Designing a circuit with a "short-lived" switch with 555 IC
I think you are asking what can be added to the switch input (actually your IR sensor) to make it act like a single pulse trigger and not a continuously held switch. To do this you can just use the same method you are already using between your existing 555's, use a capacitive input, (see below).  

You may need to adjust the R1,C1,C2 values a bit to get the timing just right, and the original 33k pull-up might also need to be higher. Ultimately you could even include a buffer or one shot in-line with the first 555 to give you a better input pulse shape.  If you look into 555 applications there should even be a circuit to use a 555 as a one-shot.   

As another alternative you could use a tri-state buffer gate and enable it with an output of one of your 555's. That arrangement would temporarily disable the input signal and create a single pulse, (see second circuit).







simulate this circuit &ndash; Schematic created using CircuitLab