Re: splitting voltage

On 23 Aug 2006 16:23:56 -0700, "randomname"
<randomname12345@xxxxxxxxx> wrote:

OK everyone, thanks so much for your input. I've put in so many hours
to this project and just want to see it finished. You all are a great
help.



So as I understand, here's what needs to be modified from the
schematics (please let me know if anything is wrong):

- I'm going to keep the power supply at 4x AA batteries, since the 555s
require 4.5v.
- I will add .01uF caps on each 555 going from pin 5 to ground (I still
don't get what this does.. won't the capacitor just charge up and then
sit there charged?)
Yes it just sits there charged. Its function is internal to the 555
and it stabilizes the control voltage to the internal comparators
(hopefully preventing them from changing - more useful in electrical
noise rejection then wholesale power supply voltage droop).

You should really read up on the 555 so you understand exactly how it
achieves oscillation - it is hard to get this stuff to work by just
wiring and hoping.

- The diode parallel to the motor will be an n4001, (is this the
correct diode?)

That is good enough for most purposes

- Change the resistor values on the 555 timer that controls the motor
to represent about 40% duty cycle at 100Hz [I'll experiment with this
to find correct values]. Read below to see why 40%.



And here are things I'm not sure about:

- Adding a 100uH inductor where? From + rail to the joint just above
the motor?
Noise rejection components do the most good when physically close to
the source of the noise - otherwise the length of wire between the
filters and noise source act as antenna and radiate noise.

- Adding diodes to the 555 timers. From + rail to pin 1? n4001?
That will help if used with a large value electrolytic cap to
stabilize the voltage to the 555. A cap between pin one and pin
eight. That will see that the 555 has a stable power supply to pull
from when the rest of the circuit is noisy.

- Connecting pin 4 to +. Whats the advantage?

It is just done that way in the typical oscillator application of the
555. It is to reset the 555 in other timing applications - for
instance you have a 5 minute delay but some other requirement demands
that the 555 stop its timing in mid cycle then that pin is grounded.
It should be tied to the positive supply rail to ensure that it isn't
reset - not tied to anything is just not good practice and many 555's
won't work with it floating. Tie pin four and eight together and to
the positive supply.



More about the 3v motor: When connected purely to 2 C batteries (as it
was designed to do), it pumps a small stream of water about a foot up.
However, this stream of water is too much for what I need it for... my
specs call for an even tinier stream of water. Ideally, I'd like to be
able to control how much water is flowing, which is why I'm opting for
the PWM solution, so I can effectively control the voltage, and as a
bonus get to use another potentiometer that controls something. Since
3v is too much, I'd like to try around 2 - 2.8 volts, just enough to
barely pump the water up. So I'm thinking roughly 40% duty cycle should
do it.

OK

I just purchased an assortment of diodes and a couple .01uF caps, so
I'm all set except for the inductor, which I'm hoping won't be needed
since I don't understand where to put it and what it would do. I will
put together everything tonight and post results.

An inductor opposes the rapid change of voltage in this application.
When current flows through an inductor it builds up a magnetic field
around it, when the current tries to increase or decrease the inductor
opposes these changes by taking or adding energy to the magnetic
field. Electrical noise, like what may be causing problems in your
circuit is rapidly changing voltage/current imposed on a power supply
wire where there should just be a steady voltage - the noise plays
havoc with timing.


Again, thanks a lot everyone (especially Default)

-Sam

Let us know how you do, and good luck.

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