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  • Smartphone Quick-Jack

  • Created: Jun 24, 2014

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Summary

The Smartphone Quick-Jack Solution repurposes the standard 3.5mm stereo audio jack found on most smartphones into a self-powered data channel that makes communication with these smartphones as easy as plugging a headset jack into the audio port. The hardware/software platform is designed for iPhone® and Android® smartphones. This project will make it easy to connect external devices into a phone. It gives smartphones application developers an easy way to add context-aware application features, input user and environment data, or connecting peripherals. And also provides end-product designers instant access to smartphones’ convenience, appealing user interface, and cloud connectivity.

This project is compatible with both Open Mobile Terminal Platform (OMTP) and American Headset Jack (AHJ) standards; the hardware identifies the type of headset automatically and configures the hardware accordingly. First, a comparator circuit detects the type of headset port Quick-Jack has been inserted into. The result is interpreted by the LPC800, which then configures an analog switch accordingly. The analog switch connects the right pin of the jack plug to the right signal on the circuit board (GND/MIC).

The Quick-Jack board must be able to operate solely from the power supplied by the smartphone, one of the two audio channels is dedicated to power transfer; a constant stream of audio feeds an energy harvester circuit on the Quick-Jack board. This energy harvester circuit boosts the low voltage AC signal from the audio jack to a DC voltage suitable for digital circuitry. The power supply is composed of four parts; the diode voltage multiplier, the battery, the power-source selection, and the Low-Dropout Regulator (LDO). The diode voltage multiplier circuit boosts the voltage from the audio jack (right channel, typically about 650 mVpp at Vmpp) about six times; this is the primary power source. The battery is the secondary power source, and may be needed when Quick-Jack requires more current. The power-source selection has two jumpers for connecting or disconnecting the primary and secondary power sources. The LDO regulates the incoming voltage to a stable 3.3V. The circuit is designed to only enable the LDO when a phone is connected to the audio jack, thereby reducing battery drain when Quick-Jack is not in use.

A circuit is required to be able to adapt the analog audio signals to the microcontroller’s digital signals; it uses the phone’s left audio-channel as data output and the MIC as data input. Pin TDO/ACMP0 is of the LPC812 takes care of reading the audio input. Jumper J3 allows TDO/ACMP0 pin to be connected to either the comparator’s output signal (pin L-IN) or the filtered analog signal (pin IN+). It is connected to the comparator’s output by default. The LPC812 microcontroller handles all the hardware interfacing and software protocol handling, thereby enabling Quick-Jack to communicate with the smartphone. It uses Manchester coding as line code to achieve the communication between the smartphone and the LPC812 microcontroller.

The Quick-Jack board comes with a few on-board devices that can be controlled from (or read-out with) the smartphone app: a joystick, the five LEDs as output devices, and a SE98 temperature sensor. Three connectors can be found on the Quick-Jack board: Jack-plug for connecting Quick-Jack to the smartphone, SWD port for programming the LPC812, and the 20-pin expansion connector that allows the connection for external devices (e.g. sensor, I/O boards) to the LPC812 and interfacing with the smartphone.

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