Hello I have another question regarding the 555. I read that it is triggered when the trigger pin goes from above to below a voltage level of 1/3V+. I have a positive 2V input pulse I want to trigger it. But if the voltage is already high on the trigger pin, how can my pulse trigger it? All the schematics I have of the 555 have the input trigger pin 2 pulled high. How can I accomplish this? Thank you.

 

Got to invert it if you want it to trigger at the beginning of the pulse.
If you are okay with triggering at the end of the pulse, you can use some sort of AC coupling like action.

Which is it?

 

inverter works great. Thank you.