What is inside a tap light? | Reverse engineering | Electrical engineering | Khan Academy

By Grover Koelpin 3 comments


So today, we’re going to
take a look at the tap light. We’re going to see
what it’s made out of and what’s inside of it. You just push it and it lights
up, produces a nice warm glow, and push it again
to shut it off. So the tap light looks like
an opaque, acrylic dome here that allows the light to spread
out in a nice, soft glow. And then it has a
styrene– this is a polystyrene surround or bezel. So we’re going to go ahead
and take the screws out. There’s just four screws. These are Phillips head screws. And they are zinc-coated steel. We’ll take those out. We’ve got a couple of them out. There’s still two left here. I’m going to leave the
batteries in place, because I want to
show you what it looks like with the top off
when the light’s on. OK. So we got our screws out. And this is the battery holder. You got four double
A batteries there. And so we’ll lift the bezel off. And here’s the top portion. And this is the top cover. And then we’ve got
this light reflector. And you’ll notice that
it’s not perfectly clear. And that’s intentional. That allows for the light
to bounce around more and because it bounces
around more and goes through these opaque
surfaces, it is more diffuse. And so the light comes off
as a softer glow rather than a very harsh, sharp,
intense light. So this is what it looks
like without the cover. And this is what it
looks like with it. And you can see how that
helps to just soften the light and spread it out. And if we take this part out-,
I’m not sure who shows up on the camera, but the
light is more intense here. And this, again,
helps to continue to scatter that like to
make it a softer glow. All right. So let’s take a look at
the parts inside here. So we’ve got some springs. And these springs cause
the dome to return to its regular position. And so we have three
different springs here. We’ll remove those. And then we have a
light emitting diode. And this is a white
light emitting diode. And a light emitting
diode or any kind of diode requires that current
travel in one direction. So it basically acts as an
electronic or a digital valve to keep the current
flowing in one direction. So I’ll move the
light blockers there. All right. So the other thing is in
order to operate in LED, you need to have or
one of these guys. And that’s called a resistor. Now, resistors
exists, basically, to keep too much current
from flowing to the LED and causing it to
burn out before it’s supposed to burn out. So one of the things
I have been able to do is to use my iPhone to find
out what the resistor code is. So it’s really a
handy way to do it. It used to be you’d have to
look it up or just memorize what the different
color band codes meant. But this particular resistor,
it’s got a green band. So we’ll put it on green. There we go. And it’s got a– it looks
like a navy blue band, and a gold band,
green– oh, wait. Actually, there’s a black one. Sorry– and then the gold one. There we go. So this is a 56 ohm resistor. And that’s the
amount of resistance that that resistor provides. And the switch right here
is just a momentary switch. All right. Sorry. It’s not a momentary switch. It’s a continuous switch. So that means when you push
it down, it stays down. And when you let
it up, it stays up. So the light will stay
on after you push it. And the circuit is
extremely simple. Basically, you’ve got the
power from the batteries. It comes in through
the loop here and the switch basically
opens and closes and stops the power flow. Or when you push down on
it, it closes and allows the power to flow in the
continuous loop there. And so that’s what’s
inside of a tap light. Let’s take the batteries
out really quick so. These are double A’s. And now, it looks like
this back panel here was injection molded. And you can see the
ejector pins there. Those are the pins that
push it out of the mold. And so it looks like it
was injection molded. And I would say– it doesn’t
have the plastic designation marking on it, but I would
guess that it’s probably either polypropylene
or ABS plastic. So you can see in here,
these contacts inside here are probably made out
of– I initially thought that they were
made out of steel. But let’s take a look. We’ve got some magnets here, so
we’ll take one of our magnets and– I don’t think they are. So they’re probably
brass contacts, because the magnets are
not attracted to them. So it’s not a ferrous metal. So then we’ve got
this loop here. And this plastic loop
prevents the positive terminal on the battery from
slipping below the contacts. So it stays in constant contact
and keeps everything together. And you can see there’s
another one on this side. And that’s pretty much it. Oh, and there’s also a
feature right here so that if you have a screw
or a nail on your wall, you can put the tap light in
and just hang it like that. But that’s pretty
much the tap light.

3 Comments

The Cartographer

Jun 6, 2013, 3:15 am Reply

This is a brilliant idea for a video series.

fattyfattyx

Jun 6, 2013, 6:57 pm Reply

I like the electronics portion of the descriptions. But, in addition I really like all the manufacturing insight as well, the injection plastic molding, etc. I find this incredibly interesting (maybe less for the tap light). So I hope despite the views or lack of comments don't discourage the channel because I would really miss watching this stuff each time.

Ashutosh Agarwal

May 5, 2016, 1:44 pm Reply

Hello, may I know from where did you buy this tap light? Which company ?

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