This document describes the operating principles of 4 alarm circuits: a frost alarm, fire alarm, burglar alarm, and time delay circuit. The frost alarm and fire alarm use thermistors so that the transistor switches on or off depending on temperature. The burglar alarm uses an LDR so the transistor switches based on light levels. The time delay circuit uses a capacitor that charges over time to switch the transistor on after a set period.

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 Frost AlarmFrost Alarm

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When the thermistorWhen the thermistor
temp is high thetemp is high the
resistance is low andresistance is low and
the transistor is off.the transistor is off.

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 Frost AlarmFrost Alarm

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When the thermistorWhen the thermistor
temp is low , thetemp is low , the
resistance is high andresistance is high and
the transistor switchesthe transistor switches
on.on.

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 Fire AlarmFire Alarm

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When the temp is hot theWhen the temp is hot the
thermistor resistance is low sothermistor resistance is low so
it takes little voltage and so theit takes little voltage and so the
resistor takes most of theresistor takes most of the
voltage and so the transistorvoltage and so the transistor
switches on.switches on.

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 Fire AlarmFire Alarm

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When the temp is cold theWhen the temp is cold the
thermistor takes mostthermistor takes most
voltage and so thevoltage and so the
resistor takes little andresistor takes little and
the transistor is off.the transistor is off.

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 Burglar AlarmBurglar Alarm

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When the L.D.R. is in theWhen the L.D.R. is in the
light ,the resistance is lowlight ,the resistance is low
so the voltage is low and theso the voltage is low and the
transistor is offtransistor is off

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 Burglar AlarmBurglar Alarm

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When the L.D.R. is in the darkWhen the L.D.R. is in the dark
its resistance is high and theits resistance is high and the
transistor switches on.transistor switches on.

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 Time Delay CircuitTime Delay Circuit

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When the capacitor chargesWhen the capacitor charges
up to a certain voltage afterup to a certain voltage after
a certain time , the transistora certain time , the transistor
switches on.switches on.

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There are 3 process devices,There are 3 process devices,
The NOT gateThe NOT gate
The AND gateThe AND gate
The OR gateThe OR gate