Four small battery-powered transistor alarm circuits - you can build yourself - 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 through D1 and into the base of Q2.

This switches Q2 on.

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When Q2 switches on - it connects the base of Q1 to ground.

This causes base-current to flow - through R2 - into emitter-base junction of Q1.

So Q1 switches on.

And current through R2 and Q1 flows into the base of Q2.

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Now - it doesn't matter if you re-close the switch and cut off the R1 current.

Q2 no longer needs the R1 current in order to remain switched on.

It has provided itself with an alternative and constant supply of base current - through R2 and Q1.

In other words - Q2 has used Q1 to latch itself on.

While it's on - Q2 connects the negative lead of the buzzer to ground - and the buzzer sounds. 

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The normally-open trigger switch has a similar effect.

When the switch is closed - base-current flows into emitter-base junction of Q1- through R2.

So Q1 will switch on - and current through R2 and Q1 will flow into the base of Q2.

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This will cause Q2 to switch on.

When Q2 switches on - it connects the base of Q1 to ground.

Now - Q1 no longer needs the R4 connection to ground in order to remain switched on.

It has provided itself with an alternative and constant connection to ground through Q2.

In other words - Q1 has used Q2 to latch itself on.

Again - while Q2 remains switched on - the buzzer will sound.

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D1 allows the R1 current to reach Q2 - but it prevents the normally-closed switch from connecting the base of Q2 to ground.

Without the diode - Q2 would turn off the moment the trigger switch was re-closed.

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

Without R4 - the normally-open switch would have to carry the current drawn by the load.

R4 limits to less than 10mA - the current through the normally-open switch.

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

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

To high frequency signals - C1 represents a short circuit.

The capacitor shorts the signals to ground - before they can switch Q2 on.

R3 connects the base of Q2 to ground. It keeps C1 discharged.

It also keeps the transistor switched off - until required.

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The standby current is less than 100uA. 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.

If you leave out R1, D1 and the normally-closed switches - and use only normally-open switches - the standby current is reduced to Zero.

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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 3 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 {A}
Construction Guide

Next fit the 4 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 two transistors, the diode and the capacitor. The pnp transistor (BC557) is the one with the emitter arrow coloured red. Pay particular attention to the orientation of the diode and capacitor. Note that the side of the diode with the bar is pointing down - and the positive terminal of the capacitor 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.

You're Now Ready To Test Your Circuit

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Transistor Alarm {A} - Support Material

Construction Guide