I'm trying to design a circuit with the following goals:
- When not active, output can be either Vcc or floating
- When triggered by pressing a button, output pulses several times to GND at some frequency F for a duration of N seconds
- After the N second duration, the pulsing stops
My first thought was to use two 555 timers: one configured as a monostable timer with period N, triggered by the button. The output of the monostable timer is connected to RESET pin on a second 555 configured as an oscillator with frequency F. The output of this timer would be the desired output.
This design achieves goals #2 and #3 above. When triggered, the output of the monostable timer drives the oscillator's RESET high causing it to pulse for the duration N. However before the circuit has been triggered and after it is finished, the output from the oscillator is driven to GND by the second 555. This is undesired, since in my application I am pulsing an active-low device. I would like to understand if there is some way to pull the output of this circuit to either Vcc or have it floating, when not pulsing.