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:

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:

V_out​ = V_in​ × R2​​ / R1 ​+ R2​

where,

• Vin is the voltage input
• R1 and R2 are the values of the resistors
• Vout is the output voltage.

Charging and Discharging of Capacitors:

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

τ = R × C

where,

• τ is the time constant in seconds
• R is the resistance in ohms
• C is the capacitance in farads

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) = V_in​ × (1− e− ^{t / RC}​)

where,

• V(t) is the voltage across the capacitor at time t
• Vin is the Input voltage
• e is the base of the natural logarithm which is approximately 2.718
• t is the time in seconds
• R is the resistance in ohms
• C is the capacitance in farads

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:

I_B​ = ​V_in​ − V_BE​​ / R_B

where,

• Base emitter voltage VBE, is usually 0.7V for silicon transistors.
• Base resistor value ohms is equal to RB.
• Base current in amperes is IB

Collector Current:

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

I_C​ = β × I_B​

Calculating FET Current:

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

ID​ = 1 / 2​ × K × (V_GS​ − V_GS(th)​)²

where,

• K is the transconductance parameter variable depending on FET type
• Gate source voltage is represented by VGS.
• VGS(th) is the voltage at the gate source threshold

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:

• Refer to the circuit diagram for the connections between the components.
• Ensure that the H21A1 module is positioned such that the infrared LED and phototransistor are on opposite sides of the smoke detection slot.

Assembly:

• Connect the H21A1 module to the rest of the circuit as per the diagram.
• Connect the transistors T1, T2, T3, T4 in the darlington pair configuration.
• Connect the SCR T3 to the darlington pair output and the MOSFET T4 to the gate terminal of the SCR.

Adjustments:

• Adjust trimpots P1, P2 and the value of resistor R1 for optimal performance.
• These components are used for sensitivity adjustment and can vary based on the specific requirements of your setup.

Testing:

• Power the circuit using a 9V DC input supply.
• Observe the behavior of the visual indicator LED D1 and the piezo sounder when smoke is introduced into the detection slot.
• Adjustments may be needed for sensitivity or functionality.

Installation:

• Once the alarm is working as expected install it in the desired location where smoke detection is required.

Note:

• Please note that building electronic circuits requires basic knowledge of electronics and safety precautions.
• If you are not familiar with electronics it is recommended to seek assistance from someone experienced in electronics assembly.

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