20 & 30 Segment, LED Voltmeters

Article Created "July 2010"
Updated "July 16 2010"

These are Basic Expanded Voltmeters based on the National LM3914 chip.
They can be Switched to either Bar or Dot Mode.

The 20 LED one has a Minimum 2.5 volt range for Full Scale and it can be adjusted to Much Higher voltage ranges with the input potentiometer.

So you could easily set it for 20 or even 200 Volts Full Scale, Using a Suitable Potentiometer on the Input. DO NOT EXCEED 30 VOLTS to PIN 5 of the LM3914's.

I designed this 20 LED PCB a few years ago and have posted it now as requested by one of my readers.

The 20 K resistor Shown across the LED is required to null out the #10 LED, when higer LED's are lit up in Dot Mode. Leaving this resistor off will cause #10 (Also #20 on the 30 LED one) to light Faintly when LED's above #10 are lit. This 20 K resistor is soldered to the Copper Side, due to space limitations!

All Capacitors Should be Tantalums, A Recommended by National Semiconductor.

I designed The 20 LED unit to operate from any Voltage Supply beween 5 to 12 volts MAXIMUM. It runs with about 10 mA per LED and Exceeding 12 Volts Supply Voltage in Bar Mode WILL cause Overheating of the LM3914. In Dot Mode you can Safely operate this circuit up to a 20 Volt supply.

By Reducing LED Currents, These units can be operated at 15 Volts in Bar Mode.

This NEW, Three LM3914 Version (Shown Below) has a Minimum 3.75 Volts for full Scale. It is Programmed for around 8 mA per LED, so it can operate from Voltages Up to 15 Volts in BAR Mode, without damaging the LM3914's.

As an EXAMPLE: If you set the Input to 15 Volts Full Scale, You will now have a reading of 1/2 Volt per LED.


I can supply the LED displays as shown, they are listed in my "Parts" area. Look at "LED's, Bar Displays"

They are $1.50 each plus Shipping Cost.

The PCB and All the other parts are also available from me. Postage cost on these parts depends on total weight and where you live.

These prices are based on US Funds.

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 The Schematic NOTE: On Tantalum Caps, The Black line is Usually Positive.

 The 20 LED Circuit Board

 20 LED Picture Overlay

 A 30 LED Version,  Shown in Dot Mode. NOW Also Available.

 The 30 LED, Schematic.

 The 30 LED, Circuit Board.

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