>> The firmware was updated on 19 Mar 2011 <<
A few months ago a friend of mine -car mechanical at profession- told me that he had problem with some car sensors. He couldn't check, with a simple multimeter, if a sensor was working properly. I advised him to buy a LCD oscilloscope instead of a normal oscilloscope, because of its small size. The use of an oscilloscope is very helpful because you can see the waveform that is produced by a "healthy" in-circuit-sensor* and you can compare it with the waveform of a "suspect" in-circuit-sensor.
After that, he told me that this oscilloscope costs a "fortune" for him as he has a small car service shop. I offered to help him by designing and constructing a small, cheap and workable LCD oscilloscope for him.
*in-circuit-sensor is the sensor which is connected on a board (PCB) or it's connected somewhere in the car. It's not a disconnected sensor.
You can download the source code, hex, schematic diagram,PCB and picture of the circuit .
Step 1: Selecting the Components
As the circuit has to be constituted by a few and cheap components, I chose the DEM128064A graphical LCD based on KS0108 controller chip to display the measured signals. The ATmega32 microcontroller is chosen because it has a lot of IN/OUT pins and a 2kB RAM size. 1kB is needed from gLCD and some bytes of the rest 1kB are used in C source code as registers. Because my oscilloscope has to read both AC and DC currents and at the same time it should have a High input resistance, I chose to use an LM358 which is a dual operational amplifier. The rest components like LM7805 , capacitors and resistors are very common and I won't refer to them.
Step 2: Designing the Schematic Diagram
After I had chosen the components, I had to draw the schematic diagram of this circuit. For this purpose I used the Splan 5 that is not freeware but it is a very cheap schematic diagram software.
Step 3: Making the Prototype
The prototype was made on a dual breadboard and the microcontroller was placed on an STK500 development board (see photo). I haven't taken any photos from the prototype board. So, I can show you only the boards without the components I used on them.
Step 4: Putting the Components in Order
The breadboard circuit was temporary. It was the time to make the PCB for my AVR oscilloscope. The software that I used to draw the PCB is the Sprint layout 4. It is a really easy-to-use, cheap and efficient software to make your own PCBs.
I made my PCBs by myself by using toxic chemicals. That's why I don't describe the procedure. It's very dangerous. I suggest you to give the transparency that you will print to a professional to make the PCB for you.
Step 5: Soldering, Calibrations and Usage.
Solder all components on PCB, starting from the smallest and go on to the biggest component. Check the PCB from soldering side for shortcuts that could have been made during the component soldering. Remember to put IC3 on a base, so the removal for future reprogramming can be done very easily.
If everything is ok, supply the circuit with 12V Dc. On the screen you will see the oscilloscope's raster with a horizontal line on it. Adjust P1 (LCD contrast) with a small screw driver up to the point you will see clearly the content of the screen. If you adjust the P2 you will see that the horizontal line (beam) will be moved up or down depending on the adjustment of P2. Adjust the P2 to set the beam at the center of the screen.
Connect a 1:10 probe at BNC connector (K1) of oscilloscope. Now you are ready to make your own signal measurements. Take care not to exceed the maximum input voltage which can be up to 24V Ac or 30V Dc on 1:10 selection prob. At 1:1 the maximum input voltage can be up to 2.5V Ac or 5V Dc. S1 gives an extra input voltage division by 2. With S2 you can select between AC or DC input signals.
Step 6: Oscilloscope Demonstration
See more of my projects at http://www.serasidis.gr
Thank you for reading
hamrique made it!