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# Electronics AS project

Extracts from this document...

Introduction

Fire Alarm Aim: The aim of this project is to create a fire alarm which will detect fire or the effects of fire and as a result, with a buzzer, aware any occupants and persons in the surrounding area. To do this, I have researched how to construct this particular circuit. Fire is a major hazard and without the proper warning indications, can lead to devastating consequences. This is a device that can solve this life-threatening problem. Research > From reading through a basic guide to fire alarms from www.firesafe.com, I have learnt that a minimum of 65 dB is required for a fire alarm in general areas or 5 dB above any background noise which persists for more than 30 seconds. If the alarm is to be placed where people are sleeping and are to be woken then 75 dB is required at the bedhead. > A thermistor with a resistance of around 100 kilo-ohms at room temperature should be used to make it sensitive to the high temperatures of fire (about 100 C) but not anything much lower. When in high temperatures such as this, the resistance of the thermistor should drop to only a few ohms, drastically increases the current. ...read more.

Middle

The same thing happens with the LDR but as the brightness increases the resistance drops and the voltage travelling through the LDR is pulled high and is passed into the AND gate; switching on a \buzzer alarm. This is the system that I will investigate and use for my system. (Fig.1) Reason for preferred choice I decided to go with option B because I would like the circuit to be able to detect the heat and light of the fire, other then just the smoke as this can be accidently set-off by anything that may produce a little smoke but have no fire. Sub-system Development My complete fire alarm system will consist of the sub-systems previously shown in the block diagram (Fig.1). The LDR voltage comparitor Ratio 10:1 Therefore, trigger point = 100? Using resistors 100K? and 10K?, a 10:1 ratio was formed to work out the trigger point. With the 1K? as R1 the trigger point was made to be 100?, which is enough light from a fire to trigger the circuit. Resistor R1 forms a voltage divider with the LDR; the voltage to the non-inverting input being larger than the reference voltage when the LDR is in the light. ...read more.

Conclusion

x 100uF R1 + R2 = 1.5 seconds = 21.4K? => R1 = 7.1K? 0.7 x 100uF T-low = 0.7 R2 C 1.0s = 0.7 x R2 x 100uF R2 = 1.0 seconds = 14.3K? => R2 = 14.3K? 0.7 x 100uF Tests on Sub-Systems Voltage comparitor - LDR Inverting/non-inverting Voltage with light ON (V) Voltage with light OFF (V) Inverting 0.72 0.79 Non-inverting 0.51 5.20 Output 7.42 1.34 This shows that when the light is on the output becomes high (7.42V) as the inverting voltage is higher than the Non-inverting voltage.The sub-system is therefore working. Voltage comparitor - Thermistor Inverting/non-inverting Voltage with high temperature (V) Voltage with low temperature (V) Inverting 0.73 0.78 Non-inverting 0.70 1.60 Output 7.29 1.37 This shows that when the thermistor is in high temperature the output becomes high (7.29V) as the inverting voltage is higher than the Non-inverting voltage.The sub-system is therefore working. AND Gate Heat sensor input (V) Light sensor input (V) Output (V) 1.37 1.34 0 1.37 7.42 0 7.29 1.34 0 7.29 7.42 6.28 This shows that when both the inputs of the heat sensor and light sensor are HIGH the output voltage then also becomes HIGH. However, when either one of the sensors is LOW then the output voltage remains LOW. The sub-system is therefor working. Scott Jenkins ...read more.

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