I'm presenting here the simplest most inexpensive way to extract power from a cellphone audio port for low-current sensor applications.
We will NOT be using any transformers or inductors and we will create a 3V DC voltage which will be able to supply up to 10mW depending on the cellphone used. The power supply can be used to power low-current sensors whose output signal could be fed back to the microphone input.
Have a look at the video to see it working.
The circuit uses a voltage quadrupler and an LDO regulator. The voltage drops on the Schottky diodes and the LDO are quite low since the current through them is very low.
Step 1: Parts Needed
Since this is for demonstration purposes we will mount the circuit on a breadboard.
We will need the following parts:
BAT54S (or other schottky diodes) (from eBay for example)
SOT-23 breakout boards (from eBay for example)
MIC5205-3.0 voltage regulator (these are available in other voltages too, see datasheet here:
10uF ceramic or electrolytic capacitors
2.2uF or bigger tantalum or aluminum capacitor
470pF ceramic capacitor
Step 2: Assembling the Circuit on a Breadboard
Now we assemble the whole circuit. The SMD parts are soldered on the breakout boards, pins also need to be soldered into the breakout boards.
The circuit is then mounted on the breadboard.
The zener diode is optional and could be put on the input of the voltage regulator. Some cellphones can produce a voltage higher than the maximum rating allowed for the voltage regulator (under no load condition)
Step 3: Connecting to the Cellphone
In order to get most power out of the audio port we don't use the ground connection but the left and right channel only whose signals are inverted.
Step 4: Driving the Circuit With an Audio Signal
To get sufficient power out of an headphone audio port we need to drive both channels with two inverted sinewaves.
The maximum power point is different for each cellphone both mostly I found it to be between 10kHz and 15kHz on the cellphones I tested.
Therefore we either need to know how to program Android apps or for test purposes we can use an app which allows us to adjust frequency, volume and phase of each channel. There are several but I used "Audio Test tone Generator" which was available from GooglePlay. To start adjust volume to 0dB, Phase to 180 deg and frequency to 10kHz. Once the generator is turned on also increase media volume on the cellphone to the maximum.
A real application obviously needs its own app since it will probably use the microphone input as a feedback input from the sensor output.