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.

 

Welcome to AAC!

It would be helpful if you posted a schematic. It sounds like you just need to AC couple whatever (a switch I assume) is providing the trigger.

 

Or you have the 555 wired as an astable oscillator rather than a monostable pulse circuit. Again, schematic.

ak

 

it's exactly wired as shown, with R*C set at about 0.5.
The trigger, which is at Vcc first, is set low and held low (that is, not a pulse), and causing the output make repeated pulses, right? But I only need one...so how should I do it?

 

I don't know what your game circuit looks like, but I've made one myself. There is a signal that goes low when any button is pressed. It is used to "lock out" all other buttons.

The solution I am about to propose doesn't use a 555, but will create the effect you are after.

I'd use this signal, negated, to generate an edge-triggered pulse. This pulse would start a monostable vibrator for 0.5 seconds.

If your output is a buzzer, that should be enough. Otherwise you could use a second 555 to generate the buzzer sound.

The edge-generate pulse circuit is shown below.

 

how should I do it?

AC couple the switch that's triggering the one shot: