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alarm circuit using lm555

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long84

New Member
Can you tell me in detail the function of lm555 (see attach file)?
Is it used to set the duration the buzzer will act?
Or to adjust the frequency of buzzer?
Still, it's said that the thyristor in this case is the same as a switch which means
not use the phase control function
Can you explain this to me ?
 

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Swanepoel

New Member
The 555 timer is in monostable mode (or oneshot) so it will only give a single pulse once the trigger (pin 2) is brought high.
So it determines the time that that buzzer will be on as the output (pin 3) will trigger the thyristor and switch on the buzzer.
To calculate the time that the buzzer will be on for use this
T = 1.1R1 x C1 where R1=10k and C1=220uF
this gives 2.42s
 

Hero999

Banned
I don't know what it's supposed to do because it won't work; the 555 has no negitive connection and the timing resistor capacitor and resistor aren't connected to the IC.

Is this college work?

Well . . .

We don't mind helping you but we won't do all of the work for you. Please post what you've already done including any ideas you've had and we might be able to make some suggestions. If you've got completely no idea then may be it's your lecuturer's fault and it isn't right that you make him/her look good by cheating and copying from the Internet.
 

long84

New Member
no, i think you both don't understand. When is works, power switch is on and exit switch is off. If a thief touch the Push, buzzer will ring. But I wondering if lm555 only makes a pulse or continuously pulses?
And the thyristor, is its function of phase control used?
 

Leftyretro

New Member
"And the thyristor, is its function of phase control used?"

The thyristor, here used in a DC function, is acting as a latch. Once turned on by the 555 the buzzer will remain energized forever until it's manual switch (or the main power switch) is turned off, then on to reset the buzzer awaiting the next trigger switch operation.
 

Hero999

Banned
Even if the missing connections were completed the 555 would still serve no purpose. You might as well connect the SCR input directly to the trigger.

If you want the buzzer to be triggered for a certain period after being triggered then obviate the SCR and use the monostable 555.
 

audioguru

Well-Known Member
Most Helpful Member
I fixed the 555 monostable and used it as a delay timer. Then I added a transistor to allow a delay before the exit switch (in a different spot in the schematic) is enabled. Yesterday, on another website's forum.
 

Hero999

Banned
Could you please post a link then.
 
S

Souper man

Guest
I was looking at the schematic and it looked like you diddnt ground pin 1...
 

audioguru

Well-Known Member
Most Helpful Member
It was good that its pin 1 was not grounded.
Its pin 7 connected directly to the positive supply and if pin 1 was grounded then the transistor at pin 7 would get too hot trying to kill the battery.

There are many mistakes in the original circuit.
 
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