Who wants to see a magic trick?
I have a run-of-the-mill standard Raspberry Pi Zero W2, and I have a TFT display with
a touch controller. It's pretty easy to connect the two, just plug them together. You will
however note that I have no battery and I've not connected it to any power source. But
what I do have is my nice Pi branded notepad.
Well what do you know, it's powered itself up. And if we give it a few seconds, it even
boots into X-Windows. Now of course the big question is, does it run Doom?
And there we go, it runs Doom pretty well.
So what's going on? What is this magic? Well fans of the channel will recognise these
wireless LEDs from previous videos. We can even power one of my wireless Christmas trees.
And it works with my wife's phone. This was a slightly nervous moment, blowing up your
wife's phone is not a good thing. By now you will have guessed it. We are playing around
with some more wireless charging technology. I've been looking at a few wireless power
options over the past year or so, but I've always found them to be slightly disappointing.
They work well for simple stuff like LEDs, but I've struggled to use them for anything
practical. So I thought why not try a proper version.
As you've seen, this one even works with your phone. Let's have a look at what it's
actually capable of. I've hooked up the transmitter PCB to my USB power meter, so we can monitor
one to the power going in. And I've got the receiver hooked up to my electronic load.
With the USB supply in 5 volts, we can get up to almost one amp on the output before
it collapses. The transmitter and receiver are both compatible
with the iPhone wireless charging standard, so we can give it a go with my phone charger.
Interestingly this is using 9 volts for the transmitter side of things.
With this we can get up to about 1.2 amps, that's pretty impressive stuff.
From the docs, my transmitter board should be able to go up to 12 volts on its input.
But when I tried this, the voltage on the receiver seemed to become quite unstable, and it would
shut off at a relatively low load. I also noticed that the receiver PCB started
to get quite warm. We're getting up to around 80 degrees centigrade on the receiver board
when we run the transmitter at 12 volts. If we run the transmitter at 5 or 9 volts,
it only goes up to a much more comfortable 50 degrees.
Interestingly, the transmitter board doesn't seem too bothered by the supply voltage at
all. There's only a temperature increase of around 5 or 6 degrees when running at 12
volts compared to 5 or 9 volts. Let's see if we can work out how efficient
it is. I've rerun the tests and noted down the power input and the power output.
We've got some interesting graphs. With the transmitter running at 5 volts,
we seem to hit peak efficiency of around 73% at a load of around 0.5 amps, and we can get
up to around 0.9 amps before we lose regulation. When the transmitter is running at 9 volts,
we get peak efficiency of just under 70% at a load of 1.1 amps, and we can pull up to
1.2 amps from the system. The boards are supposed to be able to transmit
1.5 amps, and I think this is actually probably doable under the right conditions. You would
need to get good alignment of the coils and the perfect distance between them.
I'm also using a rectangular receiver coil with a round transmitter. Now I'm not sure
how much of an effect this has, but I'm sure it's probably not ideal. There are a lot
of alternatives on AliExpress, some of them with multiple coils, which should give much
better results. I've also tried to measure the transmitter
range. I've set up the load to draw 0.5 amps, and with a 5 volt input, we get around 1 centimeter
of range. With the 9 volt input, this goes up to about
1.5 to 2 centimeters. That's pretty good, and you would normally
arrange things so that there's not much between the transmitter and the receiver coils. The
PCBs are remarkably simple. The receiver board has just one IC on it, along with some passive
components. The CP2101, which should not be confused with the very common USB to UART
bridge chip that we find on a lot of our dev boards, is doing all the heavy lifting. It's
pretty amazing how much is packed into this one chip.
On the transmitter side, we have a couple of ICs. The JW7951C is the transmitter chip.
This is responsible for driving the coil and provides information on what the coil can
see. The other chip is a bit of a mystery. It's probably some kind of microcontroller
and power delivery control chip, but I can't find any reference to it anywhere. If you
know what it is, then let me know in the comments.
Now while we're on the subject of PCBs, I should probably tell you about PCBway who
are sponsoring this video. Why not have a go at designing your own PCB and send it off
to them for manufacture? They'll even do SMD assembly for you.
But back to the video. Now obviously, you've already seen this working and powering the
Pi, but just how much power does the Pi need to boot up and play Doom?
Well, let's have a look. I've hooked it up to the USB power meter and it uses a surprising
amount of power, getting up to almost 0.6 amps when running the Doom demo. If you've
watched the previous video, then you remember that under heavy load the Pi Zero can draw
around 400mA, so our screen is adding quite a bit of load to the system, probably mostly
with the backlight. However, it is well within the capabilities of the wireless power system
given what we've measured. But I found it surprisingly hard to get it to run consistently,
and it would often brown out during booting, launching X windows or running Doom. It took
quite a few tries to get a good end to end take.
The main issue is that when you can't see the coil, it's quite hard to get good alignment.
This does explain why a lot of wireless chargers have locating magnets to snap your phone into
position. Without them, you get a pretty ineffective charging system. Something to think about for
the future projects.
Now what can we do with this? Obviously, we can't just have our Raspberry Pi only working
when it's sat on top of the transmitting coil. That would be a bit rubbish.
So let's make a complete wireless charging system with a battery. I've got a bunch of
these basic boards that are based around the 4056 charge controller I see. We could hook
one of these up with the battery and get it to charge. But I've also got these slightly
nicer boards. These have the 4056 charge controller, and they have an adjustable boost converter
which can output from 4.5 volts to 24 volts. In theory, this means that we should be able
to run our Pi even when the battery voltage is quite low.
So I've hooked it up to a battery, it's measuring just over 4 volts. But we have a
bit of a problem. The output voltage is too high. It's 5.66 volts, and I can't seem
to adjust it any lower. We should be able to go down to 4.5 volts, but the trim pot is
already at the minimum. The Pi 0 might be just about ok with this slightly higher voltage.
Looking at the datasheet of the power management IC, it will take up to 5.5 volts, and we're
pretty close to that. But it would be nice to get exactly 5 volts. What's going wrong
with our PCB? If we look at the datasheet for the boost converter on our board, we can see
that the voltage output is controlled by this resistor divider. The top of this divider
is this resistor. This has the code 104, which should be 100 kOhms. Let's disconnect the
battery and measure what we've actually got in circuit.
Well, for the top resistor, we're measuring 57 kOhms. And for the trim pot, when it's
at its minimum, we've got 10 k. It's not surprising that our output voltage is completely
wrong. We need to either decrease the value of the bottom resistor, or increase the value
of the top resistor. Fortunately, we do have these through holes where we can add some
additional resistors. I'm going to put a 100 k resistor in parallel with the top resistor.
This should give us a parallel resistance of around 78.5 k. So with that done, we just
need to reconnect the battery. And after a bit of fiddling with the trim pot, we can get
almost exactly 5 volts on the output. I have no idea what's up with these resistors, but
I've measured them on a bunch of the boards I've got, and they're all completely wrong.
Some of them are closer to 100 k, maybe within the 10% tolerance, but it's pretty rubbish.
Well, let's see how much current we can get out of the step up converter. I've connected
it up to the electronic load, and we can easily draw 1 amp without the voltage dropping. So
if that's not bad at all, this will easily power our Pi 0 even with the screen and under
heavy load. So I've hooked the board up to the wireless power receiver, and it's charging
nicely. I can see the voltage on the battery slowly going up, so that's working pretty
well. And eventually, our battery is fully charged. Now if we hook the board up to our
Raspberry Pi with a nice switch so we can turn it off and on, everything works as normal.
We've got a fully wireless Raspberry Pi. We'll give it a few seconds, and it boots
into X-Windows, and we can even run DOOM without any problems. So I've been running
it for a while, and I wanted to check just how hot the boost board was getting when running
the Pi at full pelt. And it's actually not too bad. The diode is getting a bit warm,
but it's definitely not hot, so that's going to be pretty good for our future project.
I did find some more boost boards in my stock of bits. These will boost up to 5 volts, and
I've been thinking a bit about what I've built so far. It may be better to separate
the boost circuit from the charge circuit. This would let us put a switch before the
boost converter. The advantage of this is it will stop the boost circuit draining our
battery even when we're not using the Pi, but we'll do that in the next video.
Since we've been looking at Raspberry Pi's, then check out this video. It's using a Pi
Zero for radar motion detection, face detection, and it's got face recognition. It's definitely
worth a watch. And if you're a fan of wireless power, then there's a whole bunch of videos
in the playlist which should be up on your screen right now.
Next video, we'll add sound and a controller to our Pi, and we'll have a fully portable
DOOM machine to play with.