Introduction: ATtiny Wearable Device - PCB Edge Connector
this is the second part of the series programming tool's for wearables, in this tutorial I explain how to create a PCB edge wearable device, which can be used with my Arduino ATtiny programming shield.
In this example, I used an ATtiny85 uC in SOIC package. You can use this tutorial as a reference and create boards with other SMD packages as well.
Let's define the project constraints:
- Arduino ATtiny programming shield compatible
- Compatible with ATtiny variants in SOIC/TSSOP =>SMD packages
- 1 ATtiny85 in SOIC package
- 1 RED SMD LED , for Status indication. I am using Kingbright 3.2mmx2.5mm SMD CHIP LED LAMP
- 1 SMD Resistor (3225 package), 400 Ohm
- 1 Coin cell battery holder
- CAD Tool for schematics and PCB design, I am using Kicad 5.1.5
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Step 1: Creating the PCB Edge Connector Footprint
Let's recall a little bit ... we want to insert our wearable device in an edge connector similar to the green one above.
For this, we need to create a male connector footprint matching the dimensions of the first female connector.
First of all, we need to have 6 PADs in our footprint. Based on the technical documentation, we can find following relevant information:
- the pitch ( distance between PADs) is 2.54mm
- the thickness of the board to be inserted can be between 1,45 and 1,82mm
- the device can be inserted 7.9 mm in the female connector
- the PADs main contact is at depth of 4.1mm
- and the width of the edge connector male has to be smaller or equal to 17.8mm
those are the constraints for our PADs.
Let's define our design steps:
- create a raster of 6 PADs with distance 2.54mm. There is an option for this in most of ECAD tools
- manufacture a PCB with thickness of 1.6 mm. Standard by many PCB suppliers
- PAD height 7mm and PAD width 1.7mm
- connector width 14.7mm
by doing this, we fulfill all constraints listed before.
Check the final footprint in last picture
Step 2: Creating the Schematic
Let's create a simple circuit by linking a LED and resistor to one of the PINs of the ATtiny85 micro.
We want respective programming/power PINs to be linked to our Edge connector, in order to enable the Arduino shield to program our device.
The logic is quite straightforward.
Step 3: Mapping Schematic to Footprint Components
In above picture you can find which footprints are used in our circuit:
- I reused one Coin Cell holder footprint from a previous tutorial
- I used the just created Edge connector footprint
- and we used a respective SOIC footprint for our SMD micro
As usual, if required I can upload respective files to this tutorial.
Step 4: Creating the PCB and Last Comments
On the top layer, we place the edge connector footprint, the micro and LED. On the bottom layer we place the battery holder.
And the last step is to define a nice shape for our device :)
In my next tutorial, I will explain how to create a Coin Cell charger.... yes I got tired to buy all the time new ones.
Hope you had fun !