Four small battery-powered transistor alarm circuits - you can build your self - using cheap off-the-shelf components.

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Circuit Description

Click Here For A Photograph Of The Prototype.

Learn More About The Complementary Latch

This alarm is based on a complementary latch. For a detailed account of how the latch works - click on the link below the schematic diagram.

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In standby mode - the current flowing through R1 is diverted to ground by the normally-closed switch.

When the switch is opened - the current is no longer diverted to ground.

Instead - it flows into the base of Q1.

This switches Q1 on - and current through R2 and Q1 flows into the base of Q3

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Q3 switches on - and connects the base of Q2 to ground.

This causes base-current to flow - through R3 - into emitter-base junction of Q2.

So Q2 switches on.

And current through R3 and Q2 flows into the base of Q3.

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Now - it doesn't matter now if you re-close the switch and turn Q1 off.

Q3 no longer needs the R2 current in order to remain switched on.

It has provided itself with an alternative and constant supply of base current - through R3 and Q2.

In other words - Q3 has used Q2 to latch itself on.

While it's on - Q3 connects the negative side of the relay coil to ground - and the relay in turn sounds the siren. 

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The normally-open trigger switch has a similar effect.

When the switch is closed - base current flows into the emitter-base junction of Q2- through R3.

So Q2 will switch on - and current through R3 and Q2 will flow into the base of Q3.

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This will cause Q3 to switch on.

When Q3 switches on - it connects the base of Q2 to ground.

Now - Q2 no longer needs the R5 connection to ground in order to remain switched on.

It has provided itself with an alternative and constant connection to ground through Q3.

In other words - Q2 has used Q3 to latch itself on.

Again - while Q3 remains switched on - the siren will sound.

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Some small alarm type switches have a maximum current rating of 20mA or less.

Without R5 - the normally-open switch would have to carry the current drawn by the relay coil.

R5 limits to less than 10mA - the current through the normally-open switch.

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Q1 and Q3 form a modified version of a Darlington-Pair.

The current gain of Q3 is multiplied by the current gain of Q1 - so the trigger circuit is very sensitive.

The wires going to the trigger switches act like a radio antenne.

They pick up high frequency electrical interference that could trip the alarm.

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C1 represents a short circuit to high frequency signals.

The capacitor shorts the signals to ground - before they can switch Q1 on.

The trigger is further de-sensatised by R4 and C2.

R4 connects the base of Q3 to ground. It keeps C2 discharged and Q3 switched off - until required.

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Relays coils and some sounders produce reverse voltage spikes that will destroy sensitive electronic components.

The two diodes are there to short circuit these voltages at source - before they can do any damage.

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The standby current is less than 2uA. This is set by the value of R1.

Depending on the gain of the actual transistors you're using - you may be able to increase the value of R1 - and so reduce the standby current still further.

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Parts List

Suppliers

Worldwide RS Components

UK & Ireland Maplin

Construction Guide

Click here if you're new to constructing stripboard projects.

The terminals are a good set of reference points. To fit them, you may need to enlarge the holes slightly. Then turn the board over and use a felt-tip pen to mark the 7 places where the tracks are to be cut. Before you cut the tracks - double check your marks - to be certain that they are all in the right place

When you're satisfied that the pattern is right - cut the tracks. Make sure that the copper is cut all the way through. Sometimes a small strand of copper remains at the side of the cut and this will cause malfunction. Use a magnifying glass - and backlight the board. It only takes the smallest strand of copper to cause a problem. If you don't have the proper track-cutting tool - then a 6 to 8mm drill-bit will do. Just use the drill-bit as a hand tool - there's no need for a drilling machine.

Transistor Alarm {C}
Construction Guide

Next fit the 5 resistors and the three wire links. Use the wire you've trimmed from the resistors - to make the links. The resistors are all shown lying flat on the board. However, those connected between close or adjacent tracks are mounted standing upright. See The Photograph Of The Prototype.

The next stage is to fit the three transistors and the two diodes. The pnp transistor (BC557) is the one with the emitter arrow coloured red. Pay particular attention to the orientation of the diodes. Note that in both cases the side with the bar is pointing up.

Transistor Alarm {C}
Construction Guide

Fit the relay and the two capacitors next. Pay particular attention to the orientation of the electrolytic capacitor. Note that the positive terminal is pointing up.

Turn the board over and examine the underside carefully - to make sure that there are no unwanted solder bridges or other connections between the tracks. If you backlight the board during the examination - it makes potential problem areas easier to spot. When you are satisfied that everything is in order - add the one solder bridge (coloured blue) to the underside of the board.

You're Now Ready To Test Your Circuit

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Transistor Alarm {C} - Support Material

Construction Guide