Posted on Feb 5, 2014

This circuit provides a cheap alternative to the LM3915 series LED displays. The meter relies on a square-wave oscillator built around two CMOS analog switches, which alternatively selects the right and left channels for monitoring and display. The selected signal is amplifted by the common-emitter stage T1, and the output is fed into the string c

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omparators which control the display. These eight comparators are from two LM324 quad op amps, each is connected to a resistor network, which has a 3dB step between each comparator. Each comparator has a positive feedback resistor to increase the hysteresis to provide a longer display, which is switched altematively at about 10 kHz. The two CMOS switches in line are biased at half the supply voltage by 1-M © resistors from a 100-k © divider, which allows them to handle analog signals up to 9 V peak to peak. As the voltage increases above the setpoint of each comparator, the output goes low and the corresponding LED lights, which produces a bar of light in response to the input voltage. For a linear response the resistor-network can be replaced by nine 10-k © resistors, giving an equal voltage gap before each LED comes on.

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