In appreciation for my help on the glass buster and other projects, my producer friend gave me a new iPad . Very generous and appreciated. This of course opens up a new world of applications and projects and a sudden interest in magazine articles about interfacing to the device.
In the current issue of Nuts&Volts, there is an article about using an iPxxx as a moisture meter for watering plants. It connects using only the audio port and uses no external power source. At a quick glance, I guessed they were using the microphone input as a ADC to measure moisture content but a longer look showed there was more to the system than that.
The moisture meter is based on a Project HiJack interface. This is a fascinating interface developed by Ye-Sheng Kuo, Thomas Schmid and Prabal Dutta at the University of Michigan and was presented at the ISLPED’10 Design Contest. The paper presented for the contest is very informative and can be found here. This incredibly cheap interface has two unique features that allow it to be used with many audio devices: 1) It derives power from an audio signal to power the required circuitry and 2) it used audio tones to create a bi-directional interface over the audio path. One of the driving goals to to create a low-cost interface that could be used with a huge range of sensors in developing countries. One of the awesome demonstration projects is a 2-lead EKG.
The audio power harvesting technique, which is described fully in the contest paper, is truly amazing when the maximum power transfer is found to occur with an audio signal of 240 mV! A step up transformer and a FET-based rectifier bridge provide a means to use the audio signal to generate about 7 mW of DC power.
To transfer data, the team used a TI MSP430 microcontroller to generate and read either a Bell-202 modem-like signal at 300 baud or a Manchester-encoded data stream at about 8 kbps. The method described should be easy to implement on a PIC18F-series device.
On the phone side, Tech Basic supports the interface allowing development of applications with the standard look and feel. Tech Basic is available at the iStore for $15 and has a wide range of features.
The HiJack interface has been sold by Seeed Studios but is out of stock with no committed date for additional stock.