek Electronics, Vancouver,for Can$2.95

Displays	3-digits .5" H ,red/orange/green ( multiplexed )	Cat # 67-1423 ND , common anode	$ 7.95
Single Display	( Panasonic ) .3"H Cat # P232-ND, .43"H Cat # P326-ND , .57"H Cat # P326-ND ( common anode )	( Can ) $ 2.44 , 2.45, 2.53
MC 14553	Cat # 07F4166	From Newark Electronics	$ 1.98 U.S , Can $ ??

Newark Electronics
U.S.A
Canada