Simple LM35 Electronic Temperature Sensor Indicator Circuit

This is a simple temperature sensor circuit diagram using LM335 temperature sensor and LM3914 IC. Here the complete temperature range becomes observable through an array of LEDs. The LM35 is an integrated circuit temperature sensor for sensing temperature in degree Celsius scale. The LM3914 is an efficient display driver cum milli volt measuring IC and it is very easy to apply as an analog meter circuit.

In this temperature monitoring system the milli volt output from the LM35 is converted to digital temperature indicator by means of LM3914 IC. The LEDs in this circuit shows the temperature levels digitally via 20 steps. This temperature indicator circuit is absolutely the simplest to build, as it is based on a single stand alone IC LM3914 by Texas Instruments, which executes the entire process of indicating the temperature range.

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Circuit diagram of digital temperature sensor

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Components required for temperature monitor circuit

1. Power supply (7V or above)
2. 1% or 2% film resistors (200Ω; 499Ω x 2; 1kΩ; 1.2kΩ; 1.5kΩ x 2)
3. Capacitors 16V or above (1µF; 20µF)
4. Potentiometers (1kΩ x 3)
5. LM35 temperature sensor
6. LM3914 driver IC
7. 20 LEDs

Working of temperature indicator

• The LM35 series are precision integrated-circuit temperature sensors, whose output voltage is directly proportional to the Celsius (Centigrade) temperature range of operating temperature (−55° to +150°C).

• The LM35 converts the temperature into milli volt range at its output pin. For each single degree variation in the temperature, the IC LM35 generates an output with a 10 mV variation.

• LM35 is capable of working with single power supplies and plus minus power supplies. It draws only 60 μA from its supply, it has very low self-heating.

• Here LM3914 display driver IC is used to convert the LM35 output voltage to indicating LEDs.

• The IC LM3914 is basically a milli volt measuring tool which is able to convert a varying milli volt input into an equivalent LED indication providing a linear analog display.

• Current through the LED is regulated and programmable, this aspect allows operation of the entire system from less than 3V and eliminating the requirement for resistors.

• Refer the block diagram of LM3914, a high input impedance buffer functions with signals from ground to 12V and is protected against overturn and over voltage signals. This comparator drives 10 different LEDs with respect to a reference voltage.

• The reference voltage is given by the resistor potential divider network.

• Here the resistor thread is coupled to the internal 1.25V reference voltage. Thus for each 125mV increase of input signal, a comparator will switch ON further LED.

• LED brightness control is another feature of this IC. The current drawn out of the reference voltage pin (pin 7) decides LED current.

• Pin 9, the Mode Select input it is used to interconnect two LM3914 ICs. Here we have connected two ICs using this mode pin.

• Under rapidly changing signal condition, the built-in hysteresis avoids the jumping of LED immediately from one to other.

• Due to the presence of internal regulated transistors, LM3914 has the ability to work with low or fluctuating voltages. It also permits the display driver to interface with logic circuitry, opto-coupled solid-state relays and low current incandescent lamps.

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Pinout

• LM35
• LM3914

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