simple, cheap, wired controller?

I'm looking for a simple hand-held tethered controller that has two levers, each of which controls one line of voltage. Each voltages will
be fed, on the other end, through a DC motor, thus providing a very crude (but hopefully effective) wired remote control for a differential-drive bot.
I thought Tamiya might make something like this -- a lot of their kits have a wired controller -- but I haven't been able to find it. I want to buy just the controller, if possible (I dream of reselling this as part of my own kit, so I don't want to pay for extra parts I don't need). OWI also tends to have this sort of thing, but again I haven't been able to find just the controller.
I'm probably overlooking something -- any tips?
Thanks, - Joe
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Naturally, right after posting, I found this:
<http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item839535265&category=4 660>
...but this isn't proportional; it's just a pair of on/off switches in a box, for which this seller wants $20 plus $6 shipping, which seems a bit ridiculous.
I also managed to find the same controller, plus two others, on this site: <http://www.patronic.com/mechanical/mechanical.html But the descriptions are poor (and don't match the item titles), making it hard to tell what we've got. I suspect that these are also just on/off switches, and the 4-channel ones simply have two switches per joystick (up/down and side/side). So also not quite right.
So, if anybody knows of a similar two-channel controller with proportional control, I'd love to hear of it!
Thanks, - Joe
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Joe Strout wrote:

Well, that's a $11 item on TowerHobbies, and their regular shipping is just $3 and change.
Be that as it may, Tamiya's stuff is all a bit expensive. Has been with the dollar/Yen exchange rates. (And the fact that Made in Japan goods are just getting more expensive.)

Proprortional control requires some way to vary a voltage, and for a basic analog joystick that would be with a potentiometer. But there's a problem. Put (say) 6 volts through to a motor that's rated at 2 amps. How many watts is that going through the pot, remember that most pots are rated at 1/4 to 1/2 watt? Looks to me 12 watts, but check my Ohm's Law for me. As Jim Carrey says in The Mask, "Smokin'!"
You might be able to create a basic proportional speed controller for the average-sized toy DC motor using some rheostats, but these are big and bulky, and no analog joystick I've ever seen had these. I'm sure they're out there, if you look hard enough.
You need to get fancy if you don't want the smoke to get out of those pots. You need to provide an H-bridge with a PWM circuit (it can be a 555), or some other method of proportionally changing the speed of the motors. Changing the direction with a single pot is doable as well using one or more op-amps.
Here's a product that does exactly the above, using a single pot (you'd substitute the pot in the kit with the pot in your analog joystick). It's KIT166:
http://www.ozitronics.com/motor.html
The schematic they provide is fairly representative of the circuits available elsewhere on the Web that do the same thing.
-- Gordon
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I don't know how complex your kit will be, but suppose your robot already had electronic motor control under the direction of a microcontroller. It would be simplicity itself to use 2 microcontroller pins to read the voltage on a couple of pots. If you use an R/C joystick you've got your left/right forwards/back control in a single device. I'd like to use a dud PC joystick I have lying about here for just that purpose.
A pair of linear slide pots could replace an expensive joystick, but you'll still need some way of driving those motors. As Gordon said, an H bridge with PWM of some description.
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I found these two controlers marked 70102 and 70106 in the Tamiya on-line catalogue:
http://tamiya.com/english/kousaku/k_item/k_mini2.htm
Good Luck.

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