You want to play microcontrollers today?

To begin with any Microcontroller IC, you need to power the things up first . So gear-up youre mind and  focus on this tutorial. There are two power connections on basic Microcontrollers : VCC and GND. What the heck is VCC? This is the label for the positive voltage. Don’t worry, after a few days of this, seeing ‘VCC’ will become very normal. GND is short for ground. All electrical current needs a way to flow back to ground. This can be called ‘common’ but is often just labeled GND.

There are thousands of different Microcontrollers out there, but 5V (five volts) is the typical VCC. 3.3V is also typical but you’ll also see 2.8V and 1.8V VCCs on more exotic Microcontrollers. For now, just worry about 5V and GND.

Where do I find this 5V?

You need to hook up 5V and GND to your Microcontrollers. Your house outlet runs at 220V AC (or 110V for many countries). AC = alternating current and is very bad for 5V DC (direct current) micros. So you’ll need to convert the 220V AC from your outlet to a useable 5V DC.

Quick note: If you reverse the connection on your Microcontrollers – bad things happen. Always make sure your 5V power supply is connected to the VCC pins and GND to GND. If you reverse this and connect 5V to GND on the Microcontrollers and GND to VCC on the Microcontrollers, things won’t explode, probably no smoke, things will probably heat up like crazy, and you’ll probably damage your $2 Microcontrollers. You probably will. I did. Many times. Try not to do it.

Ok! You need 5V. Time to build a simple voltage regulator circuit!

You can buy something called a ‘wall wart’. Don’t ask me why they call it that, ask google. A wall wart takes a higher voltage and converts it to a lower voltage. DO NOT assume a wall wart labeled ‘5V’ will output 5V. This is a major misconception – I know, I know, faulty advertising. Just hook up your multimeter to the barrel plug and see what voltage you read. Probably more like 8 or 9V. This will kill your micro so keep reading! For a more detailed explanation check out the Unregulated Power Supply Tutorial.

Let’s assume you are using a wall wart with an output of something nice like 9V. Dandy. Unfortunately this 9V output is rather noisy – meaning there is a lot of ripple. Ok what does ripple mean? You want a DC voltage meaning you want a solid voltage (the opposite of alternating). A wall wart uses some cheap tricks to get 110V AC down to 9V DC. So the DC signal coming out of the wall wart tends to alternate 100-500mV. Instead of a solid 9VDC, you see a signal that rises and falls between 8.5 and 9.5 volts. This ‘ripple’ can cause havoc with your system, and 9V is too high (we need 5V!) so we need to pass 110V through this wall wart, and send the 9V through a regulator to get down to a clean 5V DC signal. If this all sounds scary – don’t worry. After you get your 5V power system built, you’ll wonder why you were scared in the first place (it’s simple, I swear).

The most common regulator is called the LM7805. Why? I dunno. I’ve never actually touched a component with LM7805 stamped on the outside. There’s always other letters stamped on the outside like ‘LM7805’ or ‘LV78X05’ or some such crazyiness. Just know that there are many many manufacturers out there and they are all producing the same basic part, with small tweaks to each one. What you need is one of these generic parts that is designated as a ‘5V linear regulator’. If you’re playing in a breadboard, you’ll also want it in the TO-92 or TO-220 package. More about packages in a later lecture, just go with it for the moment.

You’ve got your regulator in hand, you’ve got the wall wart. Time to connect them up.

Here you can see the ‘pin-out’ of the LM7805. Say ‘IGO’ in your head and commit this to memory (input, ground, output). You’ll probably hook up a lot of these. When in doubt, always check the datasheet before hooking up a new part – or else be close to the on/off switch! Input is the input voltage of anything greater than about 7V. GND is ground. Output is the 5V output pin. Your wall wart should have two wires. One is 9V, the other is GND. All grounds need to be connected together for current to flow across the system. One more time – connect all grounds. This is the #2 reason why novii can’t get a system to work. For our breadboard, we will be inputting 9V (or whatever transformer you’ve got up to about 15V) and outputting 0V (GND) and 5V to our breadboard rails.

We are going to go through a bunch of iterations of the power supply, adding parts as we go. Shown above, we have a basic regulator configuration. 9V in, we should see a rough 5V on the output.

Schematic note: The two ground pins are not shown connected. We assume that nets (the green wires) of the same name are connected together. Schematics can get big and complex, so you won’t see all the wires together, but in your breadboard you need to connect all the GND pins together. In this case it’s the GND wire from your wall wart connected to the GND pin on the regulator.

Cool. But why doesn’t the multimeter read 5.000V? Electronics are not that good. The cheap-o regulators are +/-5% tolerant meaning you’ll see between 5.25 and 4.75V. In practice, you should see between 5.1 and 4.9V with most run of the mill regulators. You can of course spend many $$ and get tighter tolerances but 5.1-4.9V will work fine for our purposes.

Now we should be worried about ripple. There is noise coming in the input pin, the regulator tries hard, but some of that noise gets onto the output pin. Your multimeter says 5.08V, but that’s because it’s averaging many readings together and showing you only the average. Do you know someone with a oscilloscope? If so, show them this tutorial and ask them to show you the noise on your 5V rail. With no filtering caps, you could see as much as 200mV of noise.

Whoa – what’s a filtering cap? Filtering capacitors are large bulky capacitors that help smooth out ripple. There’ve been lots of analogies about capacitors so here’s another one for ya:

Capacitors act like water tanks. When your circuit pulls a bunch of water out of the system, the capacitor helps hold the voltage up temporarily until the power system can catch up. For example: you may live in a city with water and water pressure. If you take a shower you affect the pressure in the municipal water system ever so slightly. If everyone turned on their shower and flushed every toilet in the city, odds are the water pressure would fluctuate quite a bit! A big water tank helps minimize these pressure fluctuations. A big cap helps minimize the voltage fluctuations on your breadboard.

Is this something you can see happen? Unfortunately not really. You can probably run your system without filtering caps, but it’s not good engineering practice. Give it a whirl without caps! But when things don’t work, you’ll wonder if it’s the caps, or your code, or your timing, or maybe you blew out the sensor. Too many unknowns will make you crazy. My recommendation: just use a couple basic caps…

100uF (one-hundred micro farad) on the input and 10uF on the output. You will use a lot of 100uF and 10uF around power systems and you will eat 0.1uF (point one micro farad) caps like candy around micros. These two caps should smooth the input into the regulator and will smooth the output nicely.

Capacitors cannot deliver their stored energy instantaneously. Larger caps (1ouF and 100uF) store more energy, but they react more slowly. The smaller the capacitor, the faster it can deliver its stored energy.  If you have a large power outage (power dips for 10-100ms), a big cap (100uF to 1000uF) will help ‘hold up’ the falling voltage. A smaller cap (0.1uF) will help suppress higher frequency noise and shorter power dips (noise in the 1us to 100us range). Therefore, 0.1uF caps are located near the microcontroller to help with short bursts, where 100uF and 10uF caps are used on the power rails.

Now you see the schematic symbol looks a bit odd. What’s with + and curved lines? This schematic component is indicating that the 100uF and 10uF cap are polarized. Oh jeebus, what’s that? Time for a capacitor breakdown:

  • Electrolytic caps: These are larger caps capable of storing 10uF to 1,000,000s of farads. They are cheap and great for bulk capacitance. They are polarized meaning there is a positive pin and a negative pin.

The cap has a minus ‘-‘ sign on the cover indicating that pin needs to go to GND.

  • Ceramic caps: These are the cheapest and most common cap you’ll play with on a breadboard. They are NOT polarized so you can stick em in the breadboard any way you want. Ceramic caps cannot handle as large of capacitance as electrolytics so you’ll need both on your breadboard system.

  • There are many more different kinds of capacitors but for the sake of your head exploding, we won’t cover them here.

Okay – now you need to work through some logic here. You know the positive part of the 100uF cap needs to be connected to the input pin, but only the negative pin is marked. Yes it’s confusing – but you’ll get used to it. Negative marked pin goes to ground, the other goes to the input pin.

What happens if you get them switched? Well here’s where things may go poof.

From the left: Bad, good, ugly

This is what happens when you over-voltage or reverse voltage a polarized capacitor. The middle cap is normal. The cap on the left, you can see the top is slightly raised up. This is what happens when the electrolyte inside expands. And the cap on the right shows us what happens when this pressure is so great, it busts through the metal top. Notice the ‘+’ imprinted into the tops of these caps? That imprint is there so that if the pressure does build up, the cap will fail like the unit on the right – rather than blowing the top half of the cap across the room.

This picture was taken from the inside of an old Gateway computer (circa 1999). Gateway had used some ‘marginal’ 1000uF/16V capacitors. The /16V means they are rated to 16V. A 16V rating means they can withstand voltages up to 16V but no more. These caps were sitting on the 12V rail to smooth out the ripple but obviously they where failing. Gateway was trying to save $0.50 by using a capacitor that was too close to the maximum. Manufacturing is not perfect! With any production run, the population of capacitors and their tolerance looks like a bell curve. The majority of the 16V rated caps can withstand 16V. Some can 18V, even 22V! But the tolerance bell curve goes both ways; a small number of the capacitors rated at 16V will fail at 10V, some at 8V. You get a big enough ripple on the 12V line and you could pop the 16V rated cap. This is why most engineers talk of ‘de-rating’ capacitors. If you have a 5V rail, you do not stick a 5V rated cap on the rail! A good rule of thumb is to de-rate any capacitor by 50%. So a 12V cap is good to be used on 6V rail, 24V cap on a 12V rail, etc.

Guess what happens when an electrolytic cap fails like the ones above? They quit working. In most cases, they ‘fail safe’ meaning they won’t work as a capacitor anymore but they won’t short to ground. The real fun begins when the failure is so bad that the internals fuse together and you get a short to ground – then you can have some fun melt downs! In the case of this computer, the motherboard had all sorts of bad software failures because the power supply had too much ripple! The big filtering caps on the power supply had failed so the 12V was all over the place.

Similar failures can happen if you reverse the polarization of the cap. If the voltage is low (less than around 25V) the cap will probably just be damaged a bit. If you’ve got a vacuum bell sitting around and you want to really cause some damage, ask a trained professional to hook up 10V cap backwards to 10,000V. It should instantaneously blow up like a pop corn kernel.

For your power supply filtering caps, I recommend using a 25V rated 100uF cap (100uF/25V) on the input and a 10uF/10V cap on the output. Engineers will tell you to ‘derate’ the cap by 50% meaning if the label says 100V don’t trust it past 50V. This is generally good practice. Following this idea, our 100uF/25V is good for inputs up to about 12.5V before we should worry that we may pop the electrolytes. Again, not mandatory, just don’t expect a 5V rated cap to withstand a 9V input.

Back to our power supply! Don’t worry about blowing things up just yet, you should be at low enough voltages you won’t do any harm. Again, if things heat up/smoke/spark, just unplug or turn off the system. Speaking of turning things off – we need a power switch!

This will allow you to turn on/off the system. Handy. It can get really annoying pulling and inserting the power wires to power/kill your system.

Inside the small black enclosure, is a switch. The switch has three pins. It looks like a see-saw inside. The center pin is always connected to the middle of the see-saw and as you slide the switch back and forth, the see-saw rocks up and down. Slide the switch forward and the see-saw shorts from the center pin to the forward pin. Slide the switch back and the see-saw disconnects from the forward pin and shorts to the rear pin. We recommend you connect power to the center pin of the switch. When you slide the switch forward, power will short to an unconnected pin and do nothing (no power to your system). Slide the switch back and the center power pin will short to the wire running into your regulator, delivering power to your system (power on!).

Remember all the warning about reversing VCC and GND and how that is bad? Well if you connect your power supply backwards, that’s bad. So let’s protect ourselves!

That’s a diode (marked D1). A diode lets current flow in one direction (in the direction of the arrow) and it blocks current from flowing in the opposite direction. This will allow 9V to flow in the right direction, and if you accidentally hook your power supply up the wrong way, it will block current from flowing backwards and damaging your system. Is it overkill? Pretty close. But we always design them into our development boards because we don’t know what type of power supply you knuckleheads (also known as our paying customers) will plug on to our boards. If you plug the wrong type of wall wart onto a board, we want to protect you from yourself.

There are some down sides to a protection diode:

  • All diodes have a voltage drop, meaning 9V on one side will drop to about 8.5V on the other. So your 9V wall wart just became 8.5V.

  • Diodes have a current rating. If you try to suck 1A (1 amp) through a 0.1A (one hundred mili-amp) rated diode, the diode will quickly heat up and fail. For reverse protection, we recommend a 1A 1N4001 diode. These are dirt cheap and very common.

Note that diodes are polarized. They have a direction that you need to pay attention to. Many diodes have a band indicating the cathode. What’s a cathode? Go google. All you really need to know is that the line on the schematic part is the same as the line on the diode. If you can’t remember which is which, remember ‘arrow is for anode’. Cheesy, yes.

So if you want to install this ‘reverse protection diode’, the 9V from your wall wart goes into the end of the diode without the band (the anode). The banded end (cathode) goes into your switch. Your switch then goes into the input. Throw the switch and you should see 5V on the output using your multimeter. Nifty. But I am tired of using my multimeter each time to check the 5V output. There must be a better way! Time to wire in the power LED.

Light emitting diodes (LEDs) are bits of silicon that light up when current flows through them. Go google for the science. As a general rule of thumb, LEDs can have 20mA max current flowing through them before they begin to fail.

So if you hooked up your LED like in the above schematic, it would light up very bright for a split second and then burn out. That’s cause the LED is a diode and the current will flow from the anode (arrow) to the cathode (line) to ground – uncontrolled! The silicon will flow current at something like 1 amp for a split second and burn up. To limit this current flow to 20mA, we need Ohm’s law. Yea, the book worms in the room suddenly perked up:

V = IR (this is Ohm’s law)

If we have 5V, and we only want 20mA flowing through the LED:

5V = 0.02 * R

R = 250 Ohm

Now this is not completely true because the LED has a forward voltage drop, but don’t worry too much about that. Hooking up LEDs is very common with micros. All you need to remember is that you’re going to need to limit the current. The most basic way to do this is with a resistor. 220 Ohms will work (LED will be brighter), 330Ohm is also good (LED a bit dimmer), 1K (1000) will work as well. 220, 330, and 1K are more common resistor values.

I highly recommend you get your hands dirty. Hook up an LED to a 1k resistor, then a 330, then a 220, 100, 50, then finally blow the thing up by hooking it with no resistor. That was fun right? Good. You had a back-up right? Once the bit of silicon inside the LED is burned out, it is no good and the LED can be thrown away.

Eagle schematic / PDF

Our final power supply circuit. It seems like a lot of work, but once you set this up on your breadboard, you might never take it off. This is the basis for all things micro. The input voltage may change, the output voltage may change (to 3.3V for example), but the basics are all there. Flip the switch and you should have a nice 5V rail and an LED letting you know that everything is a-ok. If the LED does not light up, that means that something else on the 5V rail is sucking so much current that the LED cannot light up. This is a very strong indicator something is wrong. If you turn on your system and the Power LED does not turn on, immediately turn off the system and check your wiring.

You may be wondering if the resistor/LED order matters. It does not. The resistor can come first and then the LED or as shown. Either configuration will correctly limit current through the LED.

If you think you may have blown up your LED then your LED will never turn on. You may want to check your power system with a multimeter instead.

Good, you’ve made it this far. Now for some technical info about ripple/noise and why it’s bad.

If you’ve got major ripple on your power rail, say 500mV or more, this can cause your micro to latchup. This means that it was running fine at 4.8V, but at 4.3V it’s not happy and will go into an unknown state. When the rail returns to 4.8V (because the ripple is bouncing the rail up and down), the micro goes from unknown to possibly latching up or freezing up. This is pretty rare these days because the chip manufacturers have done a good job of internally protecting against this, but in general, ripple is bad.

Say you’ve got 500mV of ripple on your system and you’re doing analog to digital conversions off of a temperature sensor. The temp sensor has an output pin that will output an analog voltage that will vary 100mV for every 1 degree C. So at 25 degrees C (room temperature) the sensor will output 2500mV or 2.5V. If your micro is doing analog-to-digital conversions on this signal, it has to compare what it ‘thinks’ is a solid power rail of 5V against this changing analog signal from the temperature sensor. Well if your 5V ‘solid’ rail has 500mV of ripple, the micro doesn’t know this, and will report a regular 2.5V reading as varying between ~3.0V (3000mV = 30C) and ~2.0V (2000mV = 20C). This is wildly bad. You need a good ‘clean’ power rail if you are doing anything with analog signals.

Now some notes and photos on breadboards:

Go read Tom Igoe’s breakdown of the breadboard. In short, the power rails (the red/blue rows) are connected internally. The columns within the main area of the board are interconnected. So you can insert a wire into one hole and it will be electrically connected to a neighboring hole (vertical connections for the numbered columns, and horizontal connections for the blue/red power rails).

Historically, the blue rail or the horizontal row of holes next to the blue line is ‘GND’ or ground. You can connect all the ground pins of all your components to this rail. Similarly, the red rail is for VCC. In our case, this is 5 volts.

Power jack, switch, LM7805, power LED

Here you can see power from the barrel jack being delivered to the slide switch, and then to the input pin of the v-reg. When the switch is thrown to the on position, the yellow LED turns on.

I cheated a bit.

Do you see that odd thing in the upper right corner of the picture? That is my wall wart plugged into a DC barrel jack. Most wall warts are terminated with a round connector called a ‘barrel’. The outside metal sheath is ground, and the inside metal is 9V. The two metal contacts are isolated. The DC barrel jack accepts this wall wart barrel (wall wart barrel slides into the jack with some friction to hold it in place). I don’t like hacking the ends off power supplies and inserting the bare wires into a breadboard. Having energized bare wires bothers me. If the wires get pulled out of the breadboard because you kicked out the power cord, you’ll have some tense moments until you get the power brick unplugged. So I soldered some short leads to the barrel jack so that I can plug/unplug my power cable from the breadboard. Easier to transport.

See the orange wire at the end of the barrel jack? That pin inside the DC barrel jack connects to the center of the wall wart barrel. The center of our wall wart barrel connectors are ‘+’ or ‘hot’ or ‘9V’, whatever you want to call it. So the end of the DC barrel jack is soldered to an orange wiring meaning it is ‘+’. This orange wire is then connected to the center pin of the power switch.

All ground connections are connected together. You will see a small black wire underneath the DC barrel jack. This is the pin that connects to the outside sheath of the wall wart barrel. This is the ground connection on the wall wart. This small black wire connects the ground of the wall wart to the ground on the breadboard.

I did not install a reverse protection diode. I only use center positive power supplies so I know I’m safe. If you do anything similar, check your wall wart carefully with a multimeter before doing any testing.

Note: Our breadboard will have 5V and 0V rails. The blue rail is GND (considered 0V). Red is VCC (or called 5V).

Note on LEDs: LEDs are a polar device meaning you’ve got to hook them up in the correct direction. Light emitting diodes (LED) have a cathode and an anode. How do you tell the difference? Imagine the schematic element:


Do you see the arrow? Do you see the flat line? A is for arrow. A is for anode. The physical LED will have a flat side corresponding to the flat line (the cathode) in the schematic picture. And there you go! When connecting an LED, you know that diodes only pass current in one direction (from anode to cathode – in the direction of the arrow!) so the flat side of the LED needs to be connected to ground some how (usually through a resistor first) and the other side (remember arrow) is the anode and needs to be connected to power for current to flow. If you hook it up backwards, it won’t turn on, and you might damage the LED but probably not. Just verify that you’ve got 5V on the correct rail and then flip your LED around if need be.[Source]

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