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!) _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user