Triggering a 555 timer IC

The 5V bus needs capacitors across it, near the regulator, near the TSOP4838, and another one near the 555. Without it, there is ripple on the bus. The TSOP4838 will false trigger with more than 100mV ripple. This false trigger will trigger the 555. Switching the load on the 3V bus will cause sag in the 9V bus, which will be passed to the 5V bus via the regulator, before it can compensate. You should prove this by measuring the ripple before you add capacitors.

There are at least three things that can affect this:

1. Tolerance of the components. Your resistors and capacitors may be +/- 10% or more.
2. Tolerance in voltage of the 555 -- what is the actual VCC and the actual threshold voltage?
3. Variation based on temperature or other ambient conditions.

If you are within spitting distance of the timing you need, slightly adjust the capacitor or resistors until you get where you need to be. Easiest would be to perhaps use a potentiometer to make those adjustments.

And I agree with everyone else: In real life, it's easier to use a microcontroller with built-in timers and oscillators. An AVR ATTiny85 would need zero support components to do this (although a de-coupling capacitor would be good form.) Too bad you can't use it :-)