The components used in the circuit should be widely available. However, none of them are critical. So - if you can't find the specified parts -
You're certain to find something that will do just as well.
The alarm is intended primarily for use on a motorcycle. I didn't want the circuit to drain the motorcycle battery - so I designed it to have a very low standby current. This was achieved by using normally-open trigger switches and by making sure that when the alarm is set there are no resistive connections between the positive rail and ground. In standby mode the power supply to the IC is cut - and the current flowing in the remainder of the circuit is negligible.
The low standby current means that the alarm may be powered by dry-batteries. This would make it more secure - because it could not be defeated by disconnecting the motorcycle battery. It also means that the alarm may be used in situations where a power source is not available. For example, it could be fitted inside a computer - to sound the alarm if a thief tries to pick it up and carry it away.
In case you are not familiar with it, 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 required has 12 rows with 31 holes in each; and measures roughly 8 cm by 3 cm (3 in by 1.2 in). 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.
"Mercury Tilt Switches" are generally small glass bulbs with two contacts at one end. Inside the bulb is a "ball" of mercury. When the switch is "tilted" a few degrees off the horizontal - the mercury flows to one end and connects the contacts together.
Mercury tilt switches are expensive. You may prefer to use the cheaper "non-mercury" type. The main difficulty with these is that - unlike mercury switches - you can't actually see what's happening inside. This can make them troublesome to position accurately. When setting-up the switching point, you may find it helpful to use a small buzzer - or an LED connected in series with a 2k2 resistor. Alternatively - you could
Add The LED Module to the alarm - and use it to help you position the switches.
Parts List
Construction Notes
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 21 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 8 mm drill-bit will do. Just use the drill-bit as a hand tool - there's no need for a drilling machine.
Next make and fit the
Six Wire Links. I used bare copper wire on the component side of the board. Telephone cable is suitable - the single stranded variety used indoors to wire telephone sockets. Stretching the core slightly will straighten it - and also allow the insulation to slip off.
Then fit the 9 resistors. They are all shown lying flat on the board. However, those connected between close or adjacent tracks are mounted standing upright.
The next stage is to fit the 4 transistors, the 3 diodes, the relay and the IC socket. Using a socket reduces the risk of damage to the IC; and makes it easier to replace should the need arise. Again, the diodes connected between close or adjacent tracks are mounted standing upright. Note that the PNP transistor (BC557) is the one with the emitter coloured red.
Next - fit the 4 capacitors. Pay particular attention to the orientation of the electrolytic capacitors. Make sure that the positive terminals are to the top.
Then examine the underside of the board 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 8 solder bridges to the underside of the board. These are just small blobs of solder. I've used them to connect adjacent track. They are a simple and convenient alternative to wire links.
Finish off by inserting the NE555N into the socket. Pin 1 of the IC should be in the top left-hand corner. Check that all 8 pins have entered the socket. Sometimes - instead of entering the socket - a pin will curl up under the IC.
You Are Now Ready To Test Your Alarm
