Air Force oscillator

[JParlow]

I have an old Air Force board that we had to solder together. It is a very simply oscillator but I can't get it to work in Multisim. It's probably something stupidly simple but I'm tired of looking at it. Any suggestions.

Thanks

[Sharanya_R]

Hello,

My name is Sharanya Rajaratnam, a Market Development Engineer at NI.  I just had a few questions, so that I could better assist you with your issue;

1. What is the expected output of this circuit?

2. Should the LED's be blinking?

[digitalguy]

Try setting the initial conditions to zero: simulate --> interactive simulation settings --> initial conditions. Because of the nature of simulating oscillators in SPICE, you may have to change your circuit to get it to simuate faster. Try changing the values of the feedback capacitors to 10 uf. You may have to play around with the values depending on the speed of your computer. Hope this helps.

  Howard

[JParlow]

The LED's should be blinking.

[JParlow]

Setting the initial condition to zero helped. When I took the caps down to 1uF the circuit worked properly. Why would it not work with the 100uF caps? Is it simply because of the speed of the computer? What type of problem does SPICE have with oscillators?

[digitalguy]

The problems not really with the capacitors. The real problem was with the initial conditions. When you tell SPICE to set inital conditions to zero, you are, in this case, telling it to begin simulation with zero charge on the capacitors. Just as would be the case when you turn the real circuit on. This insures that the circuit will have the proper conditions to start oscillating. By changing the values of the capacitors you are simply allowing the simulation to proceed at a faster rate as the simulator and your computer cannot simulate in real time.