555 signal delay circuit different solutions

Thread Starter

popcalent

Joined Mar 17, 2018
128
Hello,

I'm trying to design a circuit that when triggered generates a delayed pulse. That is, when a push button at the input is pressed, the circuit waits for a time D (delay), then generates a pulse for a time P (pulse). I found a solution that works but I think it is ridiculously complicated. So I decided to do some research.

After searching this forum I found this old thread:
https://forum.allaboutcircuits.com/threads/simple-555-delay.60775/

I have put that circuit together on a breadboard but there's something I don't like, when I turn it on there's a pulse on the output that lasts P. Then it goes low and the circuit does what I want: I press the push button and it does nothing for a time D, then there's a pulse for a time P. But that initial pulse bothers me and I don't know how to get rid of it. They mention it's because the capacitor on the second 555 needs time to discharge but no matter how long I wait, when I turn on the circuit there's always that initial pulse. But it makes sense for it to be there: when I turn on the circuit, the output of the first 555 is low and it triggers the second 555. How do I get rid of that pulse?

On the internet I found this:
https://www.electronics-project-design.com/timedelaycircuit.html

I don't fully understand that circuit. I understand that resistor VR1 and capacitor C generate the delay but then, what generates the pulse? Should I connect a resistor from discharge to Vcc and a capacitor from discharge to ground? Or is discharge supposed to be floating? I need help understanding that circuit. I put it together and it doesn't do anything.

Finally, my solution that works. But I'm not happy with it because it's too complicated. I'm using two 555s on monostable mode, one generates a pulse for a time D+P, the other one generates a pulse for a time D. Both are triggered by the same push button, both outputs go to a XOR gate (cmos 4030). The output of the XOR gate is a delay (D) then a pulse (P). It works, no initial pulses. But I just think it's too many integrated circuits.

Any ideas? Thanks!
 

crutschow

Joined Mar 14, 2008
34,697
Here's your first circuit, slightly modified.
I added C6 to keep U2's TRIG input high during power up, which solves the problem (which I also observed) in the simulation.
(R8 solves an observed trigger sensitivity problem due to the addition of C6).

If you still have problems try increasing the values of C2, C4, and C6 by a factor of 10.

upload_2018-3-17_22-12-1.png
 

Thread Starter

popcalent

Joined Mar 17, 2018
128
Hey! Thank you very much, crutschow! This does exactly what I want. Just one thing. What does exactly R8 do and what happens if it's not there? I put the circuit together and without R8 it just doesn't work, but I want to understand why. Thanks again!
 
Top