Hi
This is a more reliable way to trigger a mono stable

I asked this yesterday in a quick reply and either I did not submit it or it was deleted for some reason. What is R7, the 47k resistor for?

Thanks,

George

 

i found the problem
there is a home device which connected to power with it in the same socket
and when this device turn on the 555 triggered
now any help

 

Get the mains voltage off the PCB, place the filter capacitors adjacent to the regulator, and add a .1μF capacitor from pin 1 to pin 8 on the 555.

 

I had handled the similar case, but my suggested on #14 didn't get the answer.

 

i don't have o'scope but with voltmeter imeasured input = 12.5v
output= 11.8v

 

If no waveform then hard to say it's easy to get to the point, but you still can try, you can using a coil with ferrite core that the values as 22uH~50uH in series with the input of 7812, if you doing this and the 555 became normal then it's done, otherwise you have to in series another one with GND of 7812.

Toroidal Ferrite Core Choke Coil.

Ferrite Core Coil Inductors.

 

I recheck the circuit, the coil also can be in series with the output of bridge rectifier, and if you need to adding the second coil then you just in series with the GND of the output of bridge rectifier.

 

so i don't need any capacitors

 

The capacitors are for the positive voltage and some over the Vcc as 12V, it means that the pulse could be over 12V.
If the pulse are negative or low less 0V then the capacitors maybe useless, and during this time the 555 can be triggered, if we using the coil that it can be isolate the pulse to interfering the right side of the coil, and the right side of the coil will keeping in a stable status.

If you in series the coil as the described on #27 then it will become a LC filter.

 

can i use inductor which its shape look like the resistor
or i must use toroid coil

 

If you using the inductor as the resistor type, you should care about the current that is the choice at here, and the watt is enough or not, the coil is a choke coil here, against the rush current.

So you can try and to get some Experience.

 

Hi

C4 discharge through R7 to gnd