The LM3915 offers a simple way to display a logarithmic voltage level using one or more groups of ten LEDs with a minimum of fuss. If you’re wanting to make a VU meter, you should use the LM3916 which we will cover in the final instalment of this trilogy.
Instead of having each LED represent a voltage level as with the LM3914, each LED connected to the LM3915 represents a 3 dB (decibel) change in the power level of the signal. For more on decibels, check out Wikipedia. To display these power level changes we’ll run through a couple of examples that you can use in your own projects and hopefully give you some ideas for the future. Originally by National Semiconductor, the LM391X series is now handled by Texas Instruments.
You can order LM3915 ICs from PMD Way with free delivery, worldwide.
Step 1: Getting Started
You will need the LM3915 data sheet, so please download that and keep it as a reference. First – back to basics. The LM3915 controls ten LEDs. It controls the current through the LEDs with the use of only one resistor, and the LEDs can appear in a bar graph or single ‘dot’ when in use. The LM3915 contains a ten-stage voltage divider, each stage when reached will illuminate the matching LED (and those below it in level meter mode).
Let’s consider the most basic of examples (from page two of the data sheet) – a simple logarithmic display of voltage between 0 and 10V.
After building the circuit you can connect a signal to measure via pin 5, and the GND to pin 2. We’ve built the circuit exactly as above on some stripboard for demonstration purposes, with the only difference being the use of an 8.2kΩ resistor for R2.
To show this in action we use a signal of varying AC voltage – a sine wave at around 2 kHz. In the following video, you can see the comparison of the signal’s voltage against the LEDs being illuminated, and you will see the logarithmic voltage increase represented by the LEDs.
Well that was a lot of fun, and gives you an idea of what is possible with the LM3915.
Step 4: Displaying Weaker Signals
In non-theoretical situations your input signal won’t conveniently be between 0 and 10 V. For example the line level on audio equipment can vary between 1 and 3V peak to peak. For example, here’s a random DSO image from measuring the headphone output on my computer whilst playing some typical music.
Although it’s an AC signal we’ll treat it as DC for simplicity. So to display this random low DC voltage signal we’ll reduce the range of the display to 0~3V DC. This is done using the same method as with the LM3914 – with maths and different resistors.
Consider the formulae. As you can see the LED current (Iled) is simple, however we’ll need to solve for R1 and R2 with the first formula to get our required Vref of 3V. For our example circuit I use 2.2kΩ for R2 which gives a value of 1.8kΩ for R1. However putting those values in the ILED formula gives a pretty low current for the LEDs, about 8.3 mA.
Live and learn – so spend time experimenting with values so you can match the required Vref and ILED.
Nevertheless in this video we have the Vref of 3V and some music in from the computer as a sample source of low-voltage DC. This is not a VU meter!
Again due to the rapid rate of change of the voltage, there is the blue between the maximum level at the time and 0V.
Step 7: Chaining Multiple LM3915s
This is covered well in the data sheet, so read it for more on using two LM3915s. Plus there are some great example circuits in the data sheet, for example the 100W audio power meter on page 26 and the vibration meter (using a piezo) on page 18.
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