Today we’re going to have a look at an old favourite, the LM3914 Dot/Bar Display Driver. The LM3914 can measure analogue voltage levels and convert that reading to a display of ten LEDs in bar or dot format. By no means is this a new or fantastic product, but for the enthusiast and experimenter it has several qualities that make it attractive…
These qualities include:
- price – it’s cheap, less than $2
- It can interface with TTL or CMOS logic
- You can throw up to +/-35V at the inputs
- each LED can draw up to 10mA
- you can daisy chain 2 or more LM3914s to make one heck of an LED meter display
- you can use a supply voltage between 3 and 25V DC
So it is very forgiving and can produce some fun and useful displays. But first, let’s say hello…
The basic workings of the LM3914 are quite simple and ingenious. It has a ten step voltage divider and an adjustable voltage reference. This means our IC can act as a basic voltmeter, using ten or multiples of ten (each with another LM3914) LEDs to display the result. The resistors R1 and R2 work as a voltage divider, and the voltage across R2 is the range of which the meter will display. For example, if your voltage across R2 is 1.2V, with the difference “displayed” between LEDs 1 and 10 will be ~1.2V. For more information, please see the data sheet (below).
Anyhow, what are we waiting for? Let’s make a voltmeter, that can measure between 0 and ~5V DC using the following example from the data sheet.
That doesn’t look to difficult… will use a 9V PP3 battery for a supply. I have some nifty little LED bar graph modules to use, which will look nice and save time. For a test input signal, I have used an LM317T on a separate board as a variable signal source.
The LM317T has a minimum output voltage in this case of 1.28V and the highest it could generate using a 6.4V source (4 x AA cells) was 5.25v. So when you view the clip below, that is the minimum and maximum sample voltage we used. The LM3914 circuit is meant to measure voltages between 0 and 5V. Let’s see how the demonstration circuit behaved in the following clip. Note that to set the mode from bar to dot requires removing the connection between pin 9 and Vs, s we do this then repeat the voltage range measurement in the second half of the clip. No audio in clip.
Notes: In writing this post, I used information from National Semiconductor, and information from old books by Forrest Mims III. Thank you! More specific manufacturer information from National Semiconductor can be found on their website, or download their data sheet. National Semiconductor LM3914
In the meanwhile have fun and keep checking into tronixstuff.com. Why not follow things on twitter, Google+, subscribe for email updates or RSS using the links on the right-hand column? And join our friendly Google Group – dedicated to the projects and related items on this website. Sign up – it’s free, helpful to each other – and we can all learn something.