henlo bird

Hardware prototype for Klangnetze, March 2022

Some notes on power consumption

The dynavox speaker, despite its small size, is powerful enough to be also installed in relatively noisy environments (e.g. Hauptplatz Gleisdorf). Its power consumption is largely dependent on the type of sounds it plays back. Steady tones like sine / pulse / square oscillatoes, which make it continously vibrate, consume considerably more energy than other sounds. We are using a 2600mAh Li-Io battery and a stress test with a square wave at 200 Hz drained it after ca. 1 hour and 30 minutes. Another test with different sounds (those that you hear in the video above) ran a fair amount longer (more than 6 hours). With (very) loud sounds, or peaks in amplitude, it might happen that the amplifier requires more power than the powerboost can provide. In this case what happens is that everything shuts down (not gracefully). Therefore we will probably need to limit the output through alsamixer (around -9dB is stable sofar).

Solar charging

Preliminary tests show good performance even in partly cloudy conditions. The BQ24074 provides power to the raspi and recharges the battery at the same time. With the 'bird patch' running, in a partly cloudy day the battery took 5 hours to go from 3.6v to full charge 4.2v. Therefore, in similar conditions, we have a total of ~10 hours of charge (5 solar + 5 battery). But, as discussed above, this depends on the type of sound you play ;)

Known issues / improvements

When the pi is shut down, it still consumes a fair amount of energy: 50mA (as a reference, raspi running on idle with various power optimizations consumes 150mA). This means that, assuming for example a 16 hours shutdown at evening/night, we use a total of 800mAh (1/3 of the battery) for doing nothing. There's no workaround other than adding a dedicated circuit that shuts down the powerboost after the raspi. one solution / a more advanced one, with battery monitor.