Hey y'all..
so basically I am trying to build this anti-theft alarm system and I am using a 555 astable timer to make a LED light up intermittently. However, once it rises to HIGH voltage it stays at HIGH and it does not oscillate. What should i do? Using Multisim Live as I am on MacOS . Ca
All values are correct to my own specifications. it just does not want to oscillate.

 

Make R3 less than half of R2.
Welcome to AAC!

 

No surprise it doesn't oscillate for either logic output of the U3 latch.
That connection to the latch will kill the 555 operation as its output is basically a short to either ground or V+.

If you want U3 to inhibit the 555 then connect it to the 555 RST pin.
A low on that pin shuts off the 555.

 

Welcome to AAC!

You haven't told us about how the flip flop is being used.

1. What does Q# do before and after you want the timer to oscillate?
2. What is the purpose of having the flip flop connected to trigger/threshold? If you want the timer to oscillate, you need to let the capacitor charge and discharge through R2 and R3.
3. The high side of R2 should be connected to VCC, not ground. What does the switch do?

 

No surprise it doesn't oscillate for either logic output of the U3 latch.
That connection to the latch will kill the 555 operation as its output is basically a short to either ground or V+.

If you want U3 to inhibit the 555 then connect it to the 555 RST pin.
A low on that pin shuts off the 555.

Hi thanks for the reply. The flip-flop is meant to initiate the 555 astable timer. Q# is meant to be LOW at first and in order for the timer to oscillate Q# should be HIGH.
LOL i didn't even notice R2 being connected to ground, silly me! these late nights have been getting to me. Thanks for helping

 

Make R3 less than half of R2.
Welcome to AAC!

Thanks for replying! I will be sure to try this out.

 

All values are correct to my own specifications. it just does not want to oscillate.

You haven't given any specifications.

 

Thanks for replying! I will be sure to try this out.

My bad! according to my calculations and my resistor and capacitor values, it should oscillate with 50% duty cycle. Should be 0.5sec HIGH 0.5sec LOW

 

If you want U3 to inhibit the 555 then connect it to the 555 RST pin.
A low on that pin shuts off the 555.

This. Exactly.

 

Q# is meant to be LOW at first and in order for the timer to oscillate Q# should be HIGH.

As I noted, the 555 can't oscillate when Q is high as it just supplies a steady voltage to that point, which will keep the 555 output low.

 

As I noted, the 555 can't oscillate when Q is high as it just supplies a steady voltage to that point, which will keep the 555 output low.

Ive managed to get it to oscillate through using output from Q as the main power and threshold point for the 555 timer.

 

Ive managed to get it to oscillate through using output from Q as the main power and threshold point for the 555 timer.

]That circuit requires U3 to provide the power to the 555 which is not a good way to control the 555.
Why not connect it just to the RST terminal, as suggested?