Re: splitting voltage


default wrote:
On 22 Aug 2006 22:55:50 -0700, "randomname"
<randomname12345@xxxxxxxxx> wrote:


- -
| |
resistor 555
| | [pin 3]
motor LEDs
| |
+ +

It is going to take more information. Regarding the motor - just a
resistor and motor will work if you know how much current the motor
requires it is easy to calculate the value of the resistor. Half the
voltage will be wasted as heat in the resistor.

An alternative to wasting the power (and it is likely the capacity of
4 AA cells won't come close to the run time you get with two C cells)
is to chop the power to the motor. You build another 555 astable
multivibrator with a 50% duty cycle. It runs too fast for the motor
so you won't notice the speed changing, yet you only use half the
power. Similar to what you are doing to keep the LEDs from burning
out. Another advantage - if you make it variable duty cycle you can
control the pump and maybe use less battery power.

Search for: Pulse Width Modulated (or pwm) DC motor speed controller,
lot of circuits on the net

when I connect the motor, the LEDs will flicker much faster and dont
hold a steady brightness. why would this cause the 555 timer to
oscillate faster and sporadically? im confused.

-sam
You don't show your schematic but I'd expect to see that with a
battery supply and a motor load against a 555

The battery voltage changes with the motor load. If the voltage to
the 555 power supply changes it is likely to trigger later or earlier
in its normal timing cycle - you need to supply the led timer with a
seady source of power.

One cheap and easy way to do it would be to add a rectifier diode to
the 555 power input pin (pin 8 and 4 on the astable). Its purpose is
to only allow current to flow into the 555 not back to the motor. You
put a relatively large capacitor on the supply and ground on the 555
side of the diode. A few thousands of microfarads at 6 volts.

Something you should already have is a bypass capacitor on pin 5 to
ground (that helps keep the voltage steady on the internal divider
string on the 555 and keeps the trigger voltage from wandering around.
I think the application notes call for .01 microfarad, but with a slow
oscillator and big problems I'd try more - up to a microfarad.(and
this may have a deleterious effect on the start up flashing of the
LEDs so the timing may be off until the cap charges). 01/.1 won't be
noticed and may help some.

The problem is you motor is sucking too much power from the batteries
and represents a varying load (with a brush type motor especailly).
The 555 really wants a clean steady source of power, so either the
power supply has to be stiffer (large caps, large batteries) or the
motor load has to be lower, or both.

The inductance in the motor and brushes create electrical noise as
each commutator segment swings by the brush. It wouldl be a very good
idea to put a ceramic disk capacitor as close to the motor brushes as
possible (right at the housing to the motor and across its power
leads) .1uf 50 volts ought to be goood. Noise can trigger a 555.

Other tricks - good physical layout. Run a seperate wire from the 555
power and ground all the way back to the battery pack. Do the same
thing with the motor - the wire should not carry the current for both
the motor and 555, that's asking for trouble.

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Thank you so much for this info.

I've drawn up a schematic...

http://img217.imageshack.us/my.php?image=circuitcs2.jpg

I've changed the supply to 2x C batteries, and added a diode in
parallel to the motor.

Do you think this would work?

.

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