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Simple Smoke Alarm Circuit

This smoke alarm circuit is like a super smelling nose for your house.

It sniffs out smoke in the air and if it finds any it sets off an alarm.

This alarm can be a loud buzzer, a flashing light or even a signal to turn on other alarms in your house.

The smoke detector itself can work in a few ways, either with a special light that cannot see smoke very well photoelectric or with a tiny radioactive source that hates smoke ionization.

The most important thing is, this circuit keeps you safe by letting you know if there is a fire.

Circuit Working:

Parts List:

CategoryItemQuantity
Resistors1k2
100k, 100Ω, 220Ω1 each
Preset 1M2
CapacitorsElectrolytic 100μF 25V2
Electrolytic 4.7μF 25V2
SemiconductorsTransistors BC5472
FET BS1701
SCR BT1691
OtherLED Red 5mm 20mA1
Optocoupler H21A11
Buzzer1

At the core of this basic smoke alarm lies the H21A1, a photo interrupter module comprising a gallium arsenide infrared LED linked to a silicon phototransistor within a plastic casing.

The slot positioned between the infrared emitter LED and the receiver phototransistor enables the interruption of the signal by smoke toggling the module output from ‘on’ to ‘off’.

When smoke infiltrates the slot it obstructs the infrared beam that falls on the phototransistor.

Consequently, the phototransistor ceases operation causing the transistors T1 and T2 configured as a simple darlington pair to conduct thereby activating the remainder of the circuit.

Upon the darlington pairs conduction the gate voltage of SCR BT169 T3 rises triggering the SCR.

Subsequently, the gate terminal of the MOSFET BS170 T4 receives a positive voltage through resistor R4, causing the active piezo sounder to energize and latch.

The visual indicator LED D1 promptly illuminates.

The buzzer emits a sound to indicate the presence of smoke unless the circuit is switched off by disconnecting the 9V DC input supply.

For optimal performance, adjust trimpots P1, P2 and the value of resistor R1.

Formulas:

You will need to utilize some fundamental electronic formula and concepts to figure out the right values for resistors, capacitors, and other components in order to create a smoke alarm circuit .

The following are some essential calculations and formula that may be helpful for various circuit components:

Values of Resistors and Capacitors

Formula for a Voltage Divider:

The voltage divider formula can be used to set a certain voltage for signal conditioning or biasing:

Vout​ = Vin​ × R2​​ / R1 ​+ R2​

where,

Charging and Discharging of Capacitors:

The RC time constant can be used to assess capacitors in timing circuits:

τ = R × C

where,

The rate at which the capacitor charges or discharges is determined by the time constant τ.

Voltage of Capacitor Through Resistor:

The voltage across a capacitor in a basic RC charging circuit as a function of time is:

V(t) = Vin​ × (1− e− t / RC​)

where,

Calculating Transistor Biasing Base Current:

With a BC547 NPN transistor, the base resistor RB and the input voltage Vin may be used to determine the base current IB:

IB​ = ​Vin​ − VBE​​ / RB

where,

Collector Current:

The transistors current gain Ib determines how the base current, Ib and collector current Ic are connected.

IC​ = β × IB​

Calculating FET Current:

For a FET, the drain current ID at saturation may be roughly expressed as follows:

ID​ = 1 / 2​ × K × (VGS​ − VGS(th)​)2

where,

You may use the components you already have to successfully create and evaluate the smoke alarm circuit with the aid of these formulas.

How to Build:

To build a Simple Smoke Alarm Circuit please follow the below mentioned circuit assembling steps:

Circuit Design:

Assembly:

Adjustments:

Testing:

Installation:

Note:

Conclusion:

In conclusion, a smoke alarm circuit is a vital safety device that detects smoke in the air and activates an alarm to warn occupants of a potential fire.

It relies on sensors and signal processing to detect smoke particles and triggers an alarm to prompt evacuation and fire mitigation efforts ultimately helping to save lives and protect property.

References:

Smoke detector

Datasheet H21A1

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