555 timer that does not output low when reset
You just need to use the output of the 555 to drive the base (through a resistor) of an open-collector NPN transistor. Something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

In this case, I've chosen R1 to drive about 1 mA into Q1's base, assuming a Vcc of 5 V, and the need to sink less than about 50 mA. If your values are different, adjust accordingly.