Friday, August 29, 2008

Message in a balloon

Continued from "Message in a Balloon" at GeekPhysical

As part of the ongoing micropower device research we wanted to explore environments where no external power sources were available. The aim is to create a tiny model satellite that will survive for as long as possible attached to a small helium balloon. This project is an attempt to create an small and disposable radio transmitter that can be carried by a party balloon and eventually be powered by a tiny solar panel.

In its current version the transmitter morse-codes a radio amateur call sign and identifies itself. For the final project we call upon the radio amateur society to pick up signals from the satellite, log its time and position and report back to a dedicated website. The test transmitter was flown on tethered balloons for several hours to test the range and durability. The message was picked up by several pre-warned radio amateurs at various distances from the test-site.

Our future plans for this project include the use of solar panels, arranged in a triangle formation, which would always be pointing to a source of sunlight, and could thus power the device without the need for external power.

Listen to the message recorded by an radio amateur.

Wednesday, August 27, 2008

Finding Stuff in the Sky

Continued from "Finding Stuff in the Sky" on GeekPhysical

As an artist and experimenter you may come across the problem of finding something or someone in your surroundings and needing to pointing some kind of sensor at them.

First, one needs to acquire an initial position, and thereafter retain a lock on the object need to be performed. One method of achieving the initial position is to first scan the whole space where the object may be located using the sensor(s) and then choosing the position with most/best sensor signal. Once the initial position is acquired a smaller part of the surroundings may be continuously scanned to retain knowledge of the position even if the object is moved.

Antenna tracker

We are doing long distance flights with RC video planes. During these flights a high-gain receiving antenna must be pointed directly at the aircraft. The antenna is mounted on a post with servos enabling it to pan and tilt in such a way that the whole sky can be pointed at.

A microcontroller system is then connected to the servos and a receiver measures the strength of the signal received by the antenna. Initially, the microcontroller has no knowledge of the aircraft position and needs to scan the sky for a suitable signal. Since the antenna is long and flexible it is desirable to use harmonic rather than abrupt movement thus the search pattern chosen is a spiral motion starting from an arbitrary point in the sky and moving outwards.

Once a position with sufficient signal is encountered along this spiral path this is chosen as the lock position. Hereafter the antenna is moved around the initial point in a smaller circle and the point with maximum signal strength is recorded. After each circulation the initial point is moved towards the maximum point. This results in the antenna continuously improving its aim towards the transmitter.

To prototype this system an LED was used as the transmitter and a photo transistor as the antenna/receiver. The LED was modulated to avoid interference from surrounding light sources.

Self sustaining micropower devices - aka, a sexy new doorsign

Continued from GeekPhysical - Sustaining Micropower Devices

Many (interactive) electronic devices spend most of their time doing nothing or very little. The ratio between usage and inactivity often results in an extremely low average power consumption. This opens the opportunity for powering such devices from very weak but ever-present power sources.

To explore the possibilities for having an active electronic microcontroller-based device without the need for any expendable power source we have developed an LCD door sign that will run day and night powered by a tiny pocket calculator solar cell.

Miniature solar cells like those found in pocket calculators will, in favorable conditions deliver a mere 100µA @ 3-4V and since most microcontrollers require at least a couple of milliamps to run some sort of power management is required.

The microcontroller used in the project(1) has the ability to shut itself down reducing power consumption dramatically. An internal timer can be programmed to 'wake up' the controller at regular intervals to do useful work and then go back to sleep. Using a capacitor or rechargeable battery to store the energy collected between small burst of microcontroller activity allows the average current draw to be reduced to a few µA. In effect the solar panel produces an excess amount of energy over time.

The controller used(1) also has the ability to measure the voltage across the battery/capacitor thus giving a measure of the available amount of energy at any given time. The microcontroller program has the ability to change its level of activity depending on available resources. Our project contains an AVR microcontroller, a small alphanummeric LCD, two pocket calculator solar cells and a tiny 80mAh LiPo rechargeable battery.

The controller scrolls the names of the inhabitants on the LCD thus demonstrating a simple and useful device.

Apart from the display function the microcontroller performs the power management functions required to maintain active over a 24 hour power harvesting period. Further, apart from changing the interval of the activity depending on power reserve the controller has the ability to completely shut down the LCD during times of power starvation.

This project does not imply interactivity but functions could be implemented to keep the device powered during interaction. Other weak energy sources such as electromechanics-, electromagnetic-, static electric or chemical could also be applied.

(1) Most modern microcontrollers have this functionality.