PhotoTransistors (Top or Bottom?)
October 9, 2012
I am following in Mr. Oddbot's footsteps and once again, walking the road of cheap, IR-LED based obstical sensors. Put simply, I can not seem to replicate the results of the work of others. Let's do some bullet points:
- Want to build simple, 2 or 3 phototransisor object sensor
- Have been folowing This Post both to the letter, and with many variations
- I am basically (no, actually) remaking the Dagu Compound Eye, but smaller and "on edge"
- For bench testing, I am using only one "set" of parts (1-4 IR LED's, 1 or 2 Phototransistors, one ADC)
- The Compound Eye works incredibly well, indoors it can catch my hand 10" (25cm) away.
- I have replicated the Compound eye's circuit exactly (each part matching the specs of the parts used on the C.E.) but I cannot get the ADC numbers like the Compound eye. My set-up has maybe 5" (13cm) range, with a total output fluctuation of maybe 1v. I.e. 2v -3v total range from nothing in front to white paper 3cm away.
My first question is why the NPN phototransistor is on the "top" of the circuit. Is there an advantage to this location? In every schematic I see, we have collector to 5v+ and the emitter to a pull-down and ADC. Every other NPN-thing I have ever done had the collector going to the ADC/Pull-up and the emitter going to ground. Why is this different?
What's up with the transistors? I used countless "general purpose" transistors in my tests (the tests above were done with no transistors) and I could see no effect in the output. I went through 2222's, 3904's, 4401's and a darlington chip. All worked (and I know we are talking a volt/current issue here as well as impedence issues) but I could see no change in output on the O-scope or ADC numbers by adding a transistor. The total range of voltage outputted was basically the same as a straight output of the phototransisor when using say, a 2n2222. It seemed to be performing the task equally well with or without this "amplifier" circuit.
To finish this off, I basically tried every configuration of this kind of sensor. Pull-ups, pull-downs, transistors, transistor-to-transisitor-to-transistor set-ups, pull-up/down resistors from 1k to 10M, etc. etc, etc. --All in all, I could not seem to do better than this 5" (13cm) range nor did I ever see the kind of numbers I can see from the compound eye. What the hell am I doing wrong, and how do I make a simple R/L or R/C/L IR obstical sensor? Sheesh! I used to know how to do this kind of stuff.
--You guys using magic parts or something, Oddbot?