Determine suitable resistor for the base of transistor as switch (NPN)
Your problem is probably that you also need a so-called pull-up resistor between the collector of the transistor and the positive voltage (+5V).

As for the basis resistor, there are two constraints:


The resistor value should be small enough to switch the transistor completely on when the output of the 555 is high. So you have to consider the voltage on the basis (+5V - 0.7V) and the current you need through the basis, which must be greater than the current you need through the collector to pull the voltage low divided by the minimal current gain of the transistor.
The resistor value shouldn't be too small, so that you don't exceed the maximal basis current of the transistor.