Day 4: Transistors and Capacitors!

In this class, we will be using MOSFETs, a type of transistor. Transistors are tiny switches that are activated by a current. Passing a small amount of current through the gate of the transistor allows current to travel from the source to the drain. If no current is on the gate, the no current flows between the source and drain.

This allows us to control a circuit without actually physically flipping a switch. The current supplied by the LogoChip is adequate to turn the MOSFET on.

Image from the 310 lab manual

Picture taken by Hannah

By connecting the motor to the LogoChip as shown, we are able to turn the motor on and off with the LogoChip. Note that a separate battery is used to power the motor.

With this simple program and a light sensor, 

We can get the motor to turn on and off when the light sensor detects light

Video by Hannah

We also looked at capacitors! (I won't go over the details of how capacitors work.)

So we built a capacitance meter! We must try to charge the capacitor only for the linear portion of the charging curve. If it starts to level off, our meter will not work.

Image from 310 lab manual

We followed this circuit diagram...

Wrote this quick program... (It just waits for the capacitance to get to a certain level and then prints the time it took.)

Picture taken by Hannah

Wired it up...

And calibrated it!

A 0.1 μF capacitor and a 100 kΩ resistor corresponds with a 5 ms time. Both an oscilloscope and the PicoBlocks program agreed.

Two 0.1μF capacitors in parallel give a 20 μF capacitance. As predicted, this gives a 10 ms charge time.  

Two 0.1μF capacitors in series give a 5 μF capacitance. As predicted, this gives a 2.5 ms charge time.  

We also built an integrator.

It takes in an input voltage from the function generator and then integrates the input! It probably is better at integration than I am...

Picture taken by Hannah

Picture taken by Hannah

The yellow is the input voltage from the function generator; the blue is the output voltage from the integrator. 

The vertical lines indicate ringing. The output of the function generator (100 kHz) is too high frequency; if the capacitor is given time to discharge, the ringing will go away.