toy rc car steering mod

I just got a sorta good used R/C toy car for $2, pretty nice for $2, and it works too full suspension, differential drive, two speed gearbox, and the steering mechanism uses a servo but this servo has all of it's control electronics out side the gearbox and on the single PCB the car uses. I just want to hack it so that I can turn it into a 2.4ghz (freebie RF modules from cypress yay) controlled "hobby grade" car with variable speed and such this gearbox has 7 wires sticking out of it, 2 are to power the motor itself, another 2 is 6v+ and ground ( - ), the last three is a potentiometer in the mega ohms range, higher than 20 mega ohms for sure, because it goes out of my multimeter's range if it's at neutral, but goes to about 10 mega ohms when the wheels are turned now, my datasheet for my PICmicro says in the A/D converter section "The maximum recommended impedance for analog sources is 2.5 kΩ" I can't really replace the potentiometer inside the gearbox, there is room for me to fit in my own, what would you do? *add parallel resistors to the potentiometer (probably easiest) *I can have a piece of paper shaded from light to dark and have a photo resistor tell me the darkness of the paper (might get jerky) *I can probably directly attach a potentiometer outside the gearbox directly in line with the output shaft (durability issues) *or I can put a gear on the shaft and have a potentiometer above the gearbox (I would need to cut my chassis to fit it) what sounds good?

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westfw10 years ago
You ask hard questions. Your gearbox has too many wires; why does it have +/-6V when it also has three leads for the pot AND motor power. Are you SURE the pot is multi-megohm? That seems unlikely based on typical servo circuitry and typical (lack of) power efficiency in toys. Can you take apart the gearbox to see what is really inside? Assuming you're correct, though, I would think that the easiest external solution would be an analog buffer circuit coming off the pot wiper; your PIC will be measuring voltage. not resistance...
frank26080115 (author)  westfw10 years ago
"why does it have +/-6V when it also has three leads for the pot AND motor power." i have no clue i am pretty sure that it is a multi megaohm trim pot, since when the power is off, i can get different results by spinning the shaft until it goes out of range, my crappy $20 multimeter can only do 20 megaohm or less it looks like multiple contact switches when i open it up, not much room for my own pot, unless i find a way to use those tiny pcb mounted ones, too much trouble, might as well go with the external pot I didn't really want to take it apart because i figure i didn't want to ruin it or anything i think i know how to hook up my own pot, the output is the wiper, and one end of the pot goes to the power and the other to the ground, correct?
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20 Mohms is a mighty large amount of resistance. It would be an atypical analog circuits that could differentiate any greater resistances reliably (certainly not a cheap device.)

That looks more like a binary encoder, probably 4-bit (16 position resolution, one wire vcc or gnd.) Possibly a resistance network outside that to convert to an analog value...
ya_boi_k gmoon10 years ago
I got a 1969 camaro I am trying 2 mod I all ready took it apart I am at a dead end I mod da light I want 2 do something 2 da motor do u have any advice
gmoon ya_boi_k10 years ago
We talkin' real 69 Chevy here, or RC car?