Week 5 Piezzo Buzzer Arduino Project

            And For My Grand Finale................I Fried My Arduino Board!

This week we were supposed to create a project of our own using motors controlled by sensors.

Since my kit did not include a motion sensor (which is odd because I bought it from the Arduino site, I

decided to use a temperature sensor, and program it control the motor when the temperature increased (my plan

was to light a candle and move it close to the temperature sensor triggering the motor). While the concept

was cool, I ran into the problem of the motor itself not working, which I attributed to a problem in my code.

After an hour of troubleshooting and not getting anywhere, I decided to first get the motor itself to work

and then add the sensor later (probably my first mistake). I was able with the very simple code below

to get the motor to work but it started getting very hot (foreshadowing, I know). The below video

was sort of my initial tinkering before the disaster happened. I decided to take my circuit apart and calculate

how many volts the motor was using in comparison to the LED and other components I had worked with.

I then abandoned trying to use the temperature sensor altogether, and decided to research 

and use a transistor, because their purpose is to help your Arduino handle components with 

higher electrical requirements. 

I was able to get the motor to run when I mistakenly used the thermistor instead of the transistor in a

circuit with the 5V pin and motor over the two end leads of the thermistor. I had the digital pin wired to

the center lead of the thermistor with a 570 ohm resistor. I think that this worked because the center pin

of the thermistor has no input characteristics (it is essentially a one way diode connected for output only)

and the two other leads of the thermistor have very little resistance. So, the motor was essentially getting

maximum current from the Arudino board (roughly 200 mA) which was enough to engage this particular

DC motor. 

 

The output from the motorPIN went to the base pin of the transistor. I tried to cycle the output by using conditional

statements on that output to control the operation of the motor. Basically the output from the motorPIN

is a small current that switches the transistor open so that switches the transistor open so that a bigger current can

control the motor. I tried to calculate the resistance needed for the motor by calculating its voltage in comparison

to the other components I've worked with and by looking up their needed voltages online, but my math may

have been off.

However, I was not able to turn the DC motor when using a proper transistor in conjunction

with the 5V pin as a power source and the digitalpins as base current sources. I received two

transistor models with my Arduino kit - a BC547 transistor and MOSFET type transistor. Both of these

are NPN-type transistors that require a positive current on the base pin (center pin) to open the

connection between the collector pin (positive current) and emitter pin (ground current). The MOSFET

transistor is a relatively large unit which requires a high base pin current in order to open the

collector-emitter gate. The digitalpins of the Arduino kit are only rated for 20 mA with a maximum of 40

mA, so this would be insufficient for the larger MOSFET transistor. So instead I decided to use the

BC547 transistor. I connected the motor in a circuit with the BC547 transistor per diagrams I found

online, which includes a parallel diode circuit to protect the circuit from current backflows generated by

the motor spinning. When I connected the Arudino digital pin to the base pin of the BC547 under a 270

ohm resistor, which supplies approximately 18.5 mA of current, I was only able to achieve a flow of 30

mA from the collector to the emitter, which was insufficient to power the DC motor. The datasheet on the

BC547 indicates a current gain of 110 ma to 800 mA, which is considerably more than I was measuring.

My impression was that this was likely because I had a bad transistor, so I ordered 200 PN2222 NPN

transistors from Amazon (only $7). Unfortunately, I was achieving roughly the same CE current output

with the same setup on testing multiple PN2222 transistors. 

Given my lack of results, I then made the risky decision to lower the resistance to the base pin of the

transistor even more (in retrospect, this wasn't such a great idea. I tried 2- 270 ohm resistors in parallel

to get a calculated resistance of 135 ohms. This still did not allow the motor to be powered (the plot thickens)

So I tried to send current to the base pin without any resistance. This was unfortunately a colossal mistake

as I observed a spark in the digital pins and they subsequently quit working. Every digital pin no longer

supplied any current. I had “fried” the digital output part of the board and could no longer use code to

control the circuitry.  In panicking over this mistake, I started taking apart my circuit (without taking a picture)

and tried to build a simple circuit to see if I had indeed fried my Arduino board.

In doing more research, I found that I should be using an external power source to power the motor

rather than the 5V pin of the Arduino board since that will barely supply enough current (100mA) to power

a DC motor. I should probably have used a 9V battery to supply this current which would have been ample

to drive the motor. I’ve ordered a new Arduino board on amazon as well as different DC motors and plan

to keep experimenting.

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