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Now that's a project..

The Mad Professor

New Member
Plodding through the net I came across an thought provoking little snippit..
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In 1977, Forrest M. Mims reminds us in one of his "Engineer's Notebooks" that LEDs can also be used as photodiodes...
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Then I found this and my jaw dropped, open mouthed scratching the stubble on my chin... OOOOOOOOOhhhhhh yes I said to myself, I like that.

http://mrl.nyu.edu/~jhan/ledtouch/index.html
12MB of video to download
 

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DirtyLude

Well-Known Member
That is a really cool demo.

I'm guessing he's using the reflected light as a proximity sensor. It's just like all the LED/phototrasistor paired proximity sensors, only using the LED as both the light source and the detector. It's interesting, but it's obviously limited just like a regular Prox sensor. Ambient light and changes in ambient light have to be adjusted for. Shadows, anything like that can cause unwanted readings, or make it unusable.

I like how he's using actual proximity reading and not just on/off sensing.
 

philba

New Member
there's a couple threads from some guy who spent months experimenting with this. he says its' capacitive. he wound up concluding that it's not very cost effective. I'm just telling ya what he said .
 

DirtyLude

Well-Known Member
Well, it looks like the Mad Prof added a PDF file which I assume is related to the video. The PDF describes using the LED's as photo emmiters and detectors, so in this case, I'm assuming it's not ussing capacitive sensing.
 

agent420

New Member
It's funny this topic comes up, I recently obtained a collection of most of Mims' Engineer's Mini-Notebooks, and although now 20-30 years old they hold up well and are full of great ideas and circuit designs. I don't have the one that details the detection experiment, but he does make references to led light sensitivity.

I got interested and did some Googling, here are some relavent links I have found...

justDIY - LED's as sensors
Sensor Synergy
Led's as light sensors
 

The Mad Professor

New Member
Ambient light according to the text is less of a problem than anybody thought as the peak wavelength responce of the of the LED in "detector" mode is closely related to its emission characteristics. Sadly the the responce times may be a bit slow for high speed data exchange , but it certainly looks like a keypad or dare I say keyboard ,could be done.

I would be interested to hear what our resident PIC programers have to say,
and wonder how the process holds up when a Bi-Colour LED is used?
 

Hero999

Banned
I've had this idea before but I've had no application for it, however I wasn't clever enough to think about the reverse bias capacitance of the LED.
 

rockin_rick

New Member
I was thinking that another application of using a LED as a light sensor may be to generate 'random' data to reseed a random number generater using the tapped shift register method. You could have it check the light level every so often, and continously reseed the shift registers on the fly. This should eliminate generating the same pattern of random bits (unless the light level is always exactly the same). Cheap randomizer...

Rick
 

mramos1

Active Member
Great idea on the randomizer.

I have cut the tops off a power transistor and seen 1.5 volts+ at very low current. Something else to try in place of the LED?
 

fingers

New Member
I've been trying to make a project like this on a smaller scale but can't get it to work:confused:. The LED just stays at the same brightness no matter what the light levels are! It is a clear red 5mm LED. Found the circuit diagram here: http://www.edn.com/contents/images/6335303.pdf (page 5). I haven't got all the components (i.e. MC14528 & OP-90), so have modified the circuit to suit what I have available.
Would you tell me if my circuit looks wrong, please?
 

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audioguru

Well-Known Member
Most Helpful Member
Your power supply voltage is awful darn low. The LM358's output probably doesn't go high enough for the NOT gate, and the output current of the Cmos 555 will be very low.

Your circuit doesn't have a reset when power is applied.
Your circuit also keeps the 555 timing because pin 2 stays low if the NOT gate gets enough input voltage. Capacitor-couple the NOT gate to pin 2 and use a pullup resistor at pin 2 then the circuit might work.
 

bigkim100

Banned
Damn nice redesign of the original circuit guy. Have you tried the changes that audioguru suggested ?
If it does work, please send me the schematics, or post them here, as * am quite interested.
 

kchriste

New Member
Forum Supporter
You can eliminate the "not gate" (inverter) if you swap pins 5&6 on the LM358. Like AudioGuru stated, you need to capacitively couple pin 2 of the 555 which will then need a pullup resistor also. This is because the output of pin3 on the 555 will NOT return to the inactive state as long as pin 2 is low. Also, you'll need a diode in series with R5 because when pin3 of the 555 is low it will drain any voltage (through R5) that the LED produces when sensing light. A higher supply voltage should be tried if all else fails. Try 5V.
 
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audioguru

Well-Known Member
Most Helpful Member
kchriste said:
You can eliminate the "not gate" (inverter) if you swap pins 5&6 on the LM358.
Then the high output voltage of the LM358 is about 2.55V-1.2V= 1.25V which is too low for the Cmos 555 which needs at least 2.55V/3 x 2= 1.7V.

Also, you'll need a diode in series with R5 because when pin3 of the 555 is low it will drain any voltage (through R5) that the LED produces when sensing light.
Then the voltage for the LED will be even lower than it is without the diode.
I was wondering why the original circuit used the transistor instead of the Q output of the monostable. So that it doesn't load the LED when it shouldn't.
 

_nox_

New Member
This is cool! Really cool!

Hmmm... I'm curious. Could it work using infrared leds, too ?
A hyperfine matrix of infrared leds, built on one big wafer ( either silicon or polymer ), could be a great new human interface device for various applications.
 
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kchriste

New Member
Forum Supporter
audioguru said:
Then the high output voltage of the LM358 is about 2.55V-1.2V= 1.25V which is too low for the Cmos 555 which needs at least 2.55V/3 x 2= 1.7V..
He could add a pullup resistor to the output of the LM358 and probably get away with it. Or use the LMV358...

Then the voltage for the LED will be even lower than it is without the diode.
It is getting tight with all the voltage drops! Since he wants to use cheaper components it is probably best to up the supply voltage to 5V since he has it in his circuit for the inverter anyway! (and also change R1 to 330K) Not to mention how running the inverter from 5V shifts the schmitt trigger point too high (2V with 5V supply) to begin with and drives the input of the 555 above it's supply rails!
 
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audioguru

Well-Known Member
Most Helpful Member
kchriste said:
Since he wants to use cheaper components...
That is the problem with his circuit. There are two ways to make things: the correct way that works, and the other way that doesn't work.

Not to mention how running the inverter from 5V shifts the schmitt trigger point too high to begin with and drives the input of the 555 above it's supply rails!
I didn't think of that. The excessive input voltage to the Cmos 555 powers it through the input protection diode so it has enough output voltage and current to drive the LED.
 

fingers

New Member
audioguru said:
Your circuit doesn't have a reset when power is applied.
Should the reset (pin4) go to ground? It goes to Vcc in the example.
I've now put the supply voltages of all ICs to the same +5V rail.

audioguru said:
Your circuit also keeps the 555 timing because pin 2 stays low if the NOT gate gets enough input voltage. Capacitor-couple the NOT gate to pin 2 and use a pullup resistor at pin 2 then the circuit might work.
I've put a 100nF capacitor between the output of the NOT gate and the Trigger of the 555. (LED is constantly on)
I then put a 10k resistor between the trigger and the supply rail. (One flash of light and immideately goes off each time power is applied)

Thanks for your help audioguru. Which route do you suggest I take next, as I've had a few different bits of advice from yourself and kchriste?
 
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