Bargraph Voltage Indicator


This circuit lets you become a scientist and an engineer and marks the passage of that important milestone of using tools you have constructed yourself. With your own voltmeter you can measure batteries or use it to explore and understand other electronic circuits that you find in the world.

In The Kit

Photo Quantity Item
1 Bargraph Voltage Indicator PCB
1 Alcohol Prep Pad
1 Battery Clip
1 LM393N Integrated Circuit
8 1kΩ Resistor
1 100kΩ Potentiometer
4 White LED
1 Black Alligator Clip
1 Red Alligator Clip
1 White Alligator Clip
4 Rubber Foot
1 Wooden Stand

Required Tools

You'll need a couple of additional tools and materials in order to put this kit together: a soldering iron (such as this one optionally), and solder. You may like a "third hands" for holding the board while you working on it- or, if you like, you can use the wooden stand to hold the board while you solder. Batteries are not included in the kit, and you can choose to use the alligator clips to hook up a battery or power supply of your choosing, or you can assemble the SMT Battery Clip to the reverse side, in which case you'll want to supply either a 3V 2032 coin cell battery or two 3V 2016s (to form a 6V stack).

Assembly Steps

Many circuit builders find it helpful to assemble the components from shortest to tallest on the board. Insert low, flat components first, such as the 1k resistors. You may consider using the wooden stand to hold the board at a pleasing angle while you solder it. After fully inserting each resistor, bend the leads so that it stays in place while you turn the board over to solder it. Insert the LM339 chip - be careful to align the notch at the top of the chip with the notch in the outline on the board, as it is easy to solder a chip rotated 180° - and solder the #1 and #8 pins of the chip to ensure that it is fully inserted and held in place, then solder each of the next legs in order. After this, insert and solder the LEDs, and then the potentiometer. Then turn the PCB over, and as a final step, attach by soldering the SMT battery clip into place. If you like, you may also choose to clean the solder flux a bit using the alcohol pad, and you can assemble the included rubber feet either onto the circuit board (to give it a bit of grip laying flat) or onto the bottom of the wooden stand for long-term display.

How to Solder Beautifully


All four LEDs should be on when you power this circuit up, if you have built it correctly, as long as you are not using the white alligator clip on IN+ to measure a voltage. You can test the circuit by using it to measure itself - at one of the gold test points along the left-hand-side of the board. The potentiometer knob adjusts the sensitivity of the circuit (this might be useful if you are measuring a voltage that is much lower than the battery/supply voltage you are using, for example.) Remember: always measure a voltage that is less than what is powering this circuit.


Here is a fun way to use the Bargraph Voltage Indicator to measure the voltage drop across a potentiometer: