Electronics Lab - Day 3

Today's class was all about transistors, we added a MOSFET transistor into our circuit. We had previously found that because the logochip has such a high thevanin resistance that it was not a good power source for our little motor whereas the battery pack was an excellent power source for out motor. The problem is, that we can't program a battery pack like we can a logochip. So how can we control the motor with the logochip but power it with the battery pack? A transistor of course. So we added that in and also added in more batteries to create a 9v power source.

Now our little motor is so strong that it actually takes some muscle to stall it!

We were also asked to do a series of exercises that helped us understand how capacitors work and behave in circuits.

Lab 2-1

In this lab we're asked to verify how the voltage varies across a capacitor over time. It should rise parabolically and level out as it reaches the input voltage. We also have to verify that the time constant, the time it takes to reach around 63% of its full power, is T=RC. In the circuit that we set up, we had a 10 kohm resistor and a 0.01 uF capacitor so the time constant should be T=100 us.

Our time scale is 50us and it looks to be that the voltage rises to around 60% of its full voltage within two boxes which is on the scale of 100us which agrees with the calculated value for the time constant.

Logochip Capacitance Meter

In this section of the lab, we used an input pin of the logochip to monitor the voltage across the capacitor using the circuit below.

Since R = 10 kohm and C = 10 uF, the time constant for this circuit should be around 100 ms. In order to verify this, used this program:

It starts the timer when the capacitor has discharged to almost zero. It then records the time when the voltage reaches 63% of its full voltage. The timer reads in ms so whatever number the program outputs should be the time time constant in ms.

We found that when we ran the program, read 96 ms for our circuit which is just about what we calculated as the ideal time constant.

In order to calibrate the program so that it reads the value of the capacitor instead of the time constant, you simply have to divide the value on the timer by the resistance in the circuit. T=RC so C=T/R.

We were also asked to evaluate what happened to he time constant when we put two capacitors either in series or in parallel. For this portion of the lab we used 0.1 uF capacitors instead of 0.01 uF. With a 10kohm resistor and a 0.01 uF capacitor, the time constant was 1 ms. When we added another capacitor in parallel, the effective capacitance is essentially doubled so we expected the time constant to also double. This was verified when the picoblocks program read an output of 2 ms. When we added another capacitor in series, the effective capacitance in effectively halved so we expected the time constant to also be halved. However, the program continued to read 1 ms with this circuit. We guessed that because the picoblocks timer was just not sensitive enough to detect 0.5 ms and we verified that the time constant was indeed smaller than 1 ms by using an oscilloscope instead of the picoblocks program.

Lab 2-3

In this lab we constructed an integrator. This circuit essentially uses the properties of a capacitor to create an output voltage that is the integration over time of the input voltage. When we input a square wave into the integrator, the output voltage looks like a triangle wave.

And when we input a triangle wave, the output voltage looks like a series of parabolas.

When we drop the input frequency, the output voltage across the capacitor is allowed to rise to a value close to that of the input voltage and the circuit no longer acts like an integrator.