In this lab, you’re going to learn more about circuits and then you’re going to use that knowledge to help you power a motor. This lab is longer than the last labs, so leave so extra time.
1. Review what you learned in class. Read this primer on voltage, current and resistance at Sparkfun. Read about forward voltage on Adafruit and review Ohm’s Law one more time.
2. Read why your resistors are colored as they are.
3. Read about 5 ways to destroy an Arduino. #1-3 should make sense to you now, #4 you can think about later in the lab when you use a diode, and #5 won’t ever be an issue for us.
4. According to Ohm’s Law, what is the ideal resistor to use with two LEDs wired in series? Draw a draw a schematic circuit diagram (however you want — Fritzing, drawing software, a pen) and write out all your calculations. For your resistance calculations assume that there is 5V exiting the pin (it is probably less than 5V, but assume it’s 5V for this exercise). And recall that Arduino delivers a max of 40 mA per pin. What’s the forward voltage of your LED? Note that in your schematic you don’t have to draw a detailed Arduino — you can mark its place in the circuit as a 5V battery. (2 points)
5. According to Ohm’s Law, how many LEDs could you wire in series before the LED dimmed? Draw a schematic circuit diagram and write out all your calculations. (1.5 points)
6. If you modify the LEDs to be wired in parallel, how many could you power? Draw a schematic circuit diagram and write out all your calculations. (1.5 points)
7. Read about different motors.
8. Do Lesson 21, DC Motor. “Spinning a Motor.” Read through it all very carefully. Can you understand the circuit diagrams? (If not, talk them through with a classmate, Jie, or me.) Submit a photo of your board that clearly shows the wiring and a video of your serial monitor working. (2 points)
- If you’re having trouble making your motor spin, it’s probably because it’s not getting enough voltage. Your USB port allegedly provides 5V, but it’s probably less.
- One option is to power your Arduino with your 9V battery! That will work, but the motor drains the battery quickly.
- Another option is to power the Arduino through a cord like this one. (I had a few of these lying around my house — they came with different speakers.) This cord it has a voltage converter that changes the AC power coming out of the wall to DC and steps it down to 12V or 9V. Your Arduino has another voltage converter that steps it down to 5V.
9. Do Lesson 23 and then 24 (which builds on 23). In the code for Lesson 24, add some lines so you print to your serial monitor (whatever words/numbers you wish) when you click the remote. Submit a photo of your board that clearly shows the wiring and a video of the remote and serial monitor working. (3 points)
10. As always, you are encouraged to work with others, but please post your own work. Please assemble your resistor calculations, schematic circuit diagrams, and the photos into one PDF and post Canvas. This week, you don’t need to submit your code.
One reply on “Lab 3. Circuits and motors”
Regarding parallel wiring:
With a parallel circuit, you have some choices over where you put the resistor. You can put the resistor in the circuit before the current divides, after the current divides (i.e., wired in parallel), or after the current comes back together. In other words: if you think of parallel circuits being wired like a ladder, you can put the resistors on the rungs of the ladder or by the feet of the ladder.
If you wire the resistors in parallel, you can calculate the resistance you need on each branch with the equation Rtotal = 1 / (1/R1 + 1/R2).
Where you decide to put the resistor(s) is mostly determined by the size of the resistors that you have available. You’d like to choose the configuration that better matches your available resistors. Note that it’s always better to have too much resistance on the line than too little, but always it’s even better if these numbers are as closely matched as possible.