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I'm trying to design a circuit with the following goals:

  1. When not active, output can be either Vcc or floating
  2. When triggered by pressing a button, output pulses several times to GND at some frequency F for a duration of N seconds
  3. 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.

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  • \$\begingroup\$ Welcome to EE.SE. Please post a schematic! \$\endgroup\$ – winny Jul 27 at 9:44
  • \$\begingroup\$ connect the discharge pin of the first 555 to the capacitor of the second, \$\endgroup\$ – Jasen Jul 28 at 0:37
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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:

schematic

simulate this circuit – 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.

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  • \$\begingroup\$ Thank you, Dave, Nice simple solution. I will give it a try! \$\endgroup\$ – Ryan Jul 27 at 18:16

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