Now that the CatTrack collar is working, I’ve been designing the schematic for the base station.  The base station is the handheld unit I’m going to use to communicate with the collar to work out where the cat is.  Currently I’m using a CC1125 development board.  More about that here.

The idea is that the base station will mostly live at home, perhaps in my loft (to get decent range) and I’ll be able to communicate with it from my living room via Bluetooth from my laptop.  It should also be portable because if Buttons goes miles I want to be able to take the base station out and about to track him remotely.

To that end, I’ve designed the following schematic (click to enlarge):

The base station can be powered one of three different ways:

  1. Lithium ion battery (for when out and about)
  2. Micro USB (also charges the lithium ion battery when connected)
  3. +5V screw terminal (again, charges the battery when connected)

There is a switch present which disconnects the battery.  If USB or +5V are connected then the board automatically powers up, with the charging status of the battery shown on the LCD screen. The screen is a bog standard 16×2 LCD screen, found in many consumer devices and available for only a few pounds.

There are also a pair of buttons, used for controlling the base station when it’s being used as a standalone unit.  You’ll be able to use both beacon mode and the locate mode directly from the unit.  More about all the CatTrack modes here. The idea is that the results from the ‘beacon’ or the lat/lon will be displayed on the screen once Buttons has been located.

In addition to being able to control the base station from the hand held unit itself, I want to be able to talk to it remotely from my PC.  That way I can put the base station up in the loft to get maximum range, and communicate with it via my PC in the lounge.  To achieve that, I’ve added a Microchip RN41 Bluetooth module to the unit.  This module takes care of all of the difficult Bluetooth stuff for you, creating a seamless UART connection from a PC to the base station via Bluetooth.  A lot of people use HC-05 modules for this purpose, which is basically an unbranded equivalent of the RN41, but I spotted some RN41 modules going cheap on eBay, so went for that!

There are a number of other bits of interest on the board:

  1. Microchip MCP73831 – Lithium ion charging IC.  Takes care of charging the battery from the +5V input, ensuring it’s not overcharged.  The IC indicates to the microcontroller the charging status, so I can display it on the LCD.
  2. Microchip PIC16LF18855 – A 28-pin PIC microcontroller.  I’ve selected this one because it’s the cheapest one currently available that does everything I need it to.  It’s only just come out, but is widely available.  Every time Microchip bring out a new PIC they seem to make it cheaper than the previous generation, then they increase the price of all the older PICs.  This means that companies end up having to pay more and more for the Microcontrollers they’ve designed into products, as they can’t keep updating their old designs to new PICs.
  3. Microchip MCP1700T – 3V3 regulator
  4. TI CC1125 – The same transceiver IC used in the CatTrack collar, communicates with the PIC via SPI.
  5. FET circuit – Q1, D1 and R1 are there to disable the battery power when external 5V power is present.  This ensures the battery is only used when it needs to be.
  6. Jumpers – used to send the PIC UART either to the Bluetooth module or a header, used for debug purposes.

I’ve also fed the li-ion battery voltage into an ADC pin on the PIC so I can work out the percentage remaining and display it on the screen.  The +5V voltage input is also fed into the PIC, just so that the PIC knows whether it’s currently running off battery power or external +5V.

I’ll give it a good check for errors, then get on with laying out the PCB.

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