Untimed Output Module - Circuit Description

Build this DIY Alarm system yourself. It will sound once only - you decide for how long. Then it will switch off. So it won't annoy the neighbours.

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Untimed Output - Circuit Description

While the alarm is switched on - the set terminal on the main board delivers a 12-volt supply to both the relay coil and the positive output terminal.

However - because the relay is de-energized - the negative output terminal is not connected to ground.

So - any device you've connected to the 12-volt output terminals - is switched off.

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The trigger is connected to the siren output terminals - on the main board.

In standby mode - R2 keeps the transistor switched firmly off.

But when the alarm is activated - and the siren sounds - current flows into the base of Q1 through R1.

This switches the transistor on. 

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When the transistor switches on - it connects the negative side of the relay coil to ground.

So the relay energizes - and the normally open contacts connect the negative output terminal to ground.

Any device you've connected to the 12-volt output terminals - will switch on.

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The normally open relay contacts also connect the negative side of the relay coil to ground - through D2.

Now - the relay no longer needs Q1 in order to remain energized.

It has taken over - from Q1- the job of connecting its own coil to ground.

In other words - it has latched itself on.

Thus - when the siren output disappears and Q1 switches off - the relay will remain energized.

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Anything you've connected to the 12-volt output terminals - will go on working until you switch the alarm off.

When you move Sw1 to the off position - power to the positive side of the relay coil is cut.

So - the relay drops out - the 12-volt output disappears - and the circuit is unlatched.

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Relay coils and some Sounders produce large reverse voltage spikes that will destroy Cmos ICs. 

D1 & D3 are included as a precaution. They short circuit the spikes at source - before they can do any damage.

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D2 creates a one-way path.

It allows the normally open relay contacts to connect the negative side of the relay coil to ground.

But it prevents the transistor from connecting the negative output terminal to ground.

Without the diode - Q1 would have to carry the full load of any device connected to the 12-volt output terminals.

It would only have to do so briefly - but that might be enough to destroy the transistor.

So the diode is included as a precaution. 

A 1N4148 would perhaps be adequate in this position. 

But I felt that for long term reliability - it would be better to use the more robust 1N4001.

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I specified a relay with a minimum coil resistance of 270 ohms. Use a higher value if you wish.

With a 12-volt supply - the maximum current through Q1 will be about 45mA.

This is well within the limits of the BC547 - which has an IC(max) of 100mA.

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There's nothing special about the transistor.

Any small NPN transistor with a gain (hfe) greater than 100 and an IC(max) of at least 100mA should do.

But remember that the pin configuration of your transistor may be different from that of the BC547.

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Click Here For A Photograph Of The Prototype.

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One-Time-Only Alarm - Untimed Output Module