Day 1 and 2: Thevenin's Good Idea

So the week began with wiring up a LogoChip on a breadboard. I followed the steps in the 310 manual rather closely, so I will not repeat the steps taken.

We can get some pretty cool things to happen!

This video shows an LED in a cycle of dimming and brightening. Because this is done by varying the duty cycle, we can observe the voltage across the LED changing as it changes brightness.

Hannah and I easily got the LED to dim when a shadow fell over a photocell, but not to light up when the shadow fell. This is because PicoBlocks has some issues getting multiple numerical operations to occur in a single variable. Our final script looks like this

And when it is run, it looks like this!

Video by Hannah Herde

This brings us to the Thevenin Model.

For any collection of batteries, resistors, and diodes, we can replace it with a single battery and resistor with the Thevenin voltage and resistance respectively. It becomes a black box -- if we draw a box around it and look at the output, we can't tell what's inside!

Then, to find the Thevenin voltage and resistance, we can turn it into a voltage divider. We must vary Rload from 0 Ohm (a simple wire) to 47 Ohm.

With 0 Ohm Rload, Vout = Vth

With 47 Ohm Rload, using the equation Rth = Rload * Vth / Vout - Rload, we can calculate Rth.

Image from the 310 Electronics manual

Black Box	Vth	Vout	Rload	Relation of Rload to Rth	Rth
Battery Pack	4.3V	4.3V	47 Ohm	Rload >> Rth	~0 Ohm
LocoChip Output Pin	4.2V	1.6V	47 Ohm	Rload > Rth, same order	76 Ohm
Stalled Motor	4.2V	1.3 V	47 Ohm	Rload > Rth, same order	105 Ohm