I've been trying to write Arduino coding for this circuit but it still not successful. I hope anyone can help me. This circuit basically is about to detect the unknown capacitance of oil that were placed at the measurement object by using resonance method. This capacitance meter need to be measure a very low capacitance value with range of 0.1-100pF.
Hi Daya Zahid ,
do you have access to an oscilloscope? I suspect it's not (only) a software problem but hardware related: I expect the combination of C1 and D1 to do nothing much:
1. D1 = 1N4002 is a rectifier for low frequency like 50 .. 60 Hz. Certainly the frequency of the quarz is orders of magnitude higher.
2. If D1 is replaced by a suitable diode then C1 will be charged only once because there is no path for discharging. So, the coupling between oscillator and LC circuit is very weak.
If your measurements reveal that the oscillator is working but the LC circuit is not, I would replace the 1N4002 by something like 1N4148, and add a second 1N4148, cathode at the point between C1 and D1, anode at GND.
The software has to configure the 16 bit timer T1 as counter, clocked by the pulses at pin T1 = PD5 of the ATmega328P (named for unknown reasons "D5" by the Arduino developers), and has to configure one of the other two timers as timer, generating the time window during which the pulses are counted.
The relation between the frequency f of the signal of the LC circuit, the content of the counter c, and the duration of the time window t is: f = 8 * c / t
BTW, the datasheet of the LM393 doesn't specify the input capacitance but I guess it's at least a few pF. This input capacitance is parallel to C6, and affects the resonance frequency, of course.
Hi Daya Zahid ,
now I discovered your description of the purpose of the circuit; I guess you added it later.
If I understand correctly, you want to determine the relative permittivity of oil by placing the oil between the plates of the capacitor C6. The dimensions of the capacitor are such that the air filled capacitor has about 0.1 pF.
As mentioned in my earlier answer, there is already the capacitance of the comparator inputs; additionally, the wires connecting the capacitor plates to the circuit contribute to the capacitance, and even each inductance has a (small) capacitance, too. Your task would become easier if the capacitance of C6 (air-filled) would increase considerably, e. g. by increasing the area of the plates.
Secondly, pay attention to the reaction time of the comparator: It shouldn't be more than half the period of the maximum frequency. So, same as the 1N4002, the LM393 is too slow. With the TLV3201, for example, the upper frequency limit were at about 10 MHz.
Finally, I don't get the basic idea of the circuit: Given the idea of forming an LC circuit out of a known L and an unknown C, the first notion that comes to mind is building an oscillator around this resonant circuit, and measuring the frequency.
But in your solution, the LC circuit is excited by a constant frequency (the frequency of the quarz oscillator). As a consequence, you'll get a high amplitude (sufficient for the frequency counter) if the resonance frequency of the LC circuit is equal to or nearly equal to the quarz frequency, and a very low amplitude (and a zero result by the frequency counter) otherwise.
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