A Dublin Crossing Lights Emulator

Late 2015 I heard a track by Giuseppe Ottaviani. It's called Crossing Lights and features a bleepy sound made by a pedestrian crossing light. I absolutely loved this track. Then I went to Dublin. I came across a pedestrian crossing (below, borrowed from Flickr as I forgot to bring my camera) and I pressed the button on the box on the pole of the traffic lights.

It started to pip and when it went green I went berzerk. This was the exact same sound as in the house track! Listen here, here and here.

When I got home I started to design an emulator. Just for fun. I found a couple of PIC12F629s in my PIC tray. This controller has maximum six IO lines. With the master clear there are five left. Button, two speaker lines, red and green LEDs, see the schematic. I made a few recordings of the sound and with a prehistoric audio editor I analyzed its composition:

With LibreOffice Calc I came up with a simulation:

I quickly put together a kludge to test the program and this is the result:

What we see here is a spectral representation of the distinctive gliding "pew" tone the light makes as it goes green. It was quite an effort to get it right...

The final version was built into a small computer speaker, part of a set with a tiny subwoofer. There is plenty of space to accommodate a lithium battery (from a laptop power pack) and the board with the electronics. As the speaker has a low impedance of only 4 ohms I needed a driver. With the base current of the transistors of the H-bridge very frugal it drives the speaker with a modest volume (listen). This is the design now:

Final observations: the transducer in the box on the pole is actually the blue plate with the arrow. While driven by a digital signal (a square wave) it produces a nice rounded pop that the speaker can't make. Its mass is much lower than the transducer. To illustrate, compare the sounds of the pops:

The real life version is clearly different. With extra components it may be possible to improve it but since I spent a lot of effort on it so far it's OK with me now. Maybe another time...

 

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Last Update: 23 November 2015