I have a pretty bizarre problem with Multisim. I have a larger circuit which works perfectly when the inbuilt square wave generator for Multsim is used. I wish to replace the square wave generator with my own.

I have a 555 square generator circuit which works perfectly when I test it by itself, the moment I recreate that circuit in the main circuit and connect it to a 5V source the output of the main circuit just stops after a few milliseconds. This only occurs when the square wave generator is connected to a source without even connecting it to the rest of the circuit, when I remove the 5V source the circuit simulates for a few seconds and then just stops the only way to get the correct waveform is to completely[![enter image description here][1]][1] remove the 555 square wave circuit.

When I do connect the 555 timer I get an incorrect output for a few milliseconds and the circuit stops anyway. I have no idea why the presence of the timer breaks the rest of the circuit and I was hoping someone could enlighten me on this.enter image description here output when the 555 timer is disconnected from the circuit but the 5V source to it is connected

output when it is connected

The circuit is supposed to count up until the green wave form reaches the red and then it resets and starts again.


1 Answer 1


You have used earth for the 555 instead of circuit ground.

enter image description here

Figure 1. There is no ground return path for the 555 oscillator.

Replace the earth symbol with a ground symbol.

Oh, and please turn off the grid for screengrabs!

  • \$\begingroup\$ Thank you, however it is still not working. I have narrowed the problem down to the 555 timer not playing nice with the staircase circuit but I am still baffled as to why. I can post a link of just those two components if you would like to play around with them yourself? \$\endgroup\$ May 1, 2018 at 12:45
  • \$\begingroup\$ I don't have MultiSim. Sorry. I'm not sure what's supposed to happen at the right side of C1. Are you missing a pull-up or -down resistor? \$\endgroup\$
    – Transistor
    May 1, 2018 at 15:01

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