Multisim not simulating 555 timer correctly

[dm7190]

I am trying to use the 555 timer to light an led high for 2 minutes and low for two minutes and i feel like my math is right but when i simulate it in multisim it is not working correctly. I have used the 555 calculators and have done it myself but its still not right in multisim does anyone know why this is and maybe how to fix it. Thanks Dylan

 

[crutschow]

Post a schematic of your simulation.

 

[dm7190]

this is it the problem is that the high and low times arent what they are supposed to be in multisim.

 

[simonbramble]

Can you not simulate it at a discrete level from the block diagram:http://ecelab.com/555-block-diagram.htm?

 

[SneaKSz]

Strange value's for R5 and C5

 

[dm7190]

I didnt think to do it like that im not sure exactly what you mean at a discrete level though. do you mean build the 555 timer instead of using the chip that multisim provides.

What do you mean strange like those values wouldnt be available regularly or what. Because i figured about the resistor but i thought the capacitor value would be or at least close it doesnt have to be exact.

 

[simonbramble]

No - simulate it with individual comparators, SR latch, resistive divider and transistor. All the 555 timer does is measure the voltage across a charging cap and trip a comparator accordingly. You can simulate this discretely.

Failing that, LTSpice has a 555 timer block in it. if you want to get started with LTSpice, here an LTSpice tutorial:
http://www.simonbramble.co.uk/lt_spice/ltspice_lt_spice.htm

 

[dm7190]

Yea i tried but it didnt work i cant even find the right latch to use in multisim, and ltspice i cant find the block but either way i need this to be in multisim. So i guess multisim is just not able to simulate this correctly but the values are right so i geuss i can just ignore it, i just wanted to make sure it was right with multisim but that didnt work out. Thank you for replying i appreciate all the help.

 

[crutschow]

I was able to simulate the circuit correctly with a total output period of a little over 240 seconds (see below) but it takes a very long time. You can speed it up somewhat by clicking the "Simulate" tab, selecting "Interactive Simulation Settings" and setting "Initial Time Step" to 1.0.

Incidentally you typically show higher resistor values with a k (kilo) or M (Meg) suffix and capacitor values with u (micro) or p (pico) suffix.

 

[dm7190]

yea the period is around 240 but the first high time is around 190 seconds and it takes two cycles or so for it to get close to a 2 minute high and low time, but its fine i will just get it to the point it works and pause it and then show my teacher because he knows multisim doesnt work right. Also i did know about using the suffixes with the resistor and capacitor values i was just typing it in off my calculator, but you are right my teacher would get on to me if he saw that so i will definitly try to get in the habit of doing that. Again thank you all for the responses.

 

[crutschow]

The first time high of 190 seconds will likely occur in the actual circuit as shown in the simulation. That's how long it takes for the initial charge-up of the capacitor.

Why do you think multisim doesn't work right?

 

[dm7190]

ok i did not know that i thought multisim didnt work right because it was supposed to be 2 minutes high and low and i didnt think there would be an initial charge time so i thought it wasnt working right but i guess not. how would i be able to make it so that it would be 2 minutes high and low or at most 10 seconds off.

 

[crutschow]

ok i did not know that i thought multisim didnt work right because it was supposed to be 2 minutes high and low and i didnt think there would be an initial charge time so i thought it wasnt working right but i guess not. how would i be able to make it so that it would be 2 minutes high and low or at most 10 seconds off.

If you monitor the voltage at C5 (C1 on my schematic) you will see the problem. The capacitor voltage starts at zero, goes to about 8V and then discharges to about 4V. The normal oscillation occurs between 4V and 8V on the capacitor.

To avoid the initial long pulse you can precharge the cap to 4V before you close S1 to start the oscillations with a resistor network and diode as shown below.

 

[dm7190]

ok i see now thank you for helping. i didnt realize that about the oscillation i need to look more into how the 555 timer works.