[Thur Feb 9] Hysteresis, capacitor

Today I finally understood how hysteresis works, at least at my level of engineering.

We started the class reviewing what we did on Monday, which I didn't understand at the time. We went over the operational amplifier and the equations. Then Oscar showed us again how the graph regarding Vo and V- looks like. I was really uncomfortable not knowing why it voltage of Vo jumps between 12V and -12V instead of change gradually if there is any change. Oscar gave me an integral equation:
V out =  ∫ A (V+ - V-) dt
and that made a lot of sense to me. Later Oscar explained that in engineering, I don't actually need to know why it works this way. I just need to know it works and know how to use it next time. I think that I am going to be uncomfortable for things that I don't know for a while until I get used to the engineering way of thinking. I wonder whether my engineer friends know everything they are doing because they seemed to in high school.

Then Oscar used the scenario of bending a card to the left or right. It's the same kind of trick adding a force (or voltage in the electrical world) on a stable object until it moves and shifts to the next stable state until another force comes and changes its status again. Light switch works the same way. I was really amazed by that. That reminded me of my conversation with an engineer major who worked for a corporate and sat next to me on a plane. He said engineering was inspiring in the sense that it made him understand how things around him work.

Anyways, then Oscar introduced the concept of a capacitor. Basically capacitor accumulates current and expresses it as voltage. In a circuit with a resistor, it will charge itself until the voltage difference between two ends of the resistance is 0 and so is the current. The bigger the voltage difference is, the faster the capacitor charges. Therefore the voltage of the capacitor over time is a curve concave down that eventually hits the voltage of the power source and stays stable. Here is a circuit diagram with a capacitor:

After the break, we started to build our circuit. It consists of a hysteretic comparator, two potentiometers and a capacitor. Oscar did us a big favor by drawing the connections of the circuit on the board:

We still struggled a little but this time the debugging process wasn't as painful as last time. In the end, we got the graph showing that V- is oscillating between 6V and -6V. Here is our result from the oscilloscope:




Oscar explained the reasons behind the oscillation in class but I hope that we can go over it a couple more times. Oscar also told us if we touch the capacitor, then it's like we are increasing the capacitance so the frequency goes down (I am not sure why...) Everyone finished fairly quickly (thanks to the diagrams), and we started to put in a speaker in the circuit to make "electronic sounds" (like DJs...) It was great hearing a room of annoying little sounds :)

Some detailed pictures of the circuit:

This is our circuit with the speaker. Also I need to repaint my nails...