Re: controlling dc motor direction (help a newbie, please)


theneonlobster@xxxxxxxxx wrote:
wow. let me first say thank you for all the responses and help. i'm
already gaining a better understanding of what i'm doing.

chris, you assumed right, except i don't want to vary the speed of the
motor. i'm leaning towards running it at 3vdc/0.7A, but i might go
with 1.5v (or somewhere in between). from my understanding of PWM, i
won't be needing one (it's for speed control, right?)

the motor (and direction) would be manually activated by pressing down
on a microswitch. as the microswitch won't be depressed throughout the
entire process (those 5 seconds), i need the timer to simply be
activated by the switch.

i haven't read many schematics, yet, but theese are making some sense.
i thought i would need a 556 timer, (one 555 for each switch, to send
different signals to the dpdt for cw/ccw).

i could probably say more, ask more questions, but i'm going to read
some more about timers.

thanks again!
david

Hi, David. I guess you're sold on the PWM concept. The series
resistor (switchable with SW3 in the above ASCII schematic) does waste
power. You can do that with a 556 like this (view in fixed font or M$
Notepad):
|
| .--------------------.
| | |
| VCC | VCC |
| + | + |
| | | | |
| .-. VCC VCC | .-. VCC |
| | | + + | | | + |
| | | 8| 4| | | | 8| 4|
| '-' .---o----o---. | '-' .---o----o---.
| | | RST | | | | RST | SW3
| >--)-----o | | o-----o |
| | | | | | | |
| | | |3 | .-. | |3 1.5V To
| o-----o o----o | | | o-----o Q1
| | | 555 | | | | | 555 | __--o->
| | | | | '-' | | .--o
| o-----o | | | | | | 3V
| | | | | o-----o | |
| | | | | | | | |
| | | | | o-----o | |
| | | | | | | | |
| --- '---o----o---' | --- '---o----o---' |
| --- 1| 5 | --- 1| 5 |
| | === | | === |
| === GND | === GND |
| GND | GND |
| '---------------------------'
|
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
Your switch gives you the option of the transistor being on all the
time (the 3V position, Q1 being driven by the 1st 555) or about half
the time (the 1.5V position, Q1 being driven by the second 555). You
use the RST pin on the 555 (pin 4) to only have the 2nd 555 on when the
1st one is on (being driven by SW1 or SW2).

Just insert this into the circuit where the one 555 is in the original
circuit, and remove the series resistor R and SW3 in the first circuit,
and you're good to go. You've reduced the current draw by not burning
up 1/3 the drive power with a series resistor when you want to drive
the motor with 1.5V.

I hope this is clear -- feel free to post again if it's not.

Either way, you've got a good working circuit that isn't too profligate
with power, and should work reliably for motors up to an amp (if you
use the 4.5V 3 "D" batteries and the darlington transistor. If you
would grace us with more information, we could possibly save one of
those "D" batteries -- like what's the current draw of the motor? But
this will also add to the circuit complexity (possibly leave the 2nd
555 on all the time, and use it as a charge pump to get a gate drive
voltage for a power MOSFET) and expense.

Sometimes it's best to just git 'r done. Your call.

Cheers
Chris

.

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