BarGraph Ccts

Posted on Feb 6, 2014

The LED meter is simpler and smaller than it`s analogue counterpart, and is very common in audio equipment. This version is based on a National Semiconductor IC, and uses the logarithmic version. Each LED operates with a 3dB difference from the previous one, and a jumper is provided to allow dot or bar mode. This project is also an essential part of the expandable analyser to be published soon,

BarGraph Ccts
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and one meter circuit is used for each frequency band. There are many other uses for a simple LED VU meter. They are ideal as power meters on amplifiers, can be used with mixers (including the high quality mixer described in the project pages), preamps and any other application where it is important to know the signal level. The circuit is completely conventional, and is based on the application notes from National Semiconductor. The circuit is shown in Figure 1 and as you can see it uses a single IC and a few discrete components. There are two rectifier circuits so that the DC to the LEDs is almost unfiltered. This allows a higher LED current with lower dissipation than would be the case if the DC were fully smoothed, and full smoothing would also require a much larger capacitor. This increases the size and cost of the project - especially important if it is to be used for the expandable analyser, since there will be at least 10 meter circuits needed. L1 to L8 will normally be green (normal operating range) and L9 and L10 should be red (indicating overload). This gives a 6dB overload margin when the unit is calibrated as described below. As shown, full scale sensitivity (with VR1 at maximum) is 12 Volts peak (approximately 8. 5 volts RMS). This is designed for direct connection to the speaker output of an amplifier, but is still suitable for use with preamps if the sensitivity is changed. Power comes from a 15-0-15...

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