Build an UHF 915MHz SMD spy bug

Features:

- 400m range

- can be powered from as low as 0.9V

- fits into a 6.5mm pen

- voice activation

How to harvest energy from an audio port of a cellphone for low power applications

Simplest most inexpensive way to extract power from a cellphone audio port for low-current sensor applications

Several methods exist to extract power from an audio port, many of which rely on boost converters with transformers and/or inductors. While these are more efficient I was looking for an inexpensive way to power a LED or a low power microprocessor with components readily available.

A voltage multiplier with schottky diodes does exactly this. For the following examples a quadrupler was used. It requires the use of a tone generator which can drive the left and right channel with an inverted signal. This increases output voltage.

Here is how to drive a LED

The AC voltage coming from the headphone jack is increased and rectified and then driving a LED.

The voltage drop over these diodes is slightly higher than 200mV, the open circuit DC output voltage goes up to 5V with the weakest phone (Xperia) tested.

The current through the LED varies between 1.5 to 2.5mA @ 1.6V depending on the Phone.

See it in action here:

Here is an example how to use that power to flash a LED

The increased voltage goes to a oscillator with a low voltage push-pull comparator. The flashing effect stems from the fact that 1uF capacitor charges slowly during the LED OFF period and discharges fast when the output of the comparator is low, i.e. when the LED is ON.

The 100uF helps to increase luminosity when the LED is on, the zener diode is there to protect the IC, just in case. Some cellphones have pretty powerful drivers.

See it working here:

Last but not least I'll show you a simple circuit to harvest a few mW out of the audio port at 3V. That voltage could be used to power sensors for example which typically operate in the tens of uA range. The sensor output could be transformed into a PWM or frequency signal and fed back into the microphone input.

Depending on the phone I used I got between 500uA to 3mA at 3VDC at the output, that  means up to almost 10mW.

Remember, the input is connected to the left and right channel and a sinewave 180 degrees shifted is fed to the audio output of the cellphone. This circuit obviously has no means to find the maximum power

point of  the cellphone driver, I found that it's in general between 10kHz and 15kHz.