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What is inside an alarm clock radio? | Electrical engineering | Khan Academy


– [Karl] Today, we’re gonna
take apart an alarm clock radio and we’re gonna see what’s
inside it and how it works. There’s basically four systems
that we’re going to evaluate. There’s the power system. There’s the alarm clock
or the clock itself. And the structure of the device in the interface. Then we’re also gonna
take a look at the radio. The first thing is let’s take
a look at the power system. I’ve already cut apart this plug here. You can see the prongs. That’s where the power comes from. We’ve got the two wires here. The two wires connect to
what’s called the transformer. In the transformer, there
are three key components. We’ve got a primary coil, a secondary coil, and an iron core. The primary coil is wound around
a certain number of times. The secondary coil is
wound around fewer times in this particular transformer. That means it’s a step-down transformer. What this transformer does is it converts 120 volt AC down to nine volt AC. Because the components in the alarm clock need lower voltage. It steps the power down. The way it does it is this coil induces a current flow in this coil, and the iron core helps that to happen. Because this coil has fewer turns, it’s a step-down transformer
which means that the voltage is less coming out of this part. If there were more turns, it
would be a step-up transformer. The iron core, again,
facilitates that process. It’s called electromagnetic induction. This coil induces the
current flow in this coil. Anyway, the power will travel
through the cable here, the wire, and it comes to the alarm clock. Let’s take a look at the
housing, first of all. It’s fairly low cost housing. It’s made out of injection-molded plastic. Let’s see where that power
comes in and where it goes. One of the ways they’ve been
able to reduce the cost of this is they basically just
only used one fastener, one separate fastener. This is a screw. The more screws you have in things, the easier they are oftentimes to put together and take apart, but they are also more expensive. Every screw requires either a robot or a person to assemble it and it’s an expensive cost adder. The more screws you can take out, the more cost you can reduce. All these fasteners in this are actually molded into the body panels. There’s a pin or a tab there, so we can pull this top part off. This part right here is the front plate or the front vessel. It’s made out of a tinted acrylic and it’s injection-molded. There’s two parts of the
mold that come together and molded plastic is injected,
and then this comes out. The reason it’s injection-molded is that it creates very precise part. You can get a nice, clean finish. When you produce them in high volumes, you can do it for very little cost. There’s that. The reason it’s tinted it that it hides the interior components. Except when the bright
lights still show through. This is the inside. Let’s take that part out. Let’s see if we can get that there. There we go. It’s altogether. Part of the way they
kept the cost low again is there’s not a lot of separate pieces that they have to assemble. There’s one module they can just plug in to this housing. Let’s take a look at the housing before we get back to the power system. The housing was injection-molded. It’s three parts of a mold. There’s one part that comes in here, one part that comes in here, and then there’s a core that goes in here. One of the ways they’ve
been able to reduce the cost is by molding the buttons into the housing. There’s a little surround
around the button and that surround is basically a place
where there’s no material so it allows the button to flex but there’s also little
tubs that hold it in place. All the buttons were molded in and when you push on the
button, it triggers this pin which triggers a switch below. We’ll get to that in a second. In the back, you’ve got a place for the nine volt battery, so your backup battery
if the power goes out just slide in so you push
the battery in there, and this little tab holds it in place. The battery pushes up against
these two little wires right here. Very low cost connector for the battery. They don’t need an extra door or housing piece on the outside so again it reduces cost. The way this was made, you saw
how the molds came together. The plastic was actually
injected right here. You can see that little place right there. There was a piece of
plastic that stuck out once this fell out called the sprue, and they broke that off and you can see the remnants of that. By having all those
features molded into this, you reduce cost because you don’t have to
assemble separate buttons and things like that. Let’s take a look at the power again. We’ll take this top part off here where all the button
brackets and things are. Unplug this. All right. You can see the power comes in right here. If I take that off, we have four diodes in a row there. Those diodes function
as a bridge rectifier, so they convert the nine volt
AC power that’s coming in into DC power. The AC power flows like this. It’s like a sine way. It’s flowing in both directions. They convert it and they cut it so that it only flows like that. They take the sine wave
and cut it and flip it over so it goes like this, and so it’s still a little rough. It’s still a little bumpy. These capacitors help
to smooth that power out because these components don’t want power that fluctuates a lot. They want really smooth,
consistent direct current. That’s what those do. That’s how those capacitors can help out. Let’s take a look at
the clock system here. This is our clock divider,
integrated circuit chip. What it does is it takes a signal from a crystal oscillator which is a piece of quartz crystal that’s tuned to a specific frequency. When electricity is put into the quartz, it oscillates at a particular frequency and produces a voltage. It gives you a very
precise division of time. Oftentimes, those voltage divisions can be say like 60 hertz or 60 times a second. Then what you need to do is once you have all those divisions, you need to be able to
separate those divisions into minutes and hours and then send the signal from those
divisions in minutes and hours to a display. Those divisions come from here. They go through the ribbon cable. They go to this
seven-segment display here. It’s called a seven-segment
display because it has seven different segments in
each little piece right there. They display the minutes and the hours. There’s LEDs here that display AM and PM. LED stands for light emitting diode and it’s a very efficient
low-cost way to display the time. The LEDs are mounted inside this little plastic piece right here. Inside there, there are, there’s kind of a light conduit which helps to spread the light out. LED is a very intense spot of light and so this
housing helps to spread it out so it makes each segment
look completely full and the LED lights up
the whole segment there. On the back side of that is a printed circuit board. That helps to basically
direct the electrical signals coming through the
ribbon cable to the right seven-segment portion. May turn out that it’s
6:30 and so you want only certain segments to
light up because of that. We have a jumper here which helps to, you can use jumpers to
alter the functionality of the seven-segment display. This one may be programmed or maybe set up to
function in a certain way so that this jumper allows you to transfer power to the
different part of the display. Then we have these
little white spots here, and what that is is the back of this module,
this seven-segment module, has these little pieces of
plastic that stick through and there’s a hot plate
that basically pushes on those pins that stick
through and melts them, and it holds the plate against
the printed circuit board. This is just a cheap way
or an inexpensive way to fasten things together. It works pretty well. That’s the clock portion. The buttons, let’s talk a
little bit about the interface. The buttons, when you push
down on the buttons here, they trigger these pins and you can see the pins
flex pretty good right here. The pins are connected by
these little standoffs. Because the standoffs are
really thin, they can flex. The pin flexes and the pin
rests on top of the switch. When you press the button, the pin moves and the switch gets triggered. When you wanna snooze in
the morning, you push this. It shifts the pin and it causes the sleep button to be triggered. That’s kinda how that works. This is kind of ingenious
in another way too because it holds a bunch of
different things together. It’s got the pins. It holds the speaker and it also holds the ferrite rod with the
copper coil around it which functions as an antenna. That’s an antenna for AM/FM radio. The signals come from here. We got a wire broken there. Signals come from here
and they go to this thing which is a setup of
four variable capacitors and they help to tune out
frequencies we don’t want. When we turn our dial,
we can go right to 101.1 FM or 538 AMR, whichever station we want. This helps us to select those things. Those variable capacitors help us to filter out unwanted frequencies. These two things, they’re
called inductor coils and they can be used to oscillate at
a particular frequency if they’re coupled with a capacitor. That can be useful in performing
radio functions as well. This guy right here is a, it’s a radio chip. It’s an IC chip that helps to demodulate or to separate the music or the signal that you want from the actual wave. AM is amplitude modulation so that means that the wave
is changed in its height. FM is frequency modulation so that means that the wave is changed in how often it occurs in order to embed the signal that we get to listen to as radio sound. This chip basically decodes that and says, this is the original wave and then this is embedded signal. That’s able to be then sent to our speaker right here. Before it gets to the speaker, it goes past this variable
resistor right here which is also called a potentiometer. When we turn that, it changes
the resistance in the circuit and it either increases
or decreases the volume, and increases or decreases the amount of power
running to these wires. This one actually has come undone. The wires come here and there’s
a copper coil and a magnet. When the powers run to the
copper coil and the magnet, it causes the paper cone to vibrate and that produces a pressure wave and we interpret that as sound. That’s how that works. Then right here you can see there’s two different switches here. We’ve got a switch that controls
whether we’re an AM or FM and then another switch
that’s just sort of let’s us select different
functions like turn the alarm clock off or have it set
to buzzer instead of a radio, and things like that. You can also see this right
here, this is a resistor. That resist electric, current flow. That can be useful because it helps to prevent too much power from flowing to certain
components on the board and things like that. This is a transistor. These things are transistors. They can function as switches. These guys right here are filters and they can help to reduce noise or electromagnetic interference, and they can help to clean up. They’re probably used in
the radio circuit here to help to clean up the signal. On the back here, you can see again, these prongs
connect to the battery. This is the printed circuit
board here on the back. Basically, it’s a thin layer of copper that’s been applied to
this fiber glass board. Then a chemical was used. They used basically a photomotion process which is like a similar to remove … They shine a light on it and they use the photomotion process to remove certain areas of the copper
and keep other areas. They’ll use an asset or a material to etch away the areas
that aren’t protected. Those result in copper traces. Those copper traces are
basically very well ordered, little tiny wires that are very flat and they allow us to
connect all these different components in a very small
space very efficiently so we can just push the components. These are called through-hole components. Through-holes and then
solder them on the back and then they’re all wired up together. We don’t have to worry
about a lot of messy wires and things not being connected correctly. You can see there’s different components, small components on the back. The little surface mount
resistors and things like that. That’s our alarm clock radio and those are the insides. Hope you’ve enjoyed it.

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