Simple Door Alarm

Build this simple alarm circuit yourself using cheap off-the-shelf components. It's battery powered - the stand-by current is virtually zero - and it's very versatile. You can use it almost anywhere - on your gate - your shed - your caravan - your boat.

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Circuit Description

Click Here For A Photograph Of The Prototype.

Circuit Diagram For A
Cmos 4001 Door Alarm

Gates 1 & 2 of the Cmos 4001 are not used in the circuit. 

Their inputs are connected to the supply rails. 

Gates 3 and 4 of the IC are connected to form a monostable. 

Basically - a monostable provides an output for a certain length of time - and then switches itself off. 

How long the output lasts depends on the values of R2 and C2. 

With the values shown - it will last for somewhere between 3 and 5 seconds.

Increasing the value of the resistor and/or the capacitor will increase the output time.

To learn more about how a monostable works - click on the link below.

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To keep the standby current low - the value of 4M7 was chosen for R1. 

In standby mode - SW1 is closed. 

And the current through R1 is flowing to ground through the switch. 

This means that Pin 8 is low - and C1 is discharged.

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When SW1 is opened - the current through R1 is no longer being diverted to ground.

Instead - it charges C1 - and takes Pin 8 high. 

When pin 8 goes high - the monostable triggers. 

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The output from the monostable is at Pin 11. 

It supplies base-current to Q1. 

The base-current switches the transistor on. 

The transistor connects the negative lead of the Buzzer to ground - and the Buzzer sounds. 

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The Buzzer will go on sounding until C2 charges through R2.

When the capacitor takes Pins 12 & 13 high - Pin 11 will go low. 

Now - Pin 11 is no longer providing base-current for Q1.

So the transistor switches off - and the Buzzer is silenced.

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Small piezo type Buzzers generally draw a current of around 10 mA. 

However - the BC 547 will cope reliably with a current of up to 50 mA.

This is about half its Ic(max). 

So instead of using a Buzzer - you can connect the coil of a small relay to the output terminals. 

The relay contacts can then be used to operate practically any type of output device.

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R3 limit the current taken from Pin 11 to about 2 mA.

This is more than enough base-current to switch any small transistor fully on. 

I used a BC547 because they are cheap and widely available. 

But any small transistor - with a gain (hfe) greater than 100 - and an Ic(max) of at least 100mA - should do.

Just remember that the pin configuration of your transistor may be different from that of the BC547.
 
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If you're using a relay - make sure that its coil doesn't draw more than about 50 mA - otherwise the transistor might be overloaded. 

For example - if you are using a 9-volt supply - the resistance of the relay coil should be at least: - 

   9 volts ÷ 0.05 amps = 180 ohms 

However - when the alarm is activated - a higher resistance coil will reduce the drain on the battery.

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When SW1 is opened - R1 takes Pin 8 high - the monostable triggers - and the Buzzer sounds for 3 to 5 seconds. 

It's the act of taking pin 8 high that triggers the monostable. 

While Pin 8 remains high - the circuit cannot be re-triggered. 

It should be obvious that - before you can take pin 8 high again - it must first be taken low. 

In other words - you have to close SW1 first - before you can re-open it and re-trigger the monostable.

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Cmos gates react very fast. 

Stray electrical interference could cause the monostable to trigger. 

A 100nF capacitor represents a virtual short circuit to high frequency signals.

C1 shorts to ground any high frequency interference picked up by the wires going to SW1.

It makes the circuit more stable - by slowing down the response time of the trigger.
 
C3 does the similar job of removing stray interference from the supply line. 

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You'll notice a small fraction of a second delay between the opening of SW1 and the sounding of the Buzzer. 

The response time of the trigger is fixed by the values of R1 & C1. 

If you increase the values - you will slow down the response. 

If you decrease the values - you'll speed it up.

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Relay coils and some sounders produce high reverse-voltage spikes that will destroy Cmos ICs. 

D1 is there to short-circuit the spikes before they can do any damage. 

It should protect the 4001 from whatever device you choose to connect to the output terminals.

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Click Here To Learn More About How A Monostable Works

Parts List

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Construction Notes

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

Stripboard or Veroboard is a board drilled with a matrix of 1mm holes spaced approximately 2.5 mm apart and joined in rows by copper strips. The piece used has 11 rows with 21 holes in each; and measures roughly 5 cm by 2.5 cm (2" by 1"). The drawing shows the board with PCB mounting terminal blocks but - to save money and/or space - the wires may be soldered to veropins or directly to the board itself.

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 10 places where the tracks are to be cut. Before you cut the tracks, use the "actual size" drawing to Check That The Pattern is Correctly Marked .

Actual Size

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 - 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.

Battery Powered
Alarm - Layout 1

Next fit the nine components and the two links. For clarity, I've split them into three groups - but you can fit them in any order you like. I use a small piece of "Blu Tack" to help hold the components and links in place temporarily - while I solder them to the board. A little putty or modelling clay should work equally as well.

The resistors are all shown lying flat on the board. However, R2 and R3 are mounted standing upright. For the two links - use the off-cuts of wire you've trimmed from the resistors.

Simple Alarm - Layout 2

Next, examine the board very 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're satisfied that everything is in order - add the 7 solder bridges to the underside of the board.

Then double check the position and orientation of all of the components. Make sure that the positive side of C2 is connected to Pin 12 - and that the cathode of D1 (the side with the bar) is connected to Pin 14.

Finally, insert the 4001 into the socket. Make sure that pin 1 is in the top left-hand corner - and check carefully that all of the pins are correctly inserted into the socket. Sometimes - instead of entering the socket - a pin will curl up underneath the IC.

You Are Now Ready To Test Your Alarm

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Door Alarm - Support Material

Parts List

Construction Notes

Actual Size