Pedro asked me, some time ago, to do a collaboration project with a good friend of his.
The idea for the installation art project was the story "The Nightingale" from H.C. Andersen, the idea was to build/make a talking bird of sorts. Our initial plans had to change somewhat substantially... here's what we did:

My part, was to design and build the electronics that would "make the bird sing", we already had the bird's voice in a mp3 track (a danish Nightingale!!), we just needed something to play it. The original idea was to have a bird (just like in the story), but one of the places was an open air installation, so a bird cage seemed a better option.

The first constrain was time, we had only two months for doing the first "bird-o-matic". We gathered a PIR Sensor, a MP3 Player, and a LM386 amplifier IC from Solarbotics.
We wanted a reasonable sound volume so we chose a 9V battery to power the circuit, this is challenge because these batteries do not have a big capacity (max about 1000mAh) and we still wanted to play for some time without replacing the battery. This also excluded from the start using an Arduino as they consume a lot of current even if you manage to put it in standby.
I decided to use a simple monostable (LMC555) triggered by the PIR sensor that would activate the MP3 player and amplifier for a reasonable time (enough to play the nightingale sound). I used low-power MOSFETs for the power switching and one as inverter.
The first build used a LM78L05 but these have a very high quiescent current (~4mA), 10x more than the PIR sensor alone(~350uA)! The second choice was the LM2950 with only 40uA without load (increases to about the double with load) The LMC555 also has a lower quiescent current when operated at 5V (100uA) when compared to 9V (~160uA), and orders of magnitude lower than his "high power" counterpart the LM555.
Still the total current in standby was lower that 600uA, this would give 2000 hours in standby!
Playing... well "you can't make an omelet without breaking some eggs", the MP3 player consumed about 80mA and the LM386 another 80mA, giving a total play time of about 6 hours, still quite good if you think that the track was only 20 seconds... that is about 1100 times... it also exceeds the LM2950 maximum current by 50% but it works until it is too hot then the thermal shutdown "kills the bird" (battery saving on the cheap).
Here is the schematic:

Here is a close up of the inside of the bird cage:

Here a photo of the first prototype:

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