When I took apart the old broken iPad1 I ended
up with a nice LCD screen for my magic mirror and I also ended up with a couple speakers. I want to complete the salvage operation and
turn these two speakers into something useful. For most of my audio projects, I've been using
the MAX98357 breakout board from Adafruit. This is a great little I2S Class D amplifier
however, it only supports one channel. So if you want to have stereo output you need
two boards. This is fine, but it's a bit of a faff wiring
it all up. It's not a massive hardship, but let's make
our lives easier. But first I'd like to thank PCBWay for sponsoring
this video, PCBWay offer PCB Production, CNC and 3D Printing, PCB Assembly and much much
more. They are great to deal with and offer excellent
quality, service and value for money. Check out the link in the description. Looking at the Adafruit board I think there's
room for two ICs and the nice thing about I2S is that it's designed for stereo signals
so we won't need any extra pins. This should mean that whatever we design can
be made to be pin-compatible with the Adafruit board. If we look at the datasheet we can see that
it's actually a very simple IC to wire up. We just need a couple of bypass capacitors
for each IC and we need a resistor on one of the ICs so that it knows it is on the right
stereo channel. We can also take a look at the Adafrui schematic
to see what they have done. Interestingly they've added an additional
LC filter to the output. Checking with some other breakout boards we
can see that they omit the LC filter. Checking back in the datasheet we can see
that the output filter should not be needed. To save a few components, I'll omit them from
my design and rely on the speakers acting as a filter. Let's jump into EasyEDA. We'll start off with a 7 pin header and make
sure we match the Adarfuit board exactly. We need VCC, GND, SD, GAIN, DIN, BCLK and
LRCLK. With the header in place, we can now wire
up the first IC for the left channel. We'll pull in the IC and hook up the pins. It's not a difficult schematic to wire up. For the NC pins on the IC I'm connecting them
all to GND - this should help with thermal dissipation as I'm planning on making quite
a large ground plane. With the power and ground connections done,
we can just hook up the nets for the signal lines. And then we hook up the positive and negative
outputs to the speaker. To save space I'm going to use some quite
small screw headers with a pitch of 0.1 of an inch. The last thing we need is the two decoupling
capacitors. And that's our left channel completed - as
I said before it really is a simple schematic. We can just make a copy of this for the right
channel. The only real difference between the left
and the right schematic is the resistor that we place in series on the SD input. Let's have a look at the datasheet to see
how to calculate the value for this. I'm going to use the approach given in figure 5. So we'll need to pick a value for R that gives
a voltage between 0.77v and 1.4v when the GPIO is outputting 3.3v. There's an internal pull-down resistor of
100kohms with a tolerance of plus or minus 8%. I've calculated that a 200Kohm resistor with
a 1% tolerance should work for us. In the worst-case scenario, where have a 202KOhm
value for R and 92Kohm value pull-down resistor, we'll have a voltage of just over 1V. And when have 198kohms for R and 108kohms
for the internal pull-down resistor, we'll have a voltage of around 1.16 volts. This puts us pretty much in the middle of
the voltage range of 0.77V to 1.4V for right channel mode. We'll add this resistor into the schematic
for our right channel and feed the SD signal through it. We just need to make sure our output is properly
wired up and we're done with the right channel. As I've said, this is a really nice and simple
schematic to wire up. We're now ready to do the PCB layout. I'll start off with a rough layout of where
the components should go, placing them in approximately the right locations and orienting
them so that the pins and signals are nicely lined up. Since this is a breakout board, labelling
is pretty important, so I'll also do that now to make sure the text fits in nicely. I'll neaten up the layout and once we've got
everything in roughly the correct position we can add the board outline along with some
mounting holes. I'll also copy the pin labels to the bottom
of the board - this is really handy and I wish more dev boards would do this. The last thing we'll do before starting the
wiring up is label the output pins. Once again I'll duplicate these on the bottom
of the board as well We're now ready to start wiring things up. I'll turn off the silk layers for this stage
as they are getting in the way. I'll start off with the signal wires. We'll do the left channel first. I've turned off the VCC and GND nets to make
it easier to see what I'm doing. For the right channel, I don't think there's
any choice but to use the bottom of the board for some of the traces. I'm just going to tweak some of the positions
to make sure the silkscreen is still readable and adjust some of the wires. For the power, I'll use quite thick tracks the amplifiers can draw a fair amount of current when the gain is set to maximum. I'm going to use 40mil width traces which
should be more than sufficient. To get power to the left channel I'll use
a thick trace on the bottom of the board. I'll also use thick traces for most of the
wires to the speaker output. I can't use thick traces all the way as the
pins are too close together. We'll add copper pours to the top and bottom
of the board for GND and stitch them together with some vias. The thermal pads on the ICs are connected
to these ground planes so this will provide a nice route for any heat to escape. After Looking at the ground plane on the bottom
layer I've decided to reroute the signal traces for the right channel so we get a better ground
plane under the left IC. With that done I'll redo the copper areas
and we're pretty much done. Just some final tweaks on the silkscreen layer
to tidy things up. And we need to add a label to the board with
a revision number. That's it all done - I've exported the Gerber
file and we can jump onto PCBWay to get the board ordered. We'll use the PCB Instant Quote option and
upload our Gerber file. Everything should be set up for us - I'm going
to get these boards assembled so we'll complete the SMT assembly section and proceed to our
shopping cart. Here we upload the BOM file and the pick and
place file which we can export using the fabrication menu in EasyEDA. PCBWay will now source our components and
give us complete cost for our 5 boards. One of the nice things with PCBWay is that
you have a lot of options when it comes to components. You are not limited to a restricted parts
library, they will source the components for you and you can even supply your own components
if you want to. Within a day we have an email from PCBWay
with the costs for our parts. Once we've approved the costs the order goes
into production and we can track the progress online. Now, if you've watched the most recent mailbag
video, you'll know that the boards have arrived! They look really good. I've soldered one of them up and we're ready
to test. I've got a couple of speakers that I salvaged
from my old iPad1 which I took apart for the magic mirror project. I'm also using version 2 of my audio PCB so
this is a double test. Let's hope I've got the wiring set up properly... It works really nicely. The bare speakers don't sound great so I thought
I'd try 3D printing some speaker enclosures. Turns out I don't know anything about designing
speakers and it sounds pretty much the same as before. There are a couple of things that I'd like
to improve. I was a bit aggressive in making the board
as small as possible and the screw terminals are hanging off the edge of the board and
they are also pretty close to the amplifier chips and only just fit in the space. You also can't see the labels for the screw
terminals they are visible on the bottom of the board - but that's not much use when the boards are plugged into a breadboard. I'll fix these two issues and then publish
version 2 on PCBWay for other people to order. So, now we've got a stereo amplifier and some
speakers - what can we do with them? I'm going to make a really simple Bluetooth
speaker. The ESP32 supports Bluetooth A2DP which makes
this very easy. There's also a really nice library from pschatzmann
on GitHub that makes this a breeze. All we need to do is tells it the I2S configuration
and the pins to use and it will do all the heavy lifting. Once it's up and running we can pair it with
our new audio device and start playing sound. It's pretty cool! I'm really happy with both my speaker salvage
and my new audio boards. I hope you enjoyed watching the process. Thanks for watching And I'll see you in the next video!
