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Old 01-02-2003, 07:49 PM
actofgod actofgod is offline
Bit Surgeon
 
Join Date: Sep 2002
Posts: 216
The Infinite Speed Potentiometer Mod (tm)

Single Speed? Two Speed? How about a fully adjustable throttle for your bit?

This is the completion of my Rechargeable Li-ion adjustable voltage mod thread posted 12-2-02. Thanks to Akura and Ovgron for the responses on that one.

I started off with my Layman's Dual Cell Mod (tm) for the 2.4v. I could have fit 3.6v or more in there, but 2.4v is already pushing the limits of control with a fast engine IMHO. Lithium rechargeables are too hard to recharge for now anyway. This mod would work with a single cell, but I've never felt the need to "dial down" the speed of a single cell bit.

I have tried many different potentiometers (see pics) from various sources, and I'd like to save anyone else working on this project (like LEE50GURU , I just noticed) from buying any more potentiometers than they need to.

Akura was right about the necessary resistance being less than 30 ohms. To get an idea, I started out rearranging Ohm's Law as:

Resistance (ohms) = Voltage (volts) / Current (amps)

I referred to Namuna's multimeter readings for the bit char-g; he found that a single cell bit puts out 275ma, so I doubled that for a dual cell bit and got 550ma. The current per cell is 1.2v, so I used 2.4v for the dual cell.

Resistance (ohms) = 2.4v / 0.550 amps (1 milliamp = 1000th of an amp) = 4.36

Using 4.36 as a starting point, I calculated that to half the current going to the motor you would have to double the resistance. I wanted the minimum speed to be about 1/2 the speed of a single cell bit (that's 1/4 the speed of a dual cell bit), so I'd need to double the resistance twice. That gave me 4.36 * 4 = 17.45 ohms. If my math is off, someone please correct me.

Finding a resistor in that range was tricky. At first I had forgotten that the result was in Ohms while most resistors are in Kilo-Ohms, so I got a 20k resistor. I thought my calculations were wrong, so I just started trying some at random like 10k, 5k, and 1k. When even the 1k didn't work, I realized my mistake and found a 100 Ohm 25-turn potentiometer and a single turn 20 Ohm pot. While the former would allow more precision (each full revolution changes the resistance by 4 Ohms, allowing for 5 complete revolutions of control to get to 20 Ohms), I decided I didn't really need THAT much precision. I opted for the single turn 20 Ohm potentiometer, and it gives me all the control I need.

Wiring was very simple. Current is interrupted from the batteries to the entire PCB rather than just from the PCB to the motor. I desoldered the red +V wire from the PCB, leaving the other end attached to the positive battery terminal. That free end attaches to the center (wiper) pin of the potentiometer. I had to solder on a little bit of extra wire to make it long enough to reach around. I took another piece of wire and soldered to the top of the +V connection on the PCB and ran it to the left pin on the pot. Either one will work, but I wanted the speed to increase as I turned the pot clockwise and decrease as I turned counterclockwise, so in the case of this pot it was the left pin. I drilled one tiny hole for the left pin and put the center pin through the hole below where the antenna clip is (genuine bit char-g pcb cover; clones may not have this hole) and bent the right pin up out of the way. I ran the wires from the pcb up through the holes and soldered them against the exposed part of the pins, then superglued the pot to the pcb cover, leaving plenty of slack in the wires for when I need to remove the whole cover to swap batteries, etc. The Fairlady body fits perfectly on top, but since it is snug you may experience some wires grounding against the pcb. Use a tiny strip of clear plastic tape across the PCB and under the added wires if you have that problem.

Ok, enough talking.. I have pics and even a sound clip for you all. To hear the sound clip (mp3) go to http://users3.ev1.net/~actofgod/potentiometer.mp3

What you're hearing is my Fairlady 350Z getting 3v 500mah from the charger base I use. I wanted the motor (a cannon ball clone motor.. faster and torquier than a bit 2.6) to be loud enough to hear the increase and decrease in RPM. This is with orange gears and back axle in place. After "revving up" and back down, I punched the boost button and revved it again. Now I can adjust both speeds to run as fast as possible on any given track, depending on how many curves and straightaways it has.

If you'd like to do this mod, you can purchase a 20 Ohm resistor like the one I used at allelectronics.com, CAT# SVP-20

Let me know if you find this post useful or have any questions. Happy modding!

Last edited by actofgod; 01-02-2003 at 08:04 PM.
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