The main concern was to find a good combination of price, power and availability. The market is filled with cheap with IoT chips and single board computers.
Raspberry Pi Zero W
The hardware for my product had to be affordable. For the CPU I originally started the prototyping using a Raspberry Pi zero W the Wi-Fi version. My first PCB was based on this board. I used 90-degree SMD dual pin headers to connect the raspberry pi to the PCB. I went with SMD pin headers so I could keep the other side of the PCB clean and flush for the LCD-s.
The problem is that this way the Raspberry Pi board is not sitting flush with the PCB. But that is not the biggest problem. The issue is that this board can’t be ordered in batch. As of writing this post, there is a 1 piece limit/order on these small Pi boards. They are priced 15$ a board + 8$ shipping/piece. The only way to use this board for a product is by getting a approval from the Pi Foundation by contacting them about bulk orders.
Raspberry Pi Compute Module
The Pi foundation has another board that is made for better integration for PCBs. The INDUSTRIAL COMPUTE MODULE line. They have multiple RAM looking Pi boards that can be integrated into a PCB using a DDR2 SODIMM connector. The same connector that seats DDR2 RAMs in notebooks. The problem with this module is the price. The cheapest one is 30$ and its an overkill for a project like this. As this project could be powered by an Arduino calibre microcontroller.
I was looking for an Arduino type chip that has wifi. This is where the ESP8266 comes in the picture. A chip containing a single core 160MHZ CPU and 4MB of flash. Most of the development was done using this board seated on a development board. The cost of these boards is extremely cheap. We are talking about 4$ a piece.
The only drawback of this chip was that it was single core. I wanted to have a web server running in the background while the device is processing requests on the front. I needed a dual-core CPU.
This chip is the successor of the ESP8266. It packs a dual-core CPU and adds low power Bluetooth (BLE) to the pack. This is the chip I ended up using for my project. The price is around 6-8$ a piece. The only problem that might come up is the flash size that stayed 4MB. I have seen mods that allow the replacement of the memory IC to 16MB or even 32MB but the firmware have to support it.