"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 LM3914 system from less than 3V, there is a companion LM3915 device".
"The LM3914 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".
One typical application of the LM3914 is a 0V - 5V Bar Graph Meter.
LM3914 - 0V to 5V Bar Graph Meter
The only limit on the use of these versatile, yet cheap devices (or its companion LM3915) are your imagination.
Imagine a device which contains all of the following in one chip.
Its own adjustable reference and accurate 10-step voltage divider
The buffer drives 10 individual comparators referenced to the precision divider
Using the LM3914 - controller, visual alarm, and expanded scale functions are easily added on to the display system
The LM3914 circuit can drive LEDs of many colors, or low-current incandescent lamps
The LM3914 is very easy to apply as an analog meter circuit
The LM3914 can also act as a programmer, or sequencer.
Easily cascadable by adding more LM3914's to over 100 LEDs
In recent weeks I have been asked these "How can I?" questions:
I am a mechanical Engineer with some electronics background. I need a circuit that can be linked with a pressure transducer that provides 4.5 VDC output @ 200 psig pressure with 5 VDC input. The circuit needs to do the following.
a)A green LED should light up when the transducer is pressurized between 185 psig to 195 psig.
b)A red LED should light up when the transducer is pressurized over 195 psig.
c)In addition, a [email protected] 90 to 95db should sound when the transducer is pressurized between 185psig to 195 psig.
Any help will be greatly appreciated.
A further "How can I?":
I live in a rural area and rely upon a tank water supply. Is there any means of having some sort of a visual gauge to tell me the water levels in the tank?
I'm sort of OK in electronics and have constructed a few simple kits and I'm learning more from your site.
P.S. Thanks for the great site I understand things a lot better now.
For obvious reasons, it is an impossibility for me to fully answer all of these questions asked of me each and everyday.
Now with a little ingenuity and some associated circuitry, both of these projects could possibly be tackled with the LM3914 or perhaps the LM3915 (later), although I'm not suggesting that the LM3914 is the only possible answer, merely the first which springs to mind.
It was in response to both those questions that I put up this page. With the pressure transducer it may be possible to simply work off the LM3914 or LM3915 data sheets.
The water tank would certainly need some kind sensor circuit to provide an input to the LM3914. I would visualise perhaps a rod inserted in the tank which might provide say five different leads coming off which represent in actual height, the 20%, 40%, 60%, 80% and 100% fill levels. This would be part of a resistor ladder network connected to V+ so that 1V, 2V, and so on represent the inputs to the LM3914.
As I said, the only limit is your imagination and I've seen some very creative applications for both the LM3914 and LM3915 over the years.
The simplifed LM3914 block diagram [see page 6 of the data sheet] is to give the general idea of the circuit’s operation. A high input impedance buffer operates with signals from ground to 12V, and is protected against reverse and overvoltage signals. The signal is then applied to a series of 10 comparators; each of which is biased to a different comparison level by the resistor string.
In the example illustrated, the resistor string is connected to the internal 1.25V reference voltage. In this case, for each 125 mV that the input signal increases, a comparator will switch on another indicating LED. This resistor divider can be connected between any 2 voltages, providing that they are 1.5V below V + and no less than V - . If an expanded scale meter display is desired, the total divider voltage can be as little as 200 mV. Expanded-scale meter displays are more accurate and the segments light uniformly only if bar mode is used. At 50 mV or more per step, dot mode is usable.
The National Semiconductor PDF data sheet provides schematics for the following circuits:
0V - 5V Bar Graph Meter
Cascading LM3914s in Dot Mode
Zero-Center Meter, 20-Segment
Expanded Scale Meter, Dot or Bar
“Exclamation Point” Display
Indicator and Alarm, Full-Scale Changes Display from Dot to Bar
Bar Display with Alarm Flasher
Adding Hysteresis (Single Supply, Bar Mode Only)
Operating with a High Voltage Supply (Dot Mode Only)
20-Segment Meter with Mode Switch
The National Semiconductor PDF data sheet provides a number of handy application hints for using the LM3914.
• “Slow” —fade bar or dot display (doubles resolution)
• 20-step meter with single pot brightness control
• 10-step (or multiples) programmer
• Multi-step or “staging” controller
• Combined controller and process deviation meter
• Direction and rate indicator (to add to DVMs)
• Exclamation point display for power saving
• Graduations can be added to dot displays. Dimly light every other LED using a resistor to ground
• Electronic “meter-relay” display could be circle or semi-circle
• Moving “hole” display —indicator LED is dark, rest of bar lit
• Drives vacuum-fluorescent and LCDs using added passive parts
The LM3915 is a monolithic integrated circuit that senses analog voltage levels and drives ten LEDs, LCDs or vacuum fluorescent displays, providing a logarithmic 3 dB/step ana-log display. One pin changes the display from a bar graph to a moving dot display. LED current drive is regulated and programmable, eliminating the need for current limiting re-sistors. The whole display system can operate from a single supply as low as 3V or as high as 25V. Multiple devices can be cascaded for a dot or bar mode display with a range of 60 or 90 dB. LM3915s can also be cascaded with LM3914s for a linear/ log display or with LM3916s for an extended-range VU meter.
See Data Sheet - (366K) in PDF format.
LM3914 Dot/Bar Display Driver - PDF Data Sheet - 366kB
LM3915 Dot/Bar Display Driver - PDF Data Sheet - 603kB
LM3916 Dot/Bar Display Driver - PDF Data Sheet - 510kB
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