Proportional Controller EMULATOR

[dustn]

Sweet... This would be great for my latest project idea... Clone donor, full-length fiberglass chassis (not retaining any of the clone chassis), lexan slot car body (anybody know where to find these?), and either 2 1/3aaa or 3 of the cells used in the layman mod. Proportional control will be a must for getting the most out of it.

[Atomic77]

Try here for Lexan HO Slot bodies. Good luck with the mod, be sure to show some pics when you're done.

[dustn]

AHHH thank you sooo much. I'm looking right now.

[ejaf]

ph2t...

What is your source for the dual-gang potentiometers you are talking about? I have been searching for these, and am trying a different approach to "pushing the buttons" on the controller (using a Low power 556 chip).

Also, I had found an analog joystick with what I think is the correct pot on a nintendo 64 control pad, but it is dual-axis, and I was hoping to find tiny single axis joysticks, with no luck.

Any ideas?

[ph2t]

hey ejaf..

here's a link to an aussie supplier, Jaycar

I also tried using a 555 initialy, in astable mode as an oscillator. I tried using a pot on that to vary the duty cylce of the pulse, but I found that I could never achieve greater than %50 duty cylce on the square wave. ( The duty cylce equation backs this up, see page 8 of the 555 timer datasheet )
This is why i switched to a PWM circuit.

Cheers,

ph2t.

[dustn]

Hey ph2t, have you finalized anything? If you need some pots, i could send them to you. They are from an OLD airtronics 4ch plane radio.

[ph2t]

That would be great dustn, the only thing is I live in Australia!

cheers,

ph2t.

[ejaf]

ph2t...

Thanks for that bit of info. I think I may have bought useless parts :-(

One question though. The basic astable circuit that I was looking at has the capicitor between pins 1 and 2, so the capacitor charges through R1 and R2, but discharges through R2.

Also,

formula given for t1 (time between pulses)
t1 = .693(R1+R2) * C1

fromula given for t2(pulse length)
t2 = .693(R2) * C1

Therefore, I was think of using a very high value for C1, with a lower value for R2, and using a 10K pot, which, I think, may increase the duty cycle.

Take that with a MAJOR grain of salt, since I only started looking at this a while ago, and I am NOT an EE by any stretch of the imagination.

If I have a chance, I will post the picture of the book I am looking at (from RS - Timer, Op Amp & Optoelectric Circuits by Forrest Mims), if anyone is interested.

Wait...here it is attached. Hope this isn't copyright infringement:

[ph2t]

ahhhh ejaf, I used to have that book! Also the one for TTL logic circuits. They were great books.

What you're doing is exactly what I set out to do initially. After experimenting for a while I found that I could never achieve a duty cycle greater than %50.

No matter what value combination you use for R1 / R2, when the pot is turned to zero ohms you'll get %50 duty. As the pot resistance increases (10K, 100K, 1M, etc...) the duty approaches zero.

The duty cycle equation is as follows..

R2
D = -------------
R1 + 2*R2

when the pot is a max resistance (eg, R1=10K)

1
D = ------------ (assuming R1 value of 1K)
10 + 2

D = 0.0833 = 8.33% (pretty much zero)

when the pot is at zero resistance (eg, R1=0)

1
D = -----------
0 + 2

D= 0.5 = %50 duty.


Bugger!

[ph2t]

Also, here's the latest version of the mod. I have now both channels LEFT and RIGHT. Don't take the resistor values as kosher, they are no way final at the moment.

ph2t.

[ejaf]

Thanks for your insight...being a newbie to electronics, I might try it just to see the results anyway, but I will study that formula a little closer.

Also, I will definitely look at the one you are proposing a little harder now. ;-)

[ph2t]

Try increasing the frequency of the oscillator. I noticed
stepper-motor like results at low freqs, increasing this made the acceleration smoother. Even with the %50 duty cycle, at a high enough frequency it might just work.....

[ejaf]

Thanks...worth a try, just for fun.

FYI... found this, might come in handy for someone interested:

http://www.unn.ac.uk/~epje2/robotinf...eedAndDir.html

That page also mentions that there are PWM IC's, but the voltage requirements are too high for what I'm looking for.

Also, I guess the only way to truly test this would be with an o-scope, correct?

Also, this page mentions getting a 98% duty cycle out of a 556, and the circuit is totally different than the one I was looking at:

http://www.m.case.btinternet.co.uk/h...enerators.html

One other question...what is the maximum output current needed to actually push into the transmitter IC?

[ph2t]

You're right ejaf, a CRO (cathode ray oscilliscope) would be the best way to test and see the shape of the waveform you're putting out.

As for current into the TX IC. Well when the button is pushed, the pin on the TX goes to ground. So the output of the PWM needs only to go to ground to turn the IC "on" so to speak.

I was thinking of a transistor-coupled circuit using a pullup resistor on the IC pin but in the end I didn't need it. I just connected the output of the PWM to the input pin on the IC.

Currently the controller is running at approx 10Hz for the steeering. I'm working on the speed control as well, but this circuit will be running at a higher speed (possibly > 30Hz??) to reduce the "stepping" effects of motors at this speed and duty cycle.

ph2t.

[ph2t]

Please forgive the quality of the pics, I know they are crap but you can see the birdsnest I've created trying to get this thing to work!!