Simple High Temperature Alarm Circuit using IC 555

This circuit uses a special chip IC 555 to act like a temperature watchdog.

It constantly checks the temperature, and if it gets too hot, the circuit triggers an alarm usually a buzzer to let you know somethings sizzling.

This way, you can catch overheating problems before they become a big issue.

Circuit Working:

Parts List:

The high temperature alarm will emit a beep and cause the LED to blink if the devices temperature rises unusually.

This simple overheating alarm is designed to monitor heat generating devices such as motors and inverters.

The circuit employs a basic astable multivibrator using the low power CMOS t IC 555, which is a low-power version of the popular 555 IC.

The alarm is triggered by the reset pin 4 of IC1.

The astable circuit will operate only when the reset pin 4 goes high.

This pin is connected to the positive rail through a 10k NTC thermistor.

The NTC thermistor exhibits high resistance when cold which decreases to a few ohms as the temperature in its vicinity rises.

When the temperature is low as set by the thermistors position near the device, the reset pin of IC1 remains low, the astable is off and the buzzer remains silent.

As the temperature near the thermistor increases, its resistance decreases providing voltage to the reset pin of IC1, causing the astable to activate.

Note: Adjust the thermistors position near the device to maintain silence when the heat is normal.

Formula:

A high temperature alarm circuit may be created with an astable multivibrator circuit that uses an IC 555

When set up in astable mode, the IC 555 may provide an output that is a continuous square wave.

Then, you can utilize this square wave to activate an alarm or buzzer that alerts people to a high temperature.

frequency (f):

When employing the IC 555 to create an astable multivibrator, the oscillation frequency is determined by:

f = 1.44​ (R1 + 2 x R2) ×C1

where,

• f is the output frequency in hertz Hz
• R1 and R2 is the resistors in ohms Ω
• C1 is the capacitor in farads F
• 1.44 is a constant derived from the internal workings of the 555 IC

How formula works:

An output waveform that oscillates continuously is produced by setting the IC 555 in astable mode.

Using the above formula, the values of R1, R2 and C1 define the frequency of this output.

Important Notes:

In order for the formula to provide the right frequency in Hertz, the units of R1, R2 and C1 must be in ohms, ohms, and farads respectively.

An estimate of the frequency is given by the formula.

Because of several factors, including component tolerances, actual values may differ somewhat.

Although the 555 IC can be configured in several ways, this formula only applies to the astable mode.

How to Build:

To build a Simple High Temperature Alarm Circuit using IC 555 follow the below mentioned connections steps:

IC Connections:

• Connect Pin 1 to ground
• Connect Pin 2 to pin 6 with a 0.01µF capacitor in parallel to ground.
• Connect Pin 4 to the positive supply through the NTC thermistor 10k resistor.
• Pin 8 to the positive supply voltage

LED Indicator:

• Connect the anode of the LED to pin 3 through a current limiting resistor
• Connect the cathode of the LED to ground

Power Supply:

• Connect the positive terminal of the power supply to pin 8.

Testing:

• Power up the circuit and adjust the position of the NTC thermistor near the heat source to set the temperature threshold for the alarm.
• When the temperature exceeds the threshold, the alarm will sound and the LED will blink.

Adjustment:

• Fine tune the position of the NTC thermistor to ensure the alarm remains silent during normal operating temperatures.

Note:

• Ensure to double check all connections and component polarities before powering up the circuit.

Conclusion:

The High Temperature Alarm Circuit using IC 555 is a simple yet effective circuit that utilizes the popular 555 timer IC to sound an alarm and activate an LED when the temperature of a device exceeds a set threshold.

It employs an astable multivibrator configuration and an NTC thermistor to detect temperature changes.

When the thermistor detects a high temperature, it triggers the 555 timer IC causing the alarm to sound and the LED to blink, alerting users to the elevated temperature.

References:

555 timer IC