More explanation regarding what I want to achieve in my project
Hello all,
this might be a little bit messy and long, but I think I need to brainstorm here what are actually inside my head. Tell me, please, if I am doing wrong or stupid. Just point out where my mistakes are, give me clues, suggestions, or even more, answer/solution to my problem. Six weeks to go and I need to settle both electrical and programming. The mechanical design was finished with some touch-up at the end then I can go for my thesis defense. I will only post things that I have done so far, I am not a person that don't try anything out and post here, I am trying everything everyone said and found failures upon failures. I am thinking straight, but when it comes to six hours of sleeping I cannot think that straight, so I need you guys. So enough for the junk, let's go to the project.
First phase: CURRENT SOURCE
In order for my device to know how big the resistance of the test resistor, there should be a signal, which is in general known by all of my electronics, including my microcontroller. Voltage is what I need. By doing some calculations and looking back to the first chapter of electrical engineering books, Ohm's Law states V = I x R where R is not the fixed one. By supplying the constant current I, the voltage output will be constant when R is changed from one value to the other. The voltage obtained from the current source will be processed further.
Second phase: INSTRUMENTATION AMPLIFIER
The reason why I used an instrumentation amplifier is because I don't want the current flowing through the test resistor is divided in such a way it meets a branch! We know that current divides up when it meets a branch while voltage remains constant. The buffer in the first two op-amps in an instrumentation amplifier is working to ensure that the current flowing through the instrumentation amplifier is almost zero. Such thing will increase the accuracy of voltage reading on the test resistor!
The output of gained voltage may vary from 20mV up to 1640mV where 1 Ohm is the minimum value of resistor being measured and 82 Ohm is the maximum one. These values are used as an input to the microcontroller to process the work of sorting the resistor itself. Based on 10-bit ADC (I used atmel AVR ATMega8535), the resolution is around 5V:1024 = 4.88mV ~ 5mV. If we don't amplify the measured 1 Ohm resistor, it will clash with the next resistor which is 1.2 Ohm. By calculation it gives 24mV (0.020A x 1.2 Ohm). Between 20mV and 24mV there is no even one bit difference! Such things should be avoided by amplify the voltage. If we amplify by 10, then 200mV and 240mV will have around 40mV difference. This leads to 8 bits difference. Such thing, will give more precise in software approach, where I can use for example, RANGE, in which to increase the accuracy of my microcontroller reading the resistance of the test resistor.
Now I am working on this piece, because it will give me such a nice (even though it is complex) design. The relay board is bought, the coil voltage is 12VDC where eight relays are attached on a single sided PCB. I connect them with my schematics given at the beginning of my post and some problems came out. I really want to stick to my idea here, but if you guys find something strange or stupid, just tell me so. I am so open to any suggestions, but I prefer to stand on my idea, if I still could use it, with some modifications.
@Nigel:
to change a lot (by design) is a waste of time, I cannot do much in redesigning all, so please do give me humble and wise advice, please understand my position. Thank you.
To all:
thank you for reading...
Best regards,
Kelvin
Indonesia - I am sorry for my English. I am not living in such an English-Speaking-Country.