This circuit is used for the frequency counter as digit number display, that can measure the period.To show a capacitance value of the capacitor directly. According to circuit shown in Figure 1. Shows the using IC 555 as astable multivibrator. Period are as follows.
T = 0.7 (RA +2 RB) Cx.
If the capacitor is connected that not know value onto a position Cx, then the period time will alter based on the value of Cx because RA and RB are a constant value.
Figure 1 Simple capacitance measurement using IC-555
If RA and RB are the appropriate period of time to read the meter, it will be the same as the capacitance of the capacitor. In with unit is picofarads, nano or microfarads.
For example, suppose a meter long periods of time with 1 second.
It’s meant to measure the capacity of one micro Farads. They will have all the values of RA + 2RB is 1.43 M.
This circuit has a little problem, is to measure the capacity of the electrolytic capacitor with a value of about 1 microfarads. As an example:
Read value long as 1 seconds that value internal resistance of the capacitor, it will leak. As a result, the reading error.
In this case, it should take 1 second at 1000uF because value resistance of the capacitor size is less than 1000 times. If use with the frequency counter with 7 digits, it is displayed as 1000000 at 1 second. Equal to the value of 1000 microfarads. And at 1 microfarads capacity is displayed as 0.001000.
The appropriate values of RA and RB are set displayed in Table 1. Resistor should be used as a metal oxide, and the error + / – 1 percent. To the reliability of the circuit. And may not be based on value table. Be calculated. And the ability of the frequency counter.
Table 1 of Simple capacitance measurement.
When assembly is complete. And the wiring, may makes the measurement errors. In the prototype. When the measurement is completed within 36 picofarads always. When we measured it to be removed. But in the case of high-value capacitors. value error is small, it can be discarded.
If you are interested in this circuit. But there is a better than this circuit.
is The digital capacitor meter projects