Monday, 25 April 2011

Simplified shadow harp

After nearly a week of getting nowhere, we're thrilled to announce that we've (finally) got a working, consistently accurate shadow-type harp! It's much simpler than original designs, using only four LDRs (light dependent resistors) and a modified enclosure.



The original pocket- or shadow- harp designs used a pinhole "lens" to detect the position of the player's hand above the instrument. Even the original instrument's designer, Peter DeSimone, said that his original design was plagued with problems and false-triggers. To solve his false-triggering problems, Peter introduced capacitors and used the LDRs to create an oscillating input (counting the time between pulses to detect the presence - or otherwise - of a shadow).

This seemed like a lot of work for us - the problem is the LDR changes resistance continually, even by small amounts. In theory, a simple voltage divider approach should still work, we just need to eliminate the "false reads" caused by fluctuating analogue inputs.

Using a combination of hardware and software limits and sensitivity settings, we've managed to solve the problems associated with the voltage divider approach and by encasing each LDR inside a black tube, we've also managed to get rid of the cross-talk the plagued other designs.


Our LDR tube sleeves were just drinking straws covered in black crepe paper. There were no black straws in our local Morrisons, so we got these 80s style neon ones and covered them!

We're using just four LDRs and we're treating each as an individual input.
This means that we can trigger four different samples - in software we can decide whether a single sample plays or multiple (polyphonic) sounds can be produced.



Obviously four different notes/chords/samples is a bit limiting, so we're working on a dial that lets the player select a "sample set" during playing - in the same way they can change the key or chord/scale type (e.g. set to C major and the notes could be C, E, G, C, twiddle the dial and the notes could become E,G,A,B from the Em pentatonic scale).

The first thing we need to do is check we've enough spare pins for an LCD display, 4 analogue inputs, 4 LED outputs (an addition that might be useful: an indicator to show that an input has been detected) plus a few extra analogue inputs for the pots/dials. On a beefy 18F4550 that would be a doddle - we've a load of 18F2455 chips lying around from previous projects and it'd be nice to squeeze a bit more use out of those!

Check back for schematics and more videos shortly.......