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  #1  
Old 01-02-2003, 08:49 PM
actofgod actofgod is offline
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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 09:04 PM.
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  #2  
Old 01-02-2003, 08:50 PM
actofgod actofgod is offline
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view of installed potentiometer
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  #3  
Old 01-02-2003, 08:51 PM
actofgod actofgod is offline
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blurry closeup of installed potentiometer
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  #4  
Old 01-03-2003, 02:15 AM
GT-ahh GT-ahh is offline
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yeah i added a speed adjustment pot to my dual battery 2.2 bit... its good, u can wind it down to 1.0 equivalent...
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  #5  
Old 01-03-2003, 02:45 AM
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Quote:
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.
Just for reference, when you add a battery you don't increase both the amperage, and voltage. You get one or the other depending on if you wire them in series or paralell. If in series, you increase the voltage with the same amperage. If you wire them in paralell, you increase the amperage at the same voltage.

Chris
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  #6  
Old 01-03-2003, 11:17 AM
actofgod actofgod is offline
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Thanks, I was wondering about that part. In that case, the base number should be:

Resistance (ohms) = 2.4v / 0.275 amps (1 milliamp = 1000th of an amp) = 8.73

Since I couldn't find anything but 20 ohm and 100 ohm I guess 20 ohm is still the way to go. It seems to be working well.
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  #7  
Old 01-03-2003, 12:18 PM
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Off you go...
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  #8  
Old 01-03-2003, 09:37 PM
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actofgod, this is a really amazing Mod!!! Great work!
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  #9  
Old 01-03-2003, 11:17 PM
evileric evileric is offline
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Cool cool, now all we need is fully proportional steering and we'll all be happy now won't we! Good stuff, I think I'll have to try this for myself.
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  #10  
Old 01-04-2003, 03:06 AM
akura akura is offline
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Fantastic to see you got this working, wouldnt it be fantastic if you could control the voltage flow REMOTELY then! Now _that'd_ be a project to think about.
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  #11  
Old 01-04-2003, 04:59 PM
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SPEED SPEED is offline
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Just got done reading your posts, all i can say is WOW!! Thsi is a GREAT MOD to do!!!! So let me get this stright:

Red + battery wire connects to middle pin of POT
Left pin of POT connects to + connection for the battery wire on the PCB

ANd the right PIN of the POT is not used correct??? Just trying to clear things up because i may use this mod!

You said you used 20 OHM POT, can this be found at another electronic store such as Radio shack??? I seen a few POTs there.
Also, what type of POT would you have to use if you wanted to put this into a 1 cell bit??? Thanks!
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Last edited by SPEED; 01-04-2003 at 05:09 PM.
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  #12  
Old 01-04-2003, 05:28 PM
actofgod actofgod is offline
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Quote:
Originally posted by SPEED
Red + battery wire connects to middle pin of POT
Left pin of POT connects to + connection for the battery wire on the PCB, ANd the right PIN of the POT is not used correct???
That's correct. Nothing to it if you can solder a little.

Quote:

You said you used 20 OHM POT, can this be found at another electronic store such as Radio shack??? I seen a few POTs there.

The smallest rated pot I've found at radio shack is 1k, which is much too high for this project. I found the 100ohm 25 turn pot at Fry's Electronics; Allelectronics.com is the first place I found anything under 100ohms. If you buy from them I highly recommend getting the battery pack mentioned in the other post for an extra buck or so.

Quote:
Originally posted by akura
Fantastic to see you got this working, wouldnt it be fantastic if you could control the voltage flow REMOTELY then! Now _that'd_ be a project to think about.
What you'd need is an appropriate motorized potentiometer and an extra two buttons to control the motor that turns the pot. I haven't seen anything small enough though, and even if you used the booster controller's extra button to turn the motor one direction, you wouldn't be able to turn back the other direction except with some type of spring return mechanism possibly, which wouldn't allow you to lock in on one speed.

You could overcome the channel limitation by adding a second PCB of a different freq and merging the two remotes into one; however, with a second PCB and motorized pot you'd probably need a custom bus-type body due to the size. Hats off to anyone that accomplishes that, but I think I'd rather just buy a digiq, lol.

Thanks for the feedback, everyone. Post your results or questions if you try this mod.
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  #13  
Old 01-04-2003, 05:41 PM
actofgod actofgod is offline
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Quote:
Originally posted by SPEED
Also, what type of POT would you have to use if you wanted to put this into a 1 cell bit??? Thanks!
Plug in 1.2v for 2.4v:

Resistance (ohms) = 1.2v / 0.275 amps (1 milliamp = 1000th of an amp) = 4.36

To half the speed, you'd need 8.73ohms; to half it again you'd need 17.45ohms. Actually, these are the numbers I used by mistake when selecting a pot rating for the two cell mod the first time. A 20 ohm resistor would work just fine for both; you may just have a little smaller "usable speed range".

Incidentally, a 20ohm would work on a 3 cell mod too, it just wouldn't slow it down as much. You'd need 26.18 ohms to half the speed at 3.6v, so you'd be able to tune it from a little over half speed to full 3.6v speed. Still a useful addition.

I think this mod is most useful for two or more cells, though, since going below 1.2v not only sacrifices torque and acceleration but if you go low enough could affect your steering coils' effectiveness and the ability of the car to receive the signal from the transmitter. A slower engine is probably the best way to slow down a one cell car, unless you're trying to race on a single paper plate.
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  #14  
Old 01-04-2003, 06:24 PM
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SPEED SPEED is offline
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Hey thanks for the info!!! I may just go ahead and oredr the things i need and do these mods!
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  #15  
Old 01-05-2003, 01:15 AM
mrcdemon
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this mod looks pretty nice... i think i might have to try it
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