[chbot] Lego Minstorms nxt open information

[QtuTrains]

On 3/09/2008, at 8:32 , Richard Jones wrote:

> I've attached the mindstorms light sensor schematic and I've not  
> made much
> sense of it yet, anyone understand how it works? It appears to use  
> current
> mirrors and a 3.1ms time constant. I'm not even certain whether the  
> output
> to the nxt brick is current or voltage.

The wild speculation of one who understands little and fumbles in the  
dark might suggest:

Q1A & Q1B appear to be a PNP current mirror, BUT R2 damages that  
function.
As light increases, current in Q1A increases, increasing Vbe. This  
increases Ic in Q1B but this increases voltdrop on R2, thus reducing  
its Vbe relative to Vbe on Q1A. My guess is this has a logarithmic  
effect - thus increasing dynamic range for the sensor.

Q2A and Q2B appear to be another (NPN) current mirror, but with  
emitter resistors in both sides. R5/C2 will act as a filter (changes  
in Q1B/Q2A Ic will mostly be absorbed by C2, thus reducing  
instantaneous current change in Q2B. R5/R6 would appear to give 12x  
current gain and when taken in conjunction with the 10K pullup inside  
the NXT they appear to provide some negative feedback effect  
(otherwise R4 would saturate Q2B).

Q4 drives a light emitter and is controlled by the ARM and one could  
speculate that it might be driven at some frequency (perhaps 1KHz) so  
that the received analogue signal can be filtered (in software) to  
remove the effect of ambient lighting.

It looks like a nice circuit to give to a simulator package, then  
perhaps we can understand it better.

Or maybe someone here actually understands these things...

Howard
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