Abstract: iPhone dock connector can not be used unless signing a nondisclosure agreement with Apple. So, audio interface is the only way to connect the external hardware to iPhone without Jail Break. Here I show I-V characteristics of the audio interface as a power supply. Audio sinusoidal signal rectified using silicone diode voltage doubler gives open voltage 1.5 V and it drops to 1.2 V when output current is 1 mA. Its large voltage loss dues to silicone diode's large forward voltage. Microphone terminal also can be used as a power source. Its output voltage is 2.7 V and output resistance is 2.3 kohm. The amount of power is small but enough to use a ATMEL AVR low-power microprocessor which requires current 0.3 mA/MHz.
Audio output as a power source
iPhone headphone jack has two audio output channels. Its output impedance is 36 ohm and maximum voltage amplitude is 0.95 Vrms (2.7 V peak-to-peak). Therefore one can get up to 6 mW per channel.
Fig. 1: Measurement circuit.
To get enough output voltage for a logic circuit (1.8 V or 3.3 V), a measurement circuit shown in Fig.1 is a simple voltage doubler . Silicon planar diode, 1S1588 which is popular in japan, is used.
Fig. 2: Measured output current-voltage.
Measurements result is shown in Fig.2. Audio waveform is a sinusoidal and its frequency is 15 kHz generated by the sample code . Output volume is maximum. The output voltage is 1.5 V when Iout is zero. Its voltage dorp (2.7 Vpk-pk - 1.5 V = 1.2 V) is that of silicon diode forward voltage drop (0.6 V). As Fig.2 shows, the output voltage increases by only 50 mV even when C1 is changed from 4.7 uF to 330uF. The output voltage drop is caused by the diode forward voltage drop.
If the silicon diodes are replaced with schottky barrier diodes, output voltage will be 1.8 V at 1 mA output (if diode forward voltage is 0.4 V).
Microphone terminal as a power source
Microphone terminal of iPhone tetrapolar audio jack is biased to drive a capacitor microphone. Its output voltage-current measurement result is shown in Fig.3. Open-circuit output voltage is 2.7 V and its output impedance is 2.3 kohm.
Fig.3 Measured output current-voltage of a microphone terminal.
How to get power source from an iPhone headphone jack is described. Audio terminal gives 1.8 V at 1 mA, and microphone 1.8V at 300uA. It is enough for a low-power microphocessor. Higher output voltage (up to 4.5 V) can be get using L and R-channel reverse phase signal.
- Wikipedia: Voltage doubler, http://en.wikipedia.org/wiki/Voltage_doubler
- Sound modem projects (in Japanese), http://www.reinforce-lab.com/projects/iphone-software-modem
- MonoTouch Audio Unit sample code, http://blog.reinforce-lab.com/2010/08/monotouchaudiounitsamplecodes.html