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alarm circuit using lm555


long84

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This is an alarm circuit using IC lm555. But I don't really know the function of lm555.
Is it used for set the duration the buzzer will act.
Or to control the buzzer's frequency.
Still, it's said that thyristor in this circuit is not different from a switch which means not use its function of phase control. But I don't think so.
Can you explain it to me?

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This is an alarm circuit using IC lm555. But I don't really know the function of lm555.
Is it used for set the duration the buzzer will act?
Or to control the buzzer's frequency?

The 555 is wired completely wrong. I corrected it.
The thyristor makes the buzzer buzz continuously until its power is turned off.
The buzzer buzzes at its own frequency.
The 555 is supposed to be a timer to allow you to exit before the alarm goes off. I added a transistor as a gate to stop the exit switch from activating the thyristor during the delay from the 555 timer.

Still, it's said that thyristor in this circuit is not different from a switch which means not use its function of phase control. But I don't think so.

A thyristor uses phase control is used in a light dimmer that operates from AC power. This circuit does not use phase control and does not have AC power. The thyristor is used as a switch that cannot be turned off unless its power is turned off.
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The frequency of a buzzer is built into it.

An AC  light dimmer is a resistor in series and a capacitor to neutral which can delay the AC signal from the mains. If it is fed to the gate of a triac (an AC thyristor) then the triac turns on late for each half-cycle so the average power to its load is low. When the current alternates through zero then the triac turns off for each half cycle. If the resistor is a pot then it can control the amount of delay. No delay results in full power to the load, a long delay per half cycle results in a low power to the load.
The delay control is also called a phase control.

Look at Light Dimmer Circuit in Google.

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The thyristor circuit you want me to comment on uses an SCR thyristor. Like a triac, it turns on and stays on until its power is turned off.

In the first circuit you showed, if the 555 timer was wired properly then the thyristor would have turned on and stayed on immediately, without a delay.

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