High voltage pulse affecting 5v logic

Hi,

I have a serious problem which occurs randomly.

I have a low voltage 5v section with a LM555 set to "one shot" which means when i press a switch, the 555 output goes high for X miliseconds.
the 555 output goes to an optocoupler that ignites a trigger coil (ignition coil) and the coil should ignite a Xenon lamp.

the problem is, when i trigger the 555, on the scope i can see that the output rises immediately to about 4-5v, but after 50 uSec, it has a big spike downwards, even lower than 0v, and it recovers again very quickly to the high level.
the thing is, that i randomly have the flash about 100ms after triggering the 555. most of the times the flash happens immediately, but about 1 time every 20 triggers, the flash fires in some 100ms delay.

What i believe is happening, that the primary High voltage pulse is creating bad stuff on the 555, causing the spike downwards, which is causing maybe randomly the optocoupler not to fully turn on enough time, and only after the 555 output recover the spike downwards, it rises again and turns on the optocoupler fully again which triggers the lamp in delay.

both the Xenon lamp and the 555 share the same ground. they have to.
the 555 is powered via L7805 from a 12v battery which is also charging the high voltage capacitors.

on lower voltages of the xenon flash and trigger it happens much less often.

any idea how to reduce the spike or make the 555 immune to the spike ?
I already have plenty of 10uF ceramic bypass capacitors all around. tried even on the 555 output, no help...

Thanks for your replies

Did you add a cap to ground at the input and at the output of the 7805?

Ground planes and shielding?

Can you share the circuit?

Did you put a diode across the coil primary, so the diode will absorb inductive kick at end of pulse?

Also did you try powering the 555 from a separate battery than the battery powering the coil? I believe the negative leads can still be the common ground (since you say that needs to be the case).

What switch are you using? A simple push button? A debouncing type which has a capacitor somewhere that might be prone to create spikes when the supply voltage changes?

To increase noise immunity of the 555, try to power up from a 12V power supply.
Install a bypass capacitor on pin 5 (control voltage pin) and use a bypass capacitor near to the 555 power pins.

Try to eliminate any ground loops, the most common source of noise in electronics systems. Proper grounding is essential for glitch-free design.

Use large path for the GND connection.

Shield the input signal by routing it between two layers connected to GND. Create GND tracks on the opposite PCB side (i.e., creating a stripline), so the outer conducting planes can shield high-frequency radiation and improve the noise immunity of the circuit.

HI,
I do have a capacitor on pin 5, a bypass capacitor of 10uF ceramic on the supply of the 555, and on the input and output of L7805 - really everywhere
the switch doesnt matter, as the 555 is supposed to create a clean output regardless the trigger input (switch). also on scope the switch is very sharp and clean. i use a .1uF across the switch.

I thought of the diode between the primary coil of the trigger transformer - and i made a test:
i kept the trigger coil connected, and just disconnected the output HV wire from the lamp.
it stopped doing these spikes. which means to me that the spikes are caused by the HV output of the coil - not the primary.

but then i did another test - i disconnected the + of the lamp from the big capacitor (to check how the HV pulse is returning to the circuit) - it didn't help.
then i disconnected the "-" minus of the lamp from the capacitor (and common ground) leaving only the HV trigger output connected to the lamp, a few CM away from the circuit -
and then again the spike stays.!!! with no way of return to the circuit.
it seems to do the spikes when the HV output trigger is connected to a large plane (the lamp) regardless of a return path to the circuit...
MAYBE when it is connected to a large plane is acts as a load to the trigger coil and thus increasing the spike on the primary after all ?
it might be that...

As asked above, please post your circuit, sketched or otherwise, and we can take a look at it. Thanks.

How about the GND plane on the 555 circuit area?
Can you post the schematic?
Have you tried a 12V power supply for 555 circuit ?