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Circuit Classics: 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.

About This Project

Forrest M. Mims III is a trusted name in the electronics world for good reason: his charming and engaging texts have drawn millions of people into the world of electronics for the first time. I have brought three of those hand-drawn circuits projects to life by creating an exquisitely designed series of finely crafted and highly detailed boards. These are the Circuit Classics. They make a great gift for a first-time learner, an expert tinkerer, or even just as a fun conversation piece for your desk.

Star Simpson

In the kit

  • 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, 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.

Testing

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.

Experimentation

A fast, fun and easy way to use the Bargraph Voltage Indicator after you've put it together is to use the white alligator clip as a test lead to iprobe the test points along the left side of the circuit board. When you measure the test points on the Bargraph Voltage Indicator, you are using the tool to test its own internal reference voltages - the internal levels that the circuit compares input voltage against. The reference voltages are created using the voltage from whichever battery or power source you have connected to it, which is divided by the four resistors into steps of roughly 25% each of overall voltage. The instrument works by comparing the voltage on the input side to the voltage at those points. So when you measure those points, you’re comparing the voltage at that node to the main source voltage overall. That’s why each point lights up each LED in steps of about 25% of total voltage!

You can also use the Bargraph Voltage Indicator to measure other voltages, for example such as batteries that you might want to test for charge. Be careful to only measure voltages that are less than the main power source, however! For example, an AA battery should be about 1.4 volts and can be measured if your bargraph voltage indicato has a a 3, 6, or 9V source. But if you’d like to measure something with a voltage above 3 volts, you must use a 6 or 9V power supply for the bargraph voltage indicator. Additionally, remember not to use anything greater than 9V to power the Bargraph Voltage Indicator.

Finally, here is a fun way to use the Bargraph Voltage Indicator to measure the voltage drop across a potentiometer, where you use a potentiometer as a two-resistor voltage divider and measure the change in voltage as you turn the dial: