ohm's law, and 555 question

[Harry Bissell]

Hi Ed...

Eduardo Saponara wrote:

> to calculate the resistance of a circuit,  is it only the resistors and
> potentiometers values that I need to add, or do LED's and other things have
> to be added in somehow (how do you get their values).

The best way to find the power consumption is to account for all currents
flowing everywhere in the circuit. Usually the power source has one current
out, and one in  (you can consider current flow from plus to minus, or minus to

plus... its the absolute value you are interested in).  A bipolar supply will
make it
a little more complex.

Also, you need to account for the static (quiescent) and dynamic power draw
of any IC's in the circuit. This can range from very small (uA) for CMOS at low

clock speeds, to quite high (mA) for power hungry op-amps like the 5532 or
5534.
The dynamic losses depend on the actual waveforms of the circuits... very hard
to
calculate by hand. Take a good guess.

The LED is an unusual case.  You should consider it as a voltage drop.  So to
figure
a single LED, with (lets say) a 1K resistor, running on a 15V supply...
subtract the voltage drop of the LED (usually 1.7 - 2.1V) lets say... 2V
from the supply. That leaves 13V... across a 1K resistor (use ohms law) =
13mA.  If you want to get really fancy in the SEQ8 schematic... you figure out
the transistor voltage drop, and subtract that also. Usually that is about .4V
if the
transistor is saturated (turned fully on in the common emitter config as it is
shown...)

If the tranny is NOT saturated then you need to figure out the current flow in
the
tranny... and that will give you the rest of the answers.

OTOH most DIY'ers don't need that kind of accuracy.  You could use your 1A
supply
and a 1A fuse. Plug in. Blow up ???  No... must be less than 1A.  Now measure
with
a milliamp meter... actual current is found.  This can vary some as things turn
on and off.

For instance... if you have 3 LED circuits (binary) the LED current will vary
from 0mA
(all lamps off - 12% of the time) to 13mA (38% of the time) to 26mA (38%) to
39mA (12%)...  pretty big change ain't it ???

This is why an adequate supply is usually larger than you might  think you
need...  and why
its a good idea to have decoupling caps near those parts that draw varying
amounts of current...

H^) harry

Look for Data Sheets and Applications Notes from manufacturers for 555 tips. DO
not
use pin 5 for controlling the frequency. This is a loser!  You need to put that
pot feeding
from the positive supply to the timing cap...  this will have the kind of wide
range you would need. Pin 5 will only change frequency maybe 5:1 ratio.  Look
for "astable multivibrator" in those data sheets.  Try for National
Semiconductor, Texas Instruments,
Signetics (are they still in business?), etc.

The 7555 might be a better choice. MUCH less power hungry.

Yes you can get less than 50% duty. Why do you want a short pulse? (there are
many
ways to get this effect...

H^)  harry

>
>
> I have been looking at 555 astable circuits as a clock source,  and I am
> wondering what V+ Potentiometer  needs to go in on pin 5 to control tempo.
> also, anyone know how to get a pulse from it (high time shorter than low
> time) ?
>
> all help is greatly appreciated..
>
> thanks