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Basic stamp and dc motor driver SN754410 — Parallax Forums

Basic stamp and dc motor driver SN754410

Duc LeDuc Le Posts: 8
edited 2005-12-01 05:13 in BASIC Stamp
Hi,

Iam using the Basic Stamp 2px (with the Board of Education) with the SN754410 to control a DC motor to do the following:

1.· able to vary the speed with the my trimpot
2.· switch the rotation direction with a mechanical switch but I am not sure which switch and how to connect it to the circuit.· Any suggestion?

I also attached a schematic for clarification.· I really appreciate your help.
Thanks
Duc
408-623-1913
motor-exercise.jpg
720 x 540 - 17K

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-11-22 21:17
    Hello,

    ·· If that's a potentiometer you have connected there I hope it can handle the current/power of that motor.· Typically they cannot.· You wouldn't normally connect a potentiometer in this manner.· It would actually be a rheostat.· Even this isn't the typical way to control the motor.·


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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com


    Post Edited (Chris Savage (Parallax)) : 11/22/2005 9:21:18 PM GMT
  • Duc LeDuc Le Posts: 8
    edited 2005-11-22 21:34
    Hi Chris,
    Thanks for replying. What would be the right way to connect rheostat or potentiometer?
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-11-23 15:36
    Well a potentiometer would be on the control side of things...Not the load side.· But then, that would typically be used in to alter PWM or voltage going to the motor.· Either way it won't work the way you have it.· It really depends on whether you planned on using PWM or varying voltage to control motor speed.· This would also need to work for reverse conditions, making PWM and an H-Bridge the better choice.· I didn't look up the chip you're using to see what it was.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Duc LeDuc Le Posts: 8
    edited 2005-11-28 21:00
    Hi Chris,
    I have modified the design a bit and attached is a new schematic. Basically, I still have the mechanical switch to control the direction and RC timer (with variable resistor) to alter the speed. I am still unclear how to use the RC timer and convert that into PWM to control the speed. I did a little test on my RC timer with the following code:
    ' {$STAMP BS2px}
    ' {$PBASIC 2.5}
    time VAR Word
    DO
    HIGH 7
    PAUSE 100
    RCTIME 7, 1, time
    DEBUG HOME, "time = ", DEC5 time
    LOOP

    I got some number from 0 to 1,000. Now how do I use this number to convert that to PWM? Thanks very much.
    motor-exercise.jpg
    720 x 540 - 33K
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-11-28 22:26
    Hello,

    ·· After looking up this device I realized it is similar to the L293 and thus it is merely a Push-Pull driver.· You can actually get motor direction control from this chip.· But for speed control you will need a PWMPAL or similar device to drive the inputs properly.· Now, to use the RCTIME function you have for control, you will need to find the range of values you send the driver chip from the PWMPAL or whatever device you use, and scale the range accordingly.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Duc LeDuc Le Posts: 8
    edited 2005-11-29 23:45
    Yes, I was able to figure the the values mean in term of time.· Now, how do I use that to control the speed?· I guess I'd set ENABLE pin on the motor driver to be equal the PIN 7 of the RCTIME?· and for direction PIN4=HIGH=PIN1 and PIN2=0.....vice versa if I want to rotate the motor other direction?

    Thanks
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-11-30 02:39
    I'm unclear what will be generating the PWM to vary the motor speed...Are you using PWMPAL?

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Duc LeDuc Le Posts: 8
    edited 2005-11-30 05:34
    I am using the RCTIME (RC timer circuit)

    checkPot:
    PWM pin, duty, cycles
    HIGH 1 'put current out to the capacitor
    PAUSE 1 ············ 'hold a millisecond
    RCTIME 1,1, potVar 'RCTIME to get a value for potVar
    byteVar = (potVar/10000) * 255 'create byteVar, 0-255, that varies with potVar
    PWM 9, byteVar, 10 'pulse-width modulate pin 9, voltage = byteVar, for 10 milliseconds
    GOTO checkPot



    2.· I am having some problems with the running the program.· Error message: NO Basic Stamp Found.· I tried to debug the USB driver and it seemed to be OK.· I restart my computer and it didn't help.



    Thanks very much for following through
  • Duc LeDuc Le Posts: 8
    edited 2005-11-30 06:43
    It's the battery is low that's why i couldn't run the program
  • Duc LeDuc Le Posts: 8
    edited 2005-11-30 07:35
    I have a few more question:
    1. I am using the RCTIME (RC timer circuit)

    checkPot:
    PWM pin, duty, cycles
    HIGH 1 'put current out to the capacitor
    PAUSE 1 'hold a millisecond
    RCTIME 1,1, potVar 'RCTIME to get a value for potVar
    byteVar = (potVar/10000) * 255 'create byteVar, 0-255, that varies with potVar
    PWM 9, byteVar, 10 'pulse-width modulate pin 9, voltage = byteVar, for 10 milliseconds
    GOTO checkPot

    2. If I set a pin HIGH, that' means I get +5V out. Then I stick one wire from the DC motor to that same pin and the other to VSS, but somehow the motor doesnt' run. what does that mean?
  • Bruce BatesBruce Bates Posts: 3,045
    edited 2005-11-30 07:51
    Duc -

    The Basic Stamp pins were never intended to drive anything like a motor load. The total capacity of any single Stamp pin is in the neighborhood of 25-30 mA.

    Now, as to what this means:

    1. The Basic Stamp is incapable of providing the necessary current to drive your motor directly. This design limitation is also true of most micro-controllers and micro-processors.

    2. If the Stamp pin was subjected to such an overload, for any length of time, without a current limiting resistor, that Stamp pin may have been BLOWN OUT, or will be soon!

    3. Either use a motor driver chip, or at least a drive transister to take the load of the motor. The Stamp pin drives the transistor (very little current required) and the transistor controls the motor.

    Regards,

    Bruce Bates
  • Duc LeDuc Le Posts: 8
    edited 2005-12-01 05:13
    I am trying to get my RC timer to work as a PWM to control the speed and switching directions on the motor. Following is my code. I got the switching directions work but not the PWM...can you help???

    ' {$STAMP BS2px}
    ' {$PBASIC 2.5}
    potVar VAR Word
    Speed VAR Word
    main:
    HIGH 1 'put current out to the capacitor
    RCTIME 1,1, potVar 'RCTIME to get a value for potVar
    Speed = (potVar/1000) * 255 'create byteVar, 0-255, that varies w/ potVar
    PWM 9, Speed, 70 'pwm pin 9, voltage = byteVar, for 10 milliseconds
    IN15 = Speed
    DO
    IF IN14=1 THEN 'Spin the motor cw
    HIGH 12
    LOW 9
    ELSEIF IN14=0 THEN 'Spin the motor ccw
    LOW 12
    HIGH 9
    ENDIF
    LOOP
    GOTO main
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