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Mains failure detector alarm circuit design

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msmax

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mains failure alarm

What is the most simple circuit for the next:
I have a charger which can supply max. 14V and a 12V battery.

As soon as the mains is gone a lamp and a buzzer should be activated fed from the battery.
The buzzer should be silencable by a pushbutton.

Any good idea's without too much trouble?
Is it possible only using relays?
 

yes

You can do it with relays. The first computers used relays only. When an insect sat on a set of contacts and the relay flipped their squished body caused a fault. This fault was called a bug.

All you need is a flip flop which can be done with two relays. The line failing flips it and your push button flops it.
For safety purposes a third relay would be across the ac mains and used to indicate that condition to your circuit.
 

I think you can do it with just two relays.
One with mains AC coil and one with 12V DC coil.
The first trigers the second wich is connected in self retained mode. A pushbutton can be used to de-enegize this coil in the classic Start-Stop manner
hope this helps
regards
 

Thanks for your reply.

But...

The mains is gone...lamp on, buzzer starts.
I like to silence the buzzer only. Remember the mains is still gone.

In your description the circuit would be activated immediately again if I would press the silence (flop or stop) button.
 

analysis

In theory the two relay method should work. This is a sequential circuit with three states, (power on no buzzing no lamp) (power off buzzing and lamp) (power off lamp and no buzzing) Two relays can have four states between them. It is all a matter of using the multiple relay contacts to form AND and OR cirucits to control the relay coils.

The three relay method is easier to synthesize with fewer AND/OR contact circuits.
 

wire the lamp to an NC contact of the AC coil relay.

AC-coil PB DC-coil
--| |-------|/|-------()---
--| |--
DC-coil



AC-coil lamp
--|/|-----------------( )-----


DC-coil buzzer
--| |------------------( )---
 

save power and funds

If you want to save power drain on the battery and some money, use the ac mains relay to control the lamp and CD4000 logic sequential circuit to control the buzzer. One input to the logic comes from the mains relay and the other from the pusbutton.
 

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