I'm having a quiz show project with LEDs, and a buzzer. One LED must be lit up and the buzzer should work for about 0.5 sec. I am stuck on the problem that using circuit output state (which gets high and remains high till reset comes), I will set the trigger pin of timer555 low, but since it's continuously low, the buzzer, which is connected to the output of timer555, buzzes repeatedly. I'm wondering what I could do to convert a constant output to a fixed 0.5 pulse using a timer555.

  • \$\begingroup\$ Single pulse = monostable. \$\endgroup\$ – Eugene Sh. Jan 25 '17 at 16:04
  • \$\begingroup\$ I know how to make use of monostable mode generally, but with my problem the trigger part is constant low (thus not a pulse), and that makes it buzz repeatedly... \$\endgroup\$ – Sajad Jan 25 '17 at 16:07

You need to turn the input from a level to a edge trigger:

enter image description here

by adding a capacitor as shown to create a differentiator. Try using 0.1 µF to start. The pullup resistor, Rd can be 10K or so. The diode is a 1N4148.

The 555 will not re-trigger until after the input has been restored to Vcc and then goes low again, no matter how long the original pulse is.


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