[Music]
So, you may recall this device from a
previous video. We uh took it apart and
unfortunately broke it, but it’s back to
life again. So, I can now scribble as
much as I want. Click this button and it
clears. So, I’ve replaced the old broken
PCB that I blew up with my new
replacement. And it’s working really
well. So, this PCB generates 27 volts,
which is enough to clear this LCD panel.
So, now we can scribble to our heart’s
content,
and it deletes it. It’s almost a
seamless repair. You’d never know I was
here. It’s no use, Ted. You’ll never get
it absolutely right.

Well, it’s not perfect. Does need a bit
of work. I need to do a bit of surgery
to get the battery to fit in properly
and get the PCB to actually fit, but it
does work nicely. So, 27 volts clears
it. Let’s dig into the actual PCB and
the assembly. So, you may recall from
the previous video, and if you don’t,
there should be a link up in the top
right corner. We made a simple dual
thief circuit to get 27 volts from our
coin cell. We’re using a slight
variation of that simple circuit that
adds regulation to the output. So once
we reach our required output voltage, we
don’t use very much power.
Now, as always, the PCBs came from PCB
way. I didn’t order a stencil as they
are pretty simple, but I did use my
Voltera to print the solder paste. And
after a few attempts, it came out okay.
But I also wanted to practice a bit of
soldering under the microscope. So I did
a board manually as well. I definitely
need some more practice under the
microscope. I did order some additional
blister buttons, but I got ones that
were slightly too large for my PCB.
Fortunately, I was able to salvage the
one off the old board, and it works
fine. The assembled PCB is pretty
simple. On the input side of things, we
have an inrush limiter resistor of 120
ohms and a reservoir capacitor of 10
microfarads. This presents us stressing
our coin cell too much. This goes
straight to the blister button on the
other side of the board. On the output,
we have another 10 microfarad capacitor
and a 1 megga bleed resistor across
that. The heavy lifting is done by these
two inductors. They’re two 20
microhenries. They are close enough
together that they magnetically couple.
The only crucial thing is with these is
to solder them in opposite directions.
If you don’t do that, the circuit won’t
work. Regulation of the output is
controlled by this 27V zenier diode.
This feeds into a second transistor that
pulls the base of the main transistor
low once we’ve reached our required
voltage. It’s a pretty cool circuit.
So, let’s measure the actual power
consumption of our little jewel of
thief. So, I got my power profiler kit
set up here. So, in this configuration,
I’ve got it actually supplying power.
So, it was a bit uh confusing with all
these wires. We got the multimeter
hooked up to the two output pads. So
that will check that we are getting our
27 volts on the output. And then we have
the V out from the power profiler going
into our positive connection and we got
one of the grounds connected. So what
I’ll do in the actual app, I’ve got the
supply voltage set to 3.3 molts. Let’s
change that to 3.2. So 3.2 volts. And
then we turn on the output.
And then we can start monitoring. So
obviously at the moment we should have
very little current flowing. So looks
like we’re averaging well
0.112
0.12 micro amps. And we can check that
if we just select some range then yeah
0.12.
So let’s actually click the button and
see what happens. So where is my button?
So, we should see this jump up to 27
volts.
There we go. That’s one,
two,
three. Okay. So, let’s stop this and
then we can see what our consumption is.
So, let’s zoom in a bit and then scroll
across. And zoom is ridiculously
sensitive on this um on this UI, but uh
here we go. So what we can see is we
initially get a spike of current. So
when we first push the button we spike
up to 19 milliamps and we average 17
milliamps and then the regulator cuts
in. So we get up to our 27 volts and
then whilst it’s regulating we are just
drawing
on average 7.26 milliamps with a peak of
10 milliamps. So that’s quite acceptable
for a coin cell. So if we take our total
time
then if we look down here and we consume
7.29 millum of charge and we average 7
milliamps. So that’s more than
acceptable for a for a little coin cell.
But what I do want to do is try it out
with a coin cell. So let me rewire this
so that we’re not using the internal
power. We’ll route power from the coin
cell through the power profiler to our
board and then we’ll measure the current
consumption with our coin cell supply.
Okay, so I’ve switched over to using our
own power supply. So we got the coin
cell here. So now the coin cell goes
through the Nordic power meter through
the ampmeter out into our board and then
we have the measurement going. So again,
until we push the button, not much
happens. Um, so let’s try pushing the
button. See what goes on. So there we
go. Once, twice,
three times. We’re still hitting our
nice 27 volts. So let’s stop that. And
then we’ll just scroll back to one of
these things and see see if there’s any
difference.
So let’s try and zoom in. Okay. So let’s
select the first part.
So the initial part
um jumps up to 18 milliamps. Still very
acceptable for a coin cell. Average is
16 milliamps. And then when we’re
actually regulating
should be pretty similar. So 7.6
milliamps on average 2.98 millum. So we
take the total time
and 3.93 miculum to get 27 volts. So
that should last quite a long time on a
coin cell. Let’s wire it up into the
original device and see if it does
actually work.
Assuming you didn’t skip the start of
the video, you know this works already.
Um, but there we go. It works quite
nicely. So we can scribble to our
heart’s content and then push the button
and it um it clears. So you should like
and
subscribe.
So the only slight thing that’s gone a
bit wonky is obviously these two um
plates have started to come apart. Um
that’s a bit of a shame, but uh this was
very cheap, so doesn’t matter that much.
Uh, I need to do a bit of work to get
the battery to fit in nicely. And
obviously the two inductors do protrude
slightly more than the original PCB, but
um, with a bit of, uh, shuffling around,
it will fit in nicely. I have lost the
battery compartment somewhere that’s
somewhere on my desk, but I’ll find that
and I’ll remove this battery and put the
two clips from the original PCB onto
this PCB. But, um, I’m pretty pleased
with this. It’s a uh as I say, it’s a
seamless repair. So, not bad. So, yeah,
we can uh now make our shopping list and
do all sorts of stuff. So, uh what
should we have? Not sure what to
imagine. Eggs.
Yeah,
eggs. Bacon.
Well, you get the idea. Fun little
machine. Um it’s fixed. Brilliant.