555 timer used as a one shot

[Hoborob]

I have been attempting to use a 55 timer in one shot mode (monostable multivibrator). I have successfully gotten it to work in the astable mode but when I do teh circuit in what should be a one shot mode the circuit will suddenly start vibrating rather than giving one pulse and then stopping. I can upload the circuit diagram for what I am attempting but I would be satisfied with a circuit that works that I can modify with the capacitor and tresistor values I have been given provided the circuit does what it should be doing. I will admit that I am very new to the Multisim environment so I am still trying to learn the program.

[Tayyab_R]

Hi Hoborob,

Can you please post the circuit so we can take a look and see what could be causing the issue.

[Hoborob]

I did some additional testing of the circuit I had constructed today. Today the circuit works just fine while in the previous attempt it suddenly oscillated rather tan acting as a one shot. Taking this knowledge back to the original circuit that had a one shot in it, I did the same monitoring and found teh one shot circuit to be working correctly. I modified the circuit to speed up the pulse which was very long and now when I try to trigger the circuit a second time the program says there is a simulation error that is occurring. The report says that the timestep is too small, I have no idea what this means.

Attached is the circuit under test/analysis.

[Hoborob]

As an addition here is the test circuit that has the 555 timer operating as a one shot.

I do have one question to ask about the simulation however, it does seem to me that when doing things that involve timers the response times are unrealistically slow by what looks to be a foictor of around 100, is this unusual or is teh program set up that way?

[Tayyab_R]

Hi,

Sorry for the delay in replying. Firstly, you can edit the speed of the simulation by going to Simulate -> Interactive Simulation Settings and changing the timestep as shown in screen shot. The timestep determines the speed of the simulation but the defaults work well for most circuits. In your circuit you can modify it a bit to make it faster.

I think the circuit is behaving correctly. If you take a look at the attached circuits, I used the circuit wizard to create the monostable configrations. I changed one to match your circuit and you will see that it behaves correctly. Basically what happens is that when the THR voltage reaches the desired level, the output starts to oscillate.

You can try using the circuit wizard to make it easy to setup the 555 timer circuit. To use the wizard, go  Tools -> Circuit Wizards -> 555 Timer Wizard.

Hope this helps.