Circuit : Andy Collinson
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An easy to use LED bargraph display to monitor audio levels. Circuit monitors a single channel, so build two for stereo.
NotesPlease note that for convenience the circuit is not drawn with the IC in pin order. The LM3914 is an 17 pin DIL IC and its pinout is shown below left. The IC has internal current limiting so no external resistors are required.
Typical LED bargraph display, as shown right are available in many colours including red, green or yellow from suppliers such as ESR electronics in the UK.
The LM3914 requires a DC voltage applied to the input signal on Pin5. About 5.54Vdc should light every output LED. As audio signals are AC they first require changing to DC. This is easily acheived with a half wave rectifier, R4 and D11, C2 removes the DC voltage on the previous stage. As the input voltage is a little high it is boosted by a x10 single stage amplifier made with Q1 and associated components. VR1 acts as an input level control and VR2 (which can be a preset) allows adjustment for full scale signal. Should more audio gain be required, then R1 can be made a preset and the centre terminal connected to a 100u capacitor.
The output display can be made into a dot ( a single LED segment indicates the audio peak) or a bargraph where multiple segments are lit to display the input signal. This can be selected using S2.
DatasheetThe Texas Instruments datasheet (link below) includes many more circuits and application data.