Bar Graph Temperature Indicator using LM35

This is a simple temperature sensor circuit diagram using LM35 temperature sensor and LM3914 IC bar graph indicator. Here the complete temperature range is indicated through an array of 10 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. 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.

Step 1: Components Required

1. LM35

2. LM3914

3. LED's

4. Resistors

5. Capacitor 47uF

Step 2: Circuit Diagram

Working:

LM3914:

The LM3914 is a monolithic integrated circuit that senses analog voltage levels and drives 10 LEDs, providing a linear analog display. A single pin changes the display from a moving dot to a bar graph. Current drive to the LEDs is regulated and programmable, eliminating the need for resistors.This feature is one that allows operation of the whole system from less than 3V. The circuit contains its own adjustable reference and accurate 10-step voltage divider. The low-bias-current input buffer accepts signals down to ground, or V−, yet needs no protection against inputs of 35V above or below ground. The buffer drives 10 individual comparators referenced to the precision divider. Indication non-linearity can thus be held typically to 1⁄2%, even over a wide temperature range. Versatility was designed into the LM3914 so that controller, visual alarm, and expanded scale functions are easily added on to the display system. The circuit can drive LEDs of many colors, or low-current incandescent lamps. Many LM3914s can be “chained” to form displays of 20 to over 100 segments. Both ends of the voltage divider are externally available so that 2 drivers can be made into a zero-center meter. The LM3914 is very easy to apply as an analog meter circuit. A 1.2V full-scale meter requires only 1 resistor and a single 3V to 15V supply in addition to the 10 display LEDs. If the 1

resistor is a pot, it becomes the LED brightness control. The simplified block diagram illustrates this extremely simple external circuitry. When in the dot mode, there is a small amount of overlap or “fade” (about 1 mV) between segments. This assures that at no time will all LEDs be “OFF”, and thus any ambiguous display is avoided. Various novel displays are possible. Much of the display flexibility derives from the fact that all outputs are individual, DC regulated currents. Various effects can be achieved by modulating these currents. The individual outputs can drive a transistor as well as a LED at the same time, so controller functions including “staging” control can be performed. The LM3914 can also act as a programmer, or sequencer. The LM3914 is rated for operation from 0°C to +70°C. The LM3914N-1 is available in an 18-lead molded (N) package. The following typical application illustrates adjusting of the

reference to a desired value, and proper grounding for accurate operation, and avoiding oscillations.

LM35:

The LM35 series are precision integrated-circuit temperature sensors, whose output voltage is linearly proportional to the Celsius (Centigrade) temperature. The LM35 thus has an advantage over linear temperature sensors calibrated in ° Kelvin, as the user is not required to subtract a large constant voltage from its output to obtain convenient Centigrade scaling. The LM35 does not require any external calibration or trimming to provide typical accuracies of ±1⁄4°C at room temperature and ±3⁄4°C over a full −55 to +150°C temperature range. Low cost is assured by trimming and calibration at the wafer level. The LM35’s low output impedance, linear output, and precise inherent calibration make interfacing to readout or control circuitry especially easy. It can be used with single power supplies, or with plus and minus supplies. As it draws only 60 μA from its supply, it has very low self-heating, less than 0.1°C in still air. The LM35 is rated to operate over a −55° to +150°C temperature range.