Ideas for light sensor trigger/switch?

[Andy Turner]

I've got a bit of a crazy idea to try and build one of those
'invisible' MIDI harps (I think Jean Michel Jarre once used them).
Essentially there would be a bunch of light beam sensors of some sort
(I'd probably only do an octave), in the ceiling, shining *directly*
onto light sensors in the floor. As soon as the light is broken, the
sensor triggers and results in a MIDI signal being sent.

OK, so from that description, I've clearly little idea what I'm doing
and what the best way to go about this would be. I'm not adverse to
actually buying a little MIDI keyboard and pulling it apart to provide
me with some sort of latch-MIDI implementation. The trickiest bit, I'd
imagine, would be to get a system where the sensors aren't affected by
light from the adjacent bulbs - the lights would have to shine
*directly* down into the sensors and not spread at all. Or is visible
light not the way to go - should I use infra-red of some sort?

Given all that, is there any advise anyone can give me please?! Anyone
seen this project done anywhere?

Thanks


andyt

 

[John Woodgate]

I read in sci.electronics.design that Andy Turner
@4ax.com>) about 'Ideas for light sensor trigger/switch?', on Mon, 14
Feb 2005:

The trickiest bit, I'd
imagine, would be to get a system where the sensors aren't affected by
light from the adjacent bulbs - the lights would have to shine
*directly* down into the sensors and not spread at all. Or is visible
light not the way to go - should I use infra-red of some sort?

Visible red laser pointers will do quite well. Low-power ones.

 

[martin griffith]

I've got a bit of a crazy idea to try and build one of those
'invisible' MIDI harps (I think Jean Michel Jarre once used them).
Essentially there would be a bunch of light beam sensors of some sort
(I'd probably only do an octave), in the ceiling, shining *directly*
onto light sensors in the floor. As soon as the light is broken, the
sensor triggers and results in a MIDI signal being sent.

OK, so from that description, I've clearly little idea what I'm doing
and what the best way to go about this would be. I'm not adverse to
actually buying a little MIDI keyboard and pulling it apart to provide
me with some sort of latch-MIDI implementation. The trickiest bit, I'd
imagine, would be to get a system where the sensors aren't affected by
light from the adjacent bulbs - the lights would have to shine
*directly* down into the sensors and not spread at all. Or is visible
light not the way to go - should I use infra-red of some sort?

Given all that, is there any advise anyone can give me please?! Anyone
seen this project done anywhere?

Thanks


andyt

here is one I(well, we..) made earlier
http://www.walterfabeck.com/chromasone.htm


martin


"Facts are stupid things.." -- Reagan, '88

 

[Spehro Pefhany]

I read in sci.electronics.design that Andy Turner
@4ax.com>) about 'Ideas for light sensor trigger/switch?', on Mon, 14
Feb 2005:

Visible red laser pointers will do quite well. Low-power ones.

You might be able to just use one laser module- with the lens that
creates a line (available for as little as $8 USD) . Then an array of
phototransistors st when any one is shaded an object is detected. But
if this is safety-related, I suggest an tested and approved commercial
solution.


Best regards,
Spehro Pefhany

 

[Frank Bemelman]

I've got a bit of a crazy idea to try and build one of those
'invisible' MIDI harps (I think Jean Michel Jarre once used them).
Essentially there would be a bunch of light beam sensors of some sort
(I'd probably only do an octave), in the ceiling, shining *directly*
onto light sensors in the floor. As soon as the light is broken, the
sensor triggers and results in a MIDI signal being sent.

OK, so from that description, I've clearly little idea what I'm doing
and what the best way to go about this would be. I'm not adverse to
actually buying a little MIDI keyboard and pulling it apart to provide
me with some sort of latch-MIDI implementation. The trickiest bit, I'd
imagine, would be to get a system where the sensors aren't affected by
light from the adjacent bulbs - the lights would have to shine
*directly* down into the sensors and not spread at all. Or is visible
light not the way to go - should I use infra-red of some sort?

Given all that, is there any advise anyone can give me please?! Anyone
seen this project done anywhere?

You can place the fototransistor at the bottom of a tube, giving it
a bit of 'tunnel' vision. For the emitters you can use leds with a
narrow angle. If you use visible light, it is easier to align them.
Blue leds are nice, or perhaps a rainbow of color. This give a nice
effect when blowing smoke through the curtain, preferrably from
smoking some good marihuana. I don't think it is neccesairy, but
you could use modulated light, at 10Khz or so, and only amplify/rectify
the AC signal from the fototransistors, using a simple high pass filter.
That would certainly make it insensitive to other ambient lights.

Nice project BTW.

 

[nospam]

The trickiest bit, I'd
imagine, would be to get a system where the sensors aren't affected by
light from the adjacent bulbs - the lights would have to shine
*directly* down into the sensors and not spread at all. Or is visible
light not the way to go - should I use infra-red of some sort?

Multiplex LED emitters and synchronously detect with the receivers.

That eliminates crosstalk between the 'strings' and the synchronous
detection can be arranged to more or less ignore ambient light.

You can afford to drive the emitters much harder with the low duty cycle
which helps your signal to noise ratio on the receivers.

It will just work better all round unless you want to use lasers so the
strings are 'visible'.

 

[Rich Grise]

Multiplex LED emitters and synchronously detect with the receivers.

That eliminates crosstalk between the 'strings' and the synchronous
detection can be arranged to more or less ignore ambient light.

You can afford to drive the emitters much harder with the low duty cycle
which helps your signal to noise ratio on the receivers.

It will just work better all round unless you want to use lasers so the
strings are 'visible'.

Laser harp? ;-)

Cheers!
Rich

 

[Lord Garth]

Multiplex LED emitters and synchronously detect with the receivers.

That eliminates crosstalk between the 'strings' and the synchronous
detection can be arranged to more or less ignore ambient light.

That would prevent two or more "strings" from triggering the MIDI
synth simultaneously.

 

[nospam]

That would prevent two or more "strings" from triggering the MIDI
synth simultaneously.

Well if you are going to be pedantic about it, midi is a serial data stream
which can't 'trigger' two notes simultaneously anyway.

 

[Lord Garth]

Well if you are going to be pedantic about it, midi is a serial data stream
which can't 'trigger' two notes simultaneously anyway.

I'm thinking more like N-key roll over on serial keyboards...