Circuitboard design

The board for my final project is the most complex I have done sofar, It went through 3-4 generations of design, in addition to that its main features had their own initial independent designs.

Commonly multimeters have a twist knob, that physically separates the different features, and during my design of this board I understood why. But the goal of this board was to be something that can be easily replicated middle of nowhere, having to mill, and cast a special knob just to switch between features did not fit that goal. So I chose electronic approach and my limited knowledge of electronics made that approach difficult.

For future consideration, with the raspberry PI and its support for I2C, developing individual boards with the I2C network, it would be possible to turn the PI zero into a rather powerful multimeter.

The N1 version, essentially with this version I just piled everything on to the board to see how they would look on it, the benefit of this was to learn that I did need to make a separate component library item for the current measurement sensor as I didnt find the proper one in the any of the libraries for Eagle I had installed.

The N2 version, this was the first board that went all the way to be milled and stuffed, also it was with this board that I found out that I had used the wrong chip in my designs. Thus I had to buy the chip I had been using to complete my project in time.

The original idea was to have a LED on the board as well, to provide a visual cue for the shortcircuit, but since the casing of the multimeter was 3mm thick it wouldnt be visual through it and wasnt necessary.

The N3, In this version I corrected the design flaws of the N2 baord regarding my chip, while I now I had the right physical chip it did have some degree of different demands than the chip I originally thought of using. This demanded adjustment to the design. This was the board that finally ended up being in the multimeter when I presented it.

In this board also I cut down the number of pins for the display, so that only the ones that it actually needed are provided, and the connection to the right ones is made with the cables connecting the display.

The N4, this board never got milled out, altough I did make some updates to the design correcting some issues that popped up with the N3.

It has the voltage division for the 9v battery input in the upper left corner, a physical switch to cut power to the red probe making it easier to implement features.

Voltage measurement

Volt/res board, for this feature I designed an independent board, original intent was that it would have I2C network and it would through it report the values it measures. But I did not understand the I2C properly enough to make use of it at this point, and since I had no included a serial connection i had little to work with.

Idea was that the board would also be able to measure resistors, since the idea for voltage measurement and resistance measurement was practically the same.

Short check

The BEEEP board, for this feature I initially made a separate board for the explicit purpose of testing for short circuits. This allowed me to separately develop and understand the feature. On this board I had placed a LED so I could refine the code for it because I could see the LED going off when there was a short.

Voltage division board for N3

In the N3 evolution of the circuitboard I did initially use a 5v regulator, to make sure the 9v battery I planned to use for using the board didnt fry it. Problem was that the regulator didnt let enough current through for the board to work, and just got insanely hot. So I had to bypass the regulator, and use an external voltage division board I had. I corrected the problem by moving the voltage division to the circuit board in the N4 version, but my timetable did not allow me to produce the 4th version of the board.

ATmega32

The heart of the board, originally I meant the board to have a ATmega16U2 that is in the lab inventory, but I didnt pay enough attention to the differences between the chips to realize until it was too late that I had the wrong chip. Then again after looking into things, the ATmega16U2 that I was planning to use did not have a barebones library for the Arduino IDE, and would have required me to make one to properly use it.

Input Devices

Current Sensor acs712

When the + line is fed through the chip, it will measure the current flowing through it. Max capacity up to 5 amps, and the sensor is hall effect based. Originally I looked into making a separate board for this, I even bought some special resitors that could be used for making a current measuring circuit. In the end this was more accurate and much safer to implement.

Button

Basic Omron button from the lab inventory, runs a interrupt that pushes up a counter in the counter to allow switching between the different features I designed for the multimeter.

Output Devices

Piezo Speaker

Straight from the lab inventory, to give the multimeter the capacity to go beep when you hit a shortcircuit.

LCD display

This was as well from the lab inventory, and rather mandatory to have just to display the different values you can get out of the multimeter.