Idea: Self adjusting IR detector
June 9, 2010
For some time I've been playing with IR sensors, trying to get some "eyes" for my little bot here: http://letsmakerobots.com/node/19894
It is based on the IR Compond Eye design from Oddbot only smaller because I want to use two of them on the bot to get a very approximative distance measurement.
Here is a version much alike Oddbot's but with only one phototransistor and 4 IR leds, and a 10K resistor instead of 4K7 to get a little better sensitivity from the transistor :
(please ignore T1 name above, I just used one I found to get the symbol, it is not what I have, actually I have no clue what I have cause they sell them without a code here)
Later edit: and yes R1 value is wrong it has to be 120R (I have to check but is not important)
This works pretty well, I get a reading at about 10 cm in daylight and around 5 cm in darkness for some weird reason.
Now playing around with the above got me to this idea:
How about if the resistor R2 in the schematic would be replaced with an LDR (Light Dependant Resistor)?
According to my non-electrical-engineer logic it should help compensate for the environmental light, because the LDR decreases resistivity when the light increases and vice versa, directly affecting the sensitivity of the phototransistor.
I actually tried that on the schematic above and got, well not the result I had expected because the resistance of the LDR I have drops to a level so low in strong light (somewhere in the 1,5K -2K range) that kills the photo transistor.
What do you guys think? Is there a better way to attach the LDR? Could it possibly work with some adjustments?