In-class exercises #

1: Build a voltage regulation circuit #

This is an introductory exercise designed to help you get familiar with breadboard prototyping while also building a basic circuit that will be useful for later exercises and projects. It forms the basis for Project #1, but you don’t need to think about that yet. If this is all new to you, look at the pages on breadboard prototyping and multimeters.

Your goal is to put 4 components and some wires on your breadboard to achieve this goal: 12 volts go in; 5 volts come out.

The 4 components are:

  1. One power jack
  2. One L7805CV voltage regulator
  3. Two capacitors

2. Build voltage dividers #

Voltage dividers

3. Control an LED with a transistor #

Making an LED light up is pretty simple: put a resistor in series with it and apply a voltage. If you sized the resistor right and didn’t put the LED in backwards, you’re done. When we want to control things like LEDs electronically, instead of plugging them in to breadboards with our hands, we use transistors. The goal of this exercise is to use a bipolar junction transistor, the 2N3904, to turn on and off an LED.

You need 4 things:

  1. A 5 V voltage source, like the voltage regulation circuit you built previously
  2. A 2N3904 transistor (or really any transistor is fine, if you know the pinout)
  3. An LED
  4. Two appropriately-sized current-limiting resistors: one for the LED, one for the BJT

The picture below shows the concept of what you’re building. In place of the motor, we’ll use an LED.

Typical BJT circuit

4. Measure a DC gearmotor (part 1) #

Here are a few basic measurements you can make to understand your DC gearmotor better. We’ll start with current measurement.

  1. When your motor is stalled (like if you’re trying to lift something too heavy with it), it behaves like a resistor. This is when your motor draws maximum current, so checking the resistance is a good way to estimate the maximum current for whatever drive voltage you choose. Using the resistance-measuring setting of your multimeter (the omega symbol, Ω, represents ohms), measure the resistance of the coils of your motor.
  2. Make your DC gearmotor spin by connecting your motor directly to your 12 V DC power supply using alligator clips. Then, measure the current the motor draws with no load attached by putting your multimeter in series with the motor. You should switch your multimeter to the A setting (for Amps), and move the red lead to the port on the left side of the meter that is labeled “A”. The black lead stays in the black port labeled “COM”.

With these measurements, you can estimate the maximum current the motor will draw at any voltage, and the minimum current at 12 V. If you can estimate how much power a task will require, you can start to figure out what voltage this motor would need to deliver that power (assuming perfect efficiency, for now). That’s the first step toward deciding whether this is the right motor for whatever you’re building.