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I am trying to simulate a monostable multivibrator in GnuCap. I have the following simple netlist (with arbitrary, probably non-functional component values but that's not the issue):

C1 ( 1 2 )  100.u
Q1 ( 1 4 0 )  BC548 NA( 1.)
Q2 ( 3 2 0 )  BC548 NA( 1.)
R1 ( +9v 1 )  1.K
R2 ( +9v 2 )  1.K
R3 ( +9v 3 )  1.K
R4 ( 4 3 )  1.K
.model BC548 npn ( <snip> )

I'm setting the model of the transistor with build followed by .model BC548 NPN which seems to work nicely.

The thing I don't understand is how to configure the value of the +9V source? When I run the simulation the voltage seems to be (essentially) zero:

gnucap> tran 0 0.01 0.001                                                                               
#Time       v(+9v)     v(1)       v(2)       v(3)       v(4)      
 0.         19.517u    19.517u    19.517u    19.517u    19.517u   
 0.001     -10.f       14.f      -34.f      -10.f      -10.f      
 0.002     -10.f       14.f      -34.f      -10.f      -10.f      
 0.003     -10.f       13.f      -34.f      -10.f      -10.f      
 0.004     -10.f       13.f      -33.f      -10.f      -10.f      
 0.005     -10.f       13.f      -33.f      -10.f      -10.f      
 0.006     -10.f       13.f      -33.f      -10.f      -10.f      
 0.007     -10.f       13.f      -33.f      -10.f      -10.f      
 0.008     -10.f       13.f      -33.f      -10.f      -10.f      
 0.009     -10.f       13.f      -33.f      -10.f      -10.f      
 0.01      -10.f       13.f      -32.f      -10.f      -10.f

I've had a read of the documentation but it seems aimed at more experienced engineers with familiarity with electronic simulation. I'm trying to refresh the basic electronics I learned at University, and this is my first time with electronic simulation.

If someone could please let me know what the step is I'm missing, I'd greatly appreciate it.

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1 Answer

up vote 1 down vote accepted

You don't have any excitation in that circuit, do you? With no excitation, all node voltages will be zero.

I'm not familiar with Gnucap, but is the "+9v" a special node? If not, you need to add an explicit voltage source (with a value of 9 V) between your "+9v" node and your ground. Add something like this

Vsupply +9v 0 9

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That did the trick - build followed by Vsupply +9v 0 9. Now to go and read the documentation so I understand why :-) – Duncan Bayne Apr 24 '12 at 12:01
@DuncanBayne You can go directly to page 93, section 3.22 of this pdf gnu.org/software/gnucap/gnucap-man.pdf – Telaclavo Apr 24 '12 at 12:04
@DuncanBayne the 'why' is because the circuit doesn't know what your +9v symbol means: it's just another name for the net. You have to create the voltage inserting a voltage source. – clabacchio♦ Apr 24 '12 at 12:05

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