Motorcycle Alarm No.1 - Construction Guide

When one of the normally-open switches is closed - two things happen. Firstly - the negative side of the relay coil is connected to ground through D1. This causes the relay to energize. And the relay in turn sounds the siren. = Secondly - the emitter-base junction of Q1 is forward-biased. The transistor switches on - and current through R3 and Q1
Motorcycle Alarm No.1 - Construction Guide - schematic

charges C1. C1 is part of the latching circuit that keeps the relay energized after the trigger-switch has been re-opened. = When the switch is re-opened - and Q1 switches off - the charge stored in C1 keeps Q2 switched on. Q2 in turn keeps Q3 switched on. Q3 connects the negative side of the relay coil to ground. So the relay remains energized and the siren continues to sound. It will continue to sound until C1 discharges through the base-emitter junctions of Q2 and Q3. = The length of time the relay remains energized depends on the value of C1 - and the input impedance (resistance) of Q2 and Q3. A single transistor has a relatively low input impedance - so C1 would discharge quickly. But Q2 and Q3 are connected to form a modified Darlington Pair. The input impedance of a Darlington Pair is very high. Roughly that of the single transistor - squared. And R4 increases it even further. So C1 discharges relatively slowly through the two base-emitter junctions. The precise time it takes depends on the characteristics of the actual components used - particularly the gain (hfe) of the transistors. = As C1 discharges - the current through R4 and Q2 falls. So the base current through Q3 falls also. This causes the Q3 collector-emitter voltage to rise. As it does so - the voltage across the relay coil falls. When the voltage across the relay coil falls to a sufficiently low level - the relay drops out and the alarm resets. = However...

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