This is a bare unassembled circuit board for a 20 segment gain reduction bargraph based on the design published in the Pro Audio Design Forum. The picture shows an assembled unit.

The KA-Electronics GR20 board provides a simple, easy-to-use gain reduction meter for the Waveulator, compressors and control voltage displays.

SImple LM3914-based Driver

The GR20 uses two LM3914N constant-current drivers with jumper-selectable bar/dot mode, 2.4V scaling and multiple powering options. Higher full scale voltages can be displayed by inserting a series resistor in the input.

Multiple powering options

The GR20 has separate inputs for LED power and LM3914 power as well as the ability to use an on-board regulator.

When a switching power supply is used to power a device's +/-15V rails a 5V supply is also usually available. The GR20 in bar mode typically consumes 150-170 mA at full scale. (Approximately 8 mA per LED plus LM3914 current.) Use of the auxillary 5V supply not only reduces audio supply current but it also permits LED return current to flow to a non-audio supply.

When a higher-voltage supply is only available an optional 7805 three terminal regulator can be installed to reduce LED voltage and LM3914 dissipation.

The LM3914 is normally powered from the LED supply but can be externally supplied from a separate input or the unregulated LED supply input.

An auxiliary current-limited LED output is available to illuminate a power switch or separate power on indicator.

Power Supply Requirements and Dimensions

The board requires +5V minimum at approximately 160 mA for the bar graph mode. Dot display requires approximately 20 mA.

The board dimensions are: 1.0" x 3.1".

Mounting holes: #4/M3 0.125" dia on 2.3" spacing.

The construction thread for the GR20 meter is featured in the Pro Audio Design Forum.

The GR20 is sold as either a bare PC board, a "kit" including two LM3914 or an assembled and fully-tested module.

A Mouser Electronics Project Manager bill-of-materials is available for DIY builds of the GR20.