Introduction: How to Make a Fire Alarm With Analog-to-Digital & Comparator Module

This tutorial is give some basics on how to create a fire alarm using a LM35 Temperature Sensor, a buzzer and an Analog to Digital & Comparator Module. This Instructable will also show a video demonstration when fire is detected, the buzzer will ring.

Step 1: Introduction

The Analog to Digital & Comparator Module is a module that converts the analog signal to digital signal. It is using the LM339 as a comparator to compare the incoming voltage and the preset voltage. Thus, it is called comparator module. After comparing the incoming voltage and the preset voltage, it will give an output either active high or active low depending on the user’s setting. Analog to Digital & Comparator Module consists of 4 channels for the inputs and outputs. The on boards’ presets are used to set the compare voltage level.


  • Operating Voltage : 5-12 VDC
  • Output Signal : Selectable either Active High or Active Low Digital Signal
  • Board Size : 45mm x 45mm
  • Channels : 4 Inputs Channel, 4 Outputs Channel

Step 2: Material Preparation

For this tutorial, you need to have the following items:

  1. Analog to Digital & Comparator Module
  2. LM35 Temperature Sensor
  3. Female-to-Female and Male-to-Female Jumper Wires
  4. Buzzer
  5. 4 x AA Batteries

Step 3: Pin Definition

Step 4: Circuit Assembly

The Analog to Digital & Comparator Module can be used as a fire alarm by connecting the temperature sensor LM35 which is an analog type. The module converts the analog signal from LM35 to digital signal and activates the output. In this example, the buzzer act as an output.

Step 5: Change Output Mode

After completing the connection, change the output mode to Active High by removing and place back the mini jumper.

Step 6: Setting Up the Preset Temperature

To adjust the preset compare voltage and the LM35 sensitivity, tune the Voltage Level Adjustor (Potentiometer) for the desired activated temperature.

Step 7: Results

The final circuit is shown in the first picture. Then the circuit can be tested by putting a fire near the LM35.

For the result, the LED (D1) and the buzzer is on when the fire is being put near the LM35. This indicates that the voltage from the LM35 is increased due to the temperature increase. When the voltage from LM35 increases until higher than the preset compare voltage, a high signal (6V) is giving to the output pin 1 (OUT1) which will then activate the buzzer.

Step 8: Video