Solder 'n' Code
Bi-Le Dominoux
- March 10th, 2011
- Posted in Uncategorized
- By relwin
- Write comment
Experimental Le Dominoux using a PIC-08 (PICAXE) board and 2 LEDs. Watch the short video:
http://www.youtube.com/user/dirtypinball#p/a/u/0/sXNebej_zlg
This version also supports a piezo speaker and “bendable” LEDs, thus combining all 3 Le Dominoux features into one.
The LEDs are bi-directional, meaning either one is an input as well as an output. The PIC samples each LED until it detects a small voltage change on one of them, then uses the other LED as a light output. In this experiment it’ll play a tune if the blink signal is lost.
These high-efficiency LEDs can generate over 1V when exposed to bright light. However, when connected to the PIC’s high-impedance ADC the LED may continue to hold its charge long after its exposure, thus making “off” measurements difficult, and causing the PIC to lose the blink signal. To ameliorate this the LED input is briefly switched to an logic input driven low, thus discharging any remaining voltage. This is done in the code (below) whenever a blink signal is lost. This fairly simplistic method works ok in the PIC basic environment. The PICAXE manual recommends grounding unused inputs — I found that by touching an unused ADC input I caused spurious readings on the other ADC inputs; the unused ADC is now grounded.
Update: most (if not all) bi-directional LED designs use 2 GPIO’s for each LED, and a “Reverse Bias and Discharge measurement” technique. This design uses only 1 GPIO per LED. I suspect the PIC’s ADC circuitry helps out but I’m no expert here.
PIC BASIC program:
;Demonstrates using a LED as a light sensor and emitter
; automatically selects a blinking input
; input on 1 or 4, output on 4 or 1
; R. Elwin, 3/2011
symbol old1 = b1
symbol inpin = b2
symbol outpin = b5
symbol inaction = w6
symbol thresh = 8 ;ADC input drop indicates end of blink
symbol ontime = 285 ;in ms
symbol timeout = 500 ;in ms*3, when to look at inputs again
b3 = 0
gosub pinselect ;stays here upon bootup until blinking detected
inaction = 0
main:
gosub readled
if b6 = 1 then
; input off, output goes on
high outpin
pause ontime
low outpin
inaction = 0
endif
pause 1 ;short delay between samples
old1 = b0
; check if nothing blinking
inaction = inaction + 1
if inaction > timeout then
inaction = 0
sertxd ("timeout",13,10)
b3 = 1 ; play a tune
gosub pinselect
endif
goto main
; figure out which pins to use
; when swapping try to 'drain' LED's charge by setting output low
pinselect:
low 1
low 4
if b3 = 1 then
tune 0, 9,($09,$0B,$07,$27,$82) ;close encounter
else
pause 2 ;don't play tune, just pause
endif
input 1
input 4
pinselect2:
if inpin = 4 then ; check other input
inpin = 1
outpin = 4
else
inpin = 4
outpin = 1
endif
readadc inpin,old1
for b4 = 1 to 10 ; sample an input a few times for change
gosub readled
if b6 = 1 then
return
endif
pause 1 ;short delay between samples
old1 = b0
next b4
sertxd ("sampling=",#inpin,13,10)
sertxd (" data=",#b0,13,10)
goto pinselect2
; read LED on ADC, returns b6=1 if dropped below threshhold, b0=input sample
readled:
b6 = 0
readadc inpin,b0
if old1 > b0 then
b3 = old1 - b0
if b3 > thresh then
b6 = 1
endif
endif
Bi-Le Dominoux experiment, PICAXE-08M
return
555 Timer Contest entry – Le Dominoux
Reed-Solomon Error Correction Encoder/Decoder for Embedded Devices
Welcome to HackHut
Recent Posts
Archives
- February 2014 (1)
- May 2011 (1)
- March 2011 (2)
LINKS
- Copyright © 2011-2014 Solder 'n' Code
- Theme designed by mono-lab
- Powered by WordPress
Very cool!
Here’s Ben Nahill’s PIC-based Le Dominoux design notes:
http://aplusnumberone.net/domino/
I read about the bi-directional properties a few weeks ago and was very surprised by it, since it doesn’t seem to be a common usage. Once again i’m very impressed by the simple elegance in your designs.
Can we infer from the fact that green LEDs don’t detect red light, that an X coloured LED can ONLY detect X coloured light? Or is there more to it than that?
The choice of tune made me smile and nod approvingly
I’ve got to try this. Nice work sir.
A question I have is all the bi-directional LED designs use a reverse-bias & discharge measurement technique requiring 2 GPIO’s. I’ve demonstrated a single GPIO solution here. Why does it work?
After reading:
http://www.imagineeringezine.com/files/air-bk2.html
I believe I’m using the LEDs in “photovoltaic” mode which is typically not used for communications as it is too slow. However, for this experiment’s configuration pv mode works ok as the light flashes are very slow.
LEDs as receivers will detect any light of a lower wavelength. So an LED that produces red light (~700nm) will detect any light right through the spectrum down to blue light (~480nm, but green light (~540nm), will detect down to blue light, but nothing above it.
Nice to see this implemented so simply!
This principle is pretty well-understood, having been identified by Forrest Mims back in the 70’s – further reading includes this paper from my friend Bill over at Mitsubishi Electronics Research Labs in Cambridge: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.69.1570&rep=rep1&type=pdf