Build this simple motorcycle alarm circuit yourself using cheap off-the-shelf components. It can be adapted to 6-volts for your "Classic Bike" - and it won't drain your battery.
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# Motorcycle Alarm No.4 - Support Material

Free Circuit

Simulation

## Circuit Description

Motorcycle Alarm No.4 - Circuit Simulation

 When one of the switches is closed - the base of Q1 is connected to ground through D1 & R2. This switches Q1 on - and it in turn switches Q2 on. Q2 connects the positive side of the relay coil to the supply line. The relay energizes - and the siren sounds. =========== If R1 had a high value - moisture on the switches might be enough to lower the voltage - at the junction of R1 & R2 - to the point where Q1 would switch on. Using a 1k resistor for R1 eliminates this problem. =========== R2 limits the current through the emitter-base junctions of Q1 and Q2. It also controls the speed at which the capacitor charges. C1 is part of the latching circuit that keeps the relay energized after the trigger-switch has been re-opened. I wanted the relay to latch quickly - so that even a brief closing of the trigger-switch would activate the alarm. That's why R2 is a 1k resistor. It charges C1 very quickly. =========== When the trigger-switch is re-opened - it's the charge stored in C1 that keeps the transistors switched on. So the relay remains energized and the siren continues to sound. It will go on sounding until the charge in C1 falls to a level where it can no longer keep the transistors switched on. At this point - the relay will drop out and the siren will stop. =========== How long this takes to happen depends on the size of the capacitor and the value of R3. I selected both - by trial and error - to give a delay of about a minute. There is no point in trying to calculate values. The precise time it takes for the siren to stop depends on the characteristics of the actual components used. If you want to increase the time it takes for the siren to stop - use a larger value capacitor. If you want to decrease the time it takes for the siren to stop - reduce the value of R3. =========== Q1 & Q2 are wired together to form a Darlington-Pair. A single transistor has a relatively low input impedance. C1 would discharge very quickly through its emitter-base junction. So a very much larger capacitor would be needed to produce a one-minute output. The Darlington-Pair has a very high input impedance. It's roughly that of the single transistor - squared. In fact it's so high that it has almost no influence on the rate at which C1 discharges. Consequently, the time it takes for C1 to discharge is controlled almost entirely by the value of R3. =========== The low value of R1 offers some protection from the effects of condensation on the trigger-switches. However, R1 has to be prevented from discharging C1. This is the purpose of D1. The diode creates a one-way path. When a trigger-switch connects R2 to ground - D1 allows C1 to charge through R2. However - when the switch is re-opened - D1 prevents C1 from discharging through R1. =========== The transistors are used to switch the relay on and off. Most of the work is done by Q2. If the relay has a coil resistance of at least 270 ohms - then the maximum current passing through Q2 will be about 12 ÷ 270 = 45mA. The BC557 has an Ic(max) of 100mA. There is nothing special about the BC557. Any small transistors 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 BC557. =========== Relay coils and some sounders can produce high reverse voltage spikes that will destroy sensitive electronic components. D2 and D3 are there to short-circuit these spikes at source - before they can do any damage. Although there is nothing in the alarm circuit itself that's likely to be damaged - I have no idea what other electronic equipment might be connected to the same supply. So I included the two diodes as a precaution. =========== If the alarm switches are fitted properly - the circuit will reset less than 1 minute after the bike has been returned to its centre-stand or kick-stand. If it's not returned to one of its stands - and at least one of the trigger-switches remains closed - the siren will continue to sound. Generally speaking - the volume of a siren is related to the current it draws. The more current - the louder the noise. A typical siren will take about 300mA. =========== I don't advise using the bike's own horn in place of the siren. The horn is generally easily accessible. The thief can simply disconnect it. However - if you choose to use the horn - remember that the alarm relay is too small to switch the heavier load. Connect the coil of a suitably rated relay to the Siren output - and use its contacts to sound the horn. ===========

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. I also made sure that there are no purely resistive connections between the positive rail and ground.

On a modern motorcycle you will normally use a 12-volt relay and a 12-volt siren. But the circuit will work at 6-volts. So you can use it to protect your "Classic" machine. Just choose a relay and a siren suitable for the lower voltage.

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 9 rows with 24 holes in each - and measures roughly 6 cm by 2.5 cm. (2.5 in by 1 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

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 9 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.

Next fit the three resistors and the two links. I use a small piece of "Blu Tack" to 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, R1 is mounted standing upright. For the two links, use the off-cuts of wire you've trimmed from the resistors. Then fit the 2 transistors and the 3 diodes. Again, D3 is mounted standing upright

Fit the capacitor and the relay. Pay particular attention to the orientation of the capacitor. Note that the positive terminal faces downwards.

Next - double check the position and orientation of all of the components. Then examine the board very carefully - to make sure that there are no unwanted solder bridges or other connections between the tracks. When you're satisfied that everything is in order - add the three solder bridges to the underside of the board.

You can use a keyswitch or a hidden switch to operate the alarm; but add a small relay, and the alarm can be made to operate itself automatically. Below are two different circuits. In the first, every time you switch off the ignition the alarm will turn itself on - and every time you switch on the ignition the alarm will turn itself off. The second circuit works in the same way - but with the added security provided by Sw1. The ignition key alone will not turn the alarm off. You need to push Sw1 before the relay will energize.

Whichever circuit you use, the relay will only be energized while the ignition is on. When the bike is parked and the alarm is on, the relay coil is not using any current. So there is no drain on the battery.

Depending on the circuit, you'll need a single or double-pole relay with a contact rating of at least 1-amp. When you have it all wired-up correctly, protect it from the elements by wrapping it well with several layers of electrical tape.

## Add an Immobilizer to the Machine

Before fitting this or any other immobilizer to your bike, carefully consider both the safety implications of its possible failure - and the legal consequences of installing a device that could cause an accident. If you decide to proceed - you will need to use the highest standards of materials and workmanship.

Remember that the relay must be suitable for the current it's required to carry. Choose one specifically designed for automobiles - it will be protected against the elements and will give the best long-term reliability. You don't want it to let you down on a cold wet night - or worse still - in fast moving traffic!!!

Please note that I am UNABLE to help any further with either the choice of a suitable relay - or with advice on its installation.

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