Want to make a small device to detect when a single LED is lit -- Feasible?

[ptwego]

A friend who has lost his sight wants to be able to record music using a digital mixer that has buttons with small LED lights that indicate when the functions controlled by the buttons are active. I would like to make him a small handheld device that would emit an audible signal when some kind of optical sensor is positioned directly over a button whose indicator LED is lit. That would make it easier for him to confirm the state of buttons that toggle states but do not remain depressed when pushed (think the Caps Lock indicator on your keyboard).

I have a lot of experience assembling and soldering small parts, but know very little about circuit design or selecting specific pieces of hardware. Does this sound like a feasible project for someone with a limited electronics background? Any advice would be appreciated.

 

[KlausST]

Hi,

I think you need to analyze what the LEDs do.

In best case the LEDs are multiplexed, then if you put a light sensor on top (or close to) the LED and and amplify it's signal then you already get an audible signal. And even better: the closer to the LED the louder gets the signal.

Klaus

 

[betwixt]

...and if that doesn't work, a simple LDR on the reset pin of a timer IC (NE555 for example) will tell you when the light level exceeds a threshold. Try the idea Klaus suggested first as it is simpler and more immune to background light levels.

Brian.

 

[danadakk]

Do you have access to the interior of the mixer, such that you could
connect a wire to the switches ? And then run those wires external to
a circuit board ?

Another question, is there a PC generally used along with the music
"station" ?

Lastly have you ever worked with a micro or done any programming ? Like Basic,
or a programmable calculator, even Excel spreadsheet formula work....?


Regards, Dana.

 

[ptwego]

@KlausST and betwixt: I don't follow the suggestion about LEDs being multiplexed, but the idea about using an LDR makes sense to me. Possibly it could be shielded from ambient light by positioning a dark-walled hollow tube over it so you could aim the device directly at the button.

Do you have access to the interior of the mixer, such that you could
connect a wire to the switches ? And then run those wires external to
a circuit board ?

Another question, is there a PC generally used along with the music
"station" ?

Lastly have you ever worked with a micro or done any programming ? Like Basic,
or a programmable calculator, even Excel spreadsheet formula work....?


Regards, Dana.

Answers to your questions, Dana: No, the friend with the problem lives across the country from me, so I don't have access to the mixer. The mixer is also a stand-alone recording device, so no PC would be involved. Yes, I have done some programming in the past, though I'm not up to date with modern practices.

 

[betwixt]

I will explain multiplexing - perhaps it would make more sense then.

It is a 'trick' to fool our eyes but it makes the electronics far simpler in some circumstances. It uses our persistence of vision to perceive a light is always lit when it is actually flashing very fast. When lots of LEDs are wired together, such as on control panels or maybe push button indicators, they are 'scanned' in sequence very fast rather than being on or off all the time. Essentially, each LED is given a time slot and the power is turned on to light it during that time or turned off if it shouldn't be lit. Although it sounds complicated it can make a huge difference to the amount of wiring needed.

The difference it makes when sensing it is that instead of just looking at the light level to decide whether the LED is lit or not, we can see if it is flashing or not. Because each LED in a multiplex is only lit during its own time slot and not at any other time, the light from it will be in pulses and normally at a rate of around 500 pulses per second. That makes it possible to amplify the output from a light sensor and feed it to a loudspeaker because the frequency falls within the audible range. As the sensor moves closer to the LED and more light is picked up the tone gets louder. That means it may also be useful for your friend to 'home in' on the switch position although it there are several of these switches close together, it may not help to identify which is which.

Brian.

 

[danadakk]

You could use mBlock to program an Arduino that examines an LDR
looking at switch. Think a simple bar with an LDR/switch spaced
same distance as switches on mixer.

Arduino looks at each LDR, measures it, and sounds a unique tone note
for each switch detected on. If you dont want unique tone/switch thats
easy to change.

I added a cal routine for ambient light levels, and additional LDR for that.
So I set a switch LDR must exceed the ambient light level by 50 (thats
1.25 V in the A/D reading on Arduino). You would have to "play" with
this value (the "50") to get it right for the specific LDR and light coupling
to LDR bar.

Written in mBlock, where block like objects dragged into design window
and configured. When you finish and hit the upload button mBlock generates
the Arduino code from the block layout and config and programs the Arduino.
This creates a stand alone solution, run off a battery, walwart, whatever.

I would use an Arduino Nano, thats ~ $3, mBlock is free.



Regards, Dana.