The proposed capacitance is a very powerful assembly to measure in 7 ranges
capacitors of 1 pF to 999 uF, avecune excellent precision of about 10 -3 .
The range change is fully automatic. The display shows 3 digits
with decimal point. Three LEDs indicate the unit: pF nF uF or.
Power is normally with a battery or 9V battery with automatic shutoff
after 2.5 minutes. A jack for external power supply is also planned.
Click here to download the full size of the above Circuit.
The capacitor Cx to be measured determines the RC time constant of a monostable built
IC3, a 555 Cmos (t = RC)
And a capacitor of 1 nF and a load resistor (R19 + P1) 1 M W , give
a pulse of t = 10 6 x 1.10 -9 = 10 -3 s = 1 ms = 1000 microseconds.
This impulse appears in S and is measured by IC1 in mu.s, which gives a result equal to
1000 posted on 3 digits as "1.00 nF"
Monostable IC3 is triggered periodically by the astable oscillator IC2, a 555 also.
The transistor T4 requires a reset of IC2 as the impetus created by IC3 is not over.
The mu.P IC1 measure the duration of the pulse, manages the display of 3-digit multiplexed, determines
Three most significant digits to display, remove leading zeros, place the decimal point.
It also ensures the switching of the load resistance
The first 4 lines are with R19 + P1 giving a value ranging pF or nF
until 983 nF. In addition, R20 + P2 are connected in parallel so as to have a speed of
load 1000 times to go up 999 uF
The 270-pF capacitor C4 is permanently connected to the measuring input.
It allows a minimum pulse measured by the system.
Pz pusher allows for the offset of that value and then display "0 pF.
Note that, powering the "zero" is done automatically.
Powering internal battery is by a short press Pon making T6 driver.
The program starts and then passed to a PC3 So T5 driver:
The pusher may then be released.
2.5 minutes later, PC3...
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