justDIY Project Log

Thanks to the folks at Make: and Hack A Day, my research into the area of using LEDs as sensors has been receiving a lot of attention. With this attention comes questions. I like receiving questions. Only thing I don’t like about answering questions in the e-mail is the knowledge is locked up. Sure I could repost the e-mails, but it is sometimes difficult to follow the context after the thought train has left the station. With my rekindled interest, I wanted to take a short moment and summarize a few things.

These are just my opinions – and I welcome debate and feedback on them – I especially welcome anyone to be challenged by them enough to prove me wrong. LEDs as Sensors offer at least two avenues of usefulness; communications and interface. I haven’t dipped even a toe into the communications side of the pool – all my work has been on interface.

These are the applications I feel LED Sensors are a poor fit to replace:
#1 – The touchpad on your notebook. Seriously, no one but a geek would enjoy having 128 or more bright LEDs glowing continuously to replace the little capacitive discharge pad that is used 95% of the time today.

#2 – The keys on your keyboard. LED sensors are pretty slow, even a modest typist would be hindered by the response time.

#3 – Any application that needs to work outdoors. LEDs and the Sun do not get along.

These are the application I feel LED Sensors may work well in:
#1 – Keypads and interactive displays used for Art and Music. These applications fit the ostentatious nature of the interface, where the controls are as much a work of art as a functional device.

Yep, that is it… that is the only application I think LED Sensors offer any strength in.
Here is an example – Musician / DJ sound effects tablet:

Imagine a thin tablet like device, glowing brightly with powerful LEDs in an otherwise darkened club / dance hall / etc. The tablet accepts an ordinary 8×11 sheet of transparency film. Printed on the film are the names of pre-programmed effects / samples /whatever. Under the film, evenly spaced trios of LEDs are used to detect the presence of a reflective object. The tablet connects into the rest of your system using regular MIDI or whatever other interface one can think of. This tablet is no different than boxes hobbyists and musicians have been building or buying for years, except, the mechanical switches have been replaced with eye catching LEDs.

Here are a few other questions that have been raised:

Q: Do I have to use red LEDs?
A: No, technically any LED color works. Red is the cheapest and that is what I use. Along with yellow, red is almost the most sensitive.

Q: I tried IR LEDs and they seem very sensitive, why not use those?
A: The point of using LEDs as Sensors is to have an ostentatious interface. It’s not going to be very showy if the light is invisible. If you want to use infrared LEDs, use one emitter and one photodiode / phototransistor – it is a LOT easier.

Q: Does this work with organic / flexible LED displays?
A: I have no idea. Those displays are largely theoretical and prototypical in nature – maybe in five years when I can buy one for a few dollars, I’ll experiment with it. The organic compounds used to manufacturer these displays also have big problems with humidity and overall short lifespan – neither are very positive traits.

Q: Can you send me your ASM code for such and such?
A: No, no I can’t. I do not have any ASM code – I do not know how to program in assembler. I have code written in Proton Basic, which I will happily share.

Q: I want to get started with microcontrollers?!
A: Excellent – I’m not going to help you. There is a huge learning curve involved – a lot of it can be skipped by spending money (on proton basic). Check out www.sparkfun.com and www.crownhill.co.uk for good microcontroller stuff. Check out google.com for tons of info on learning microcontrollers.

Q: What microcontroller do you recommend?
A: I like the new PIC16F690 family from Microchip. It is a small inexpensive package that offers many features only found on larger processors (like dedicated i2c hardware). It also sports at least 10 ADC ports.

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In closing, I would also like to share some basic info on what I’m working on at the moment. My current project is to get a stand alone system worked out for emulating mechanical switches. I consider this a valuable “core” building block behind the technology. I have momentary switches partly working, after that, a toggle switch shouldn’t be much harder. As a future goal, I would like to get “cores” built to emulate sliders and knobs. Those three elements should cover a great deal of the artistic / musical needs that I feel this technology is well suited for.

Thank you for visiting and I welcome your comments. My gmail is gordonthree – feel free to contact me about anything.

Popularity: 73% [?]

This idea was shared with me on the electro-tech-online forums, and I made up some graphics to help myself and others understand it.

The first step involves choosing your illuminator LED – this diode will provide light for nearby sensors to ‘see’.

The second step is to reverse bias a nearby LED, preparing it to be the sensor. A diode in an adjacent row and column must be selected, to avoid electrical interference from the illuminator.

The third step is reading the voltage present on the cathode terminal of the diode, which indicates the light level the sensor detected.

The controller can then choose another nearby diode to use as a sensor, or move on to another pixel in the array, repeating the entire process.

Popularity: 31% [?]

Sorry I’ve been neglecting my blog!

I’ve been doing a lot of reading and research, as well as playing with a new toy.

Here are the top-5 items in my current project queue, sorted by priority:

1) LED backlit Rooster Project – Christmas Gift
2) High-End LED under cabinet lights – Christmas Gift
3) Rainbow LED chaser – New Toy / Fun Project
4) LEDs as Sensors – Interface Project
5) LED Matrix Display – 8×8 display modules

I’ll write more about the rainbow chaser tonight (I promise!)… The backlit rooster is just a cheezy Christmas gift I hope to do up cheap for someone who likes roosters and chickens in their kitchen decor. The high-end under cabinet lights are based on Nichia Jupiter LEDs – this project is currently in the very early design stages. I hope to get a proof of concept prototype built by xmas. LED as Sensors is sort-of stuck – I became obsessed with trying to build a large sensor array, but decided today, I don’t need a large sensor array… so I’ll regroup and start a fresh with actual application development for the sensors as I have them now, and worry about building them larger down the road. The LED matrix display will probably never get built, I just had some fun drawing up the designs… Once I priced out the cost to build them, my interest diminished (64 super flux LEDs for each module, 10-16 modules for a good sized display!).

Popularity: 13% [?]

Wow, a long time with no posts – sorry about that!

I have been busy the past week, I wanted to take Thursday and Friday off, so I had to cram a weeks worth of work into the first three days.

I’ve also been racking my brain, trying to figure out how I can integrate a matrix wired LED array into my sensor project. I think I’ve come up with a solution, partly hardware, partly software. Originally I had wanted to have all the LEDs on, and turn them off to do a measurement. This created a lot of problems, mostly due to the current demands. Well, I think I can achieve the same goals, without having all the LEDs lit simultaneously, which will reduce the current demands and should simply the circuit a little.

There is the PCB layout for my latest design… I think I’ll make one to test out, and if it doesn’t work for my sensor array, I can use it as a mini marquee. Sixteen vertical columns, anode connected, each driven by a 74HC595 serial shift register. Seven horizontal rows, cathode connected, each driven directly by the PIC’s analog input / digital output pins. To keep current demands low, I will only illuminate one column at a time. I think that still exceeds the design recommendations of the 595, but heck, its all I can think of at the moment.

Popularity: 13% [?]

My current ‘test’ setup with a 3×3 led matrix

prototype driver for the cathode rows … will never see the breadboard however, since it won’t pass-through an input signal to the PIC.

prototype driver for the anode columns… I’ll breadboard this out soon as I get some p-chan fets in.

Popularity: 33% [?]