Re: gEDA-user: NE 555 and simulation issue

[Peter Clifton <pcjc2@xxxxxxxxx>]

On Tue, 2010-03-02 at 17:00 +0530, W.H. Kalpa Pathum wrote:
> Hi,
> 
> I'm an electrical engineering student and I'm new to gEDA. I'm given a
> project and I've got several circuits. I want to simulate rather than
> soldering them all actually so that I can choose the best circuit. I
> use Fedora 12 and I have installed ng-spice also.
> 
> My circuit has a NE555 timer and I couldn't find a symbol for NE555 in
> gEDA Schemetic Editor. So how can I find a circuit.

I'm sure there will be one available somewhere - if not, you will have
to create the symbol. (Creating symbols is a normal part of electronic
design entry).

Searching "555" in gschem's schematic window found an "LM555", which
appears to have the same pinout. For future reference, you will find a
useful resource of additional symbols here: www.gedasymbols.org


As for a SPICE model (or any other simulator model), I've no idea where
you would find that. Typically, these kinds of simple circuits ought to
be designed / chosen using basic engineering approximations. The
data-sheets tell you enough about how the device operates to be able to
calculate the time periods it switches etc., calculate currents in
components.

Simulation should be a second stage - verification (if used at all), not
a primary design tool. What kind of performance differences are you
hoping to evaluate between the various circuits?

<offtopic>
So says the person looking for software to perform 3D finite volume /
MoM, in a transient simulation for marine wave / float interaction -
because I _can't_ build a huge experiment... we had this: 
http://www.tridentenergy.co.uk but it capsized :(
</offtopic>


> Next is, I designed a simple circuit with a power source, LED and a
> resistor. I wanted to simulate the circuit and see the voltage across
> the resistor. But I couldn't find how to do it. So if you can provide
> me with a simple step by step guide on how to do this that would be
> much appreciated.

Unless you need the _exact_ operating point, just calculate it.. it is
very easy. (And you'll probably find it difficult to get an LED spice
model, and / or match the parameters to your particular device).

I = (Vcc - Vf) / R

Which, e.g. might be:

Lets assume your LED has Fwd voltage drop of 2.7V (check the
data-sheet), and wants a forward current of 15mA.

The voltage across the LED is roughly fixed when the appropriate current
is flowing, so subtract that from the supply voltage and get the voltage
across the resistor. I'll assume your power source has negligible
internal impedance, but if not - that needs to account for a part of R
calculated below.

15mA = (5V - 2.7V) / R
15e-3A = 2.3 / R
R = 153 Ohms

Picking a near preferred value, let R=150 Ohms

Current will then be (5-2.7) / 150 = 15.3 mA


Seriously - simulating for things like this is not going to be the best
way to design circuits.. physical variation between parts, and
discrepancies between the model and reality, plus limited choices of
real-world resistor values will mean it is pretty pointless trying to
get any more accurate than what I've just calculated above.


Best wishes,

-- 
Peter Clifton

Electrical Engineering Division,
Engineering Department,
University of Cambridge,
9, JJ Thomson Avenue,
Cambridge
CB3 0FA

Tel: +44 (0)7729 980173 - (No signal in the lab!)



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