The SolBLETracker is a solar powered bluetooth low energy beacon with a beeper for indication to help you keep track of your kitties. It makes use of flexible batteries and solarpanels and PCB's to fuse the tracking circuitry into a flexible collar. The project is currently on its 4th design iteration. And I hope to have this design on kickstarter before the end of the year.
Above you can see the previous 3 iterations as well as the Flexible solar panels and batteries. Below you can look at the circuit and PCB schematics for the 4th iteration.
For flexibility the PCB is divided into 3 sections. The first is the BLE Module and LED. Then the Micro USB port for charging and the buzzer. Finally, the all the power circuitry.
It took a considerable number of hours to select the chips. Power consumption was the paramount concern with size as the next most important.
The solar charge control chip (BQ25504RGTT) with its quiescent current of less than 330 nA was the obvious choice.
The selection of the BLE121LR-A-M256K was driven primarily by the BGScript language. This removed the need for a secondary controller, with its low power consumption, 8dBm output power, and pulse width modulation capability to drive the buzzer it came out ontop.
Because I needed to use LiPo chemistry to have enough current to drive the beeper I needed a LDO to regulate the voltage below the BLE Modules 3.9 maximum input voltage. The TPS78330DDCR was chosen for its 500nA quiescent current.
The buzzer (SMT-0440-S-R) was chosen to be as small as possible while being lound enough to here from a a couple rooms away, as well as drawing an amount of current drivable from a small LiPo.The buzzer (SMT-0440-S-R) was chosen to be as small as possible while being loud enough to here from a a couple rooms away, as well as drawing an amount of current drivable from a small LiPo.
The lithium battery charger (MCP73831-2ACI/MC) for charging from the MicroUSB port was selected primarily for its small footprint and low external component count.
The battery protection IC (S-8261AAJMD-G2JT2G) was added because the flexible LiPo cells come without their own protection circuit. It was picked because its low 3.5uA current consumption when running and 0.1uA current consumption in shutdown mode.
The Flexible solar cell was one of the harder components to source. It was hard to obtain the smallest size flexible solar cell in limited quantity. Ultimately I had to resort to EBay. The SP3-12 was selected because it was narrow and flexible.
I was also only able to find one decent source for flexible LiPo batteries. The PGEB0054018 was selected for being narrow as well as thin.
If you add up all the quiescent current and average out the beacon current (about 20mAh for 3.5 ms every 10 seconds) draw you end up with about 7.4uA of average current draw. With that figure without solar charging using the 10mAh flexible battery you get a little over a month of beacons. The solar panel outputs 8.5 mA in ideal conditions. That's 1000 times the average current draw. Even running at 1/10 its optimal current it would only need to be lit up for about 15 minutes a day to stay charged.