I've been playing some more with the wireless LEDs
that I showed off in a previous couple of videos.
And I got to thinking, whilst playing with my DIY version,
I wonder how many LEDs I can run off one inductor?
So, I thought I'd give it a go.
We've got a strip of breadboard and I've put the required inductor and capacitor to make
the receiver circuit.
I'm using my DIY driver circuit that consists of a ESP32 - which I'm using as an oscillator
- if you wanted you could replace this with a 555 timer or even a signal generator.
We've got a driver for the MOSFET and we've got the actual MOSFET.
This switches our LC tank circuit to ground.
You can watch the previous video on the DIY version that should have appeared up in the
top right for full details.
The inductor in our tank circuit is this coil of wire.
The LC circuit is driven at around 217kHz which is the resonant frequency of the inductor
and the standard capacitor value that I've chosen to match it.
Since I'm driving my coil at around 217KHz and I'm using a 2.2mH inductor on the receiver
I'll need a capacitor of around 244pF.
A 220pF capacitor is the closest standard value
and may actually work better anyway as there will be parasitic capacitance on the LEDs.
Let's see how many LEDs we can power.
So that's one,
two,
three,
four,
five,
six,
seven - that's seven green LEDs
let's see what happens if add a red LED.
It lights up, but all the green LEDs have turned off - what's going on?
There's a pretty straight forward explanation
the forward voltage of my green LEDs are 3.2 volts.
The forward voltage of the red LED is only 2.2volts.
As soon as we add the red led in parallel to the green LEDs
the voltage drops to 2.2 volts which is too low to turn on the green LEDs.
Luckily we've got a simple solution, we're receiving an AC signal so it has both positive
and negative cycles.
We can run our red LED in the opposite direction to the green LEDs and they will both look
like they have lit up.
They are just running on opposite cycles, but the frequency is so high we don't notice.
I've added another 6 red leds which makes 13 - unlucky for some, but it's a lucky coincidence
for us as it matches the number of LEDs on my christmas PCBs.
I got these PCBs made up by PCBWay the channel sponsor - I've got a simple video on the design
and layout process for these that you can watch later if you're interested.
But, let's answer the question how many LEDs we can light up - I could just keep adding
more discrete LEDs, but honestly life is a bit too short and I'll run out of space on
my breadboard pretty quickly.
What I do have this strip of 12 volt LEDs - they do have current limiting resistors, but let's
give it a go and see what happens.
You'll notice that I've ommited the capacitor in this circuit - the LED strip has quite
a lot of parasitic capacitance already - I measured it at around 190pF - this is close
enough to the required value so we don't need to add any extra capacitance.
Amazingly it actually lights up!
There are about 120 LEDs in this strip and they have currently limiting resistors that
will be using up some of the power that could be driving more LEDs.
Unfortunately I don't have any more LED strips to test with - but this just goes to show
how amazing LEDs are - they will light up with only a tiny amount of current.
Anyway, I guess the answer to the question of how many LEDs can we drive from one inductor
is: a lot of LEDs...
So, back to our Christmas tree PCBs - I think we can make a completely wireless version
of it that will look pretty cool.
There's a couple of things to do - we aren't going to need any current limiting resistors
- we'll be driving the LEDs directly from the receiver circuit.
So I'm going to bridge over the resistors with some bits of wire.
We also need to make sure we put the green LEDs in the opposite direction to the red
LEDs.
To make it easy for me to remeber I'm soldering the green LEDs in first and putting the long
leg of the LED into the square hole.
I'll solder one pin and then make sure the LEDs are nice and flat against the circuit
board.
For the red LEDs I'm putting the long lead in the round hole.
Hopefully this means that I won't mess it up and put one in the wrong way round.
If I do mess this up it's going to be quite hard to debug.
With everything soldered up we just need to attach the inductor to the base along with
the capacitor.
One really nice thing is that the inductor is fairly heavy so we should be able to get
the Christmas tree to balance upright.
With it all done we've got our wireless Christmas tree!
It's surprisingly bright and works really well.
In fact, it works so well I made another one!
Pretty soon I'll have a forest of glowing Christmas trees.
I've linked to the previous two videos on wireless LEDs that go into more detail on
how they work and how you can make your own, but I think that might be it for wireless
LEDs for now.
Though who knows, I might get some more inspiration before Christmas comes.