Introduction: DIY Wearable Ring LED
a couple of months ago I build my first SMD PCB, so I would like to share my experience with you.
We are going to design a circular PCB with 4 RED LEDs, this board can be used as indicator or just as decoration for Halloween :). Check the magic ring above. ( Better picture to be uploaded... sorry)
Let's define the constraints of the project.
- the device shall be embedded in a housing with following volume 12mm (diameter) x 8 mm ( height)
- shall operate with low voltage and for 1h without exchanging the battery
- 4 RED SMD LEDs , I am using Kingbright 3.2mmx1.6mm SMD CHIP LED LAMP
- 4 SMD Resistors (3216 package), each 400 Ohm.
- 1 Coin Cell CR1025
- 1 Battery Holder for CR1025, I am using Keystone 3030TR
- CAD Tool for schematics and PCB design, I am using Kicad 5.1.5
I will explain in next steps how I chose above Hardware components
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Step 1: Choosing the Battery
As mentioned before, we need a low voltage battery which can supply enough energy for 4 LEDs.
This battery has to be small enough to fit in the dimensions defined before. By checking the table above, we can identify CR1025 as a good option for our project, small enough and with enough energy to provide 7.5mA for 1h to each LED. By checking it's Datasheet (Fig2), we can find that the thickness is relative small 2.5 mm. Perfect for our project as well.
Remark: The LEDs operate usually with voltage > 0.7 V and max forward current of 20 mA each. The CR1025 nominal voltage is 3 V and cut - off voltage of 2.
Step 2: Choosing the Battery Holder
This step is quite straightforward, based on google search I found a Keystone table for battery holders ( Check above figure). I choose the 3030TR because of availability at my supplier shop, but you can choose the 3050TR as well.
Remark: the height of the battery holder is 3mm, battery embedded in it => total thickness 3mm. Great!
Step 3: Other Components and Final Dimensions
Let's calculate the value of each resistor, we want to let 7.5 mA to go through each resistor.
This means ( ignoring the voltage drop in each LED). R = 3V/7.5mA == > 400 Ohm
On the other hand, we need to check the thickness of all components added. Until now, we had:
3mm due to battery holder and battery.
we need to include the PCB thickness ( usually 1.6 mm) => 4.6mm
and the LED/Resistor thickness ( 0.75 mm) => 5.35 < 8mm. Great Constraint achieved.
Step 4: Schematic and PCB
- The schematic is quite simple we connect the LEDs to respective resistors and power supply.
- I was not able to find the footprint for the Keystone 3030TR battery holder. Therefore I modified one available and used it in the project. The modification is not perfect, but it works. I will try to find a location to upload it, it was not possible here.
- I tried to follow the schematic in order to place the components, the main idea is to place the battery holder on one layer and other components in opposite one.
The last picture shows you the PCB in Kicad 3D viewer.
I sent the files to my PCB supplier to get the final work done. Hope you liked it!