R Total =


* 18.6 scales to 6.2, then each 0.25 volts = 0.0833.

** R5 and R6 are added together for required resistance of 1,640 Q, which equals 18.60 VDC.

*** R17 and R18 are added together for required resistance of 4,800 Q, which equals 12.00 VDC.

* 18.6 scales to 6.2, then each 0.25 volts = 0.0833.

** R5 and R6 are added together for required resistance of 1,640 Q, which equals 18.60 VDC.

*** R17 and R18 are added together for required resistance of 4,800 Q, which equals 12.00 VDC.

Copying to a Circuit Card

The next step is easy. To copy the circuit onto a circuit card, install and solder the components in the following order:

1. Pre-cut wires, using the breadboarded circuit as a template.

2. The DIP sockets for the op-amps and LED array (observe orientation).

3. Resistors.

4. Diodes.

6. Wire connections to the outside.

Take your time, watch the index marks, and use the correct color (length) of wire. A simple mistake takes a long time to correct once the part is soldered into place.

The completed voltage monitor on circuit card

The completed voltage monitor on circuit card


An accurate voltmeter is required to calibrate the LED display. Adjust the bench power supply to 13.5 VDC. Resistor R3 is adjusted until LED #6 just turns on. Adjust the bench power supply in 0.25 VDC steps, and verify that the LEDs follow.

A variable power supply is required to build and test this project. If you plan to build many circuits, you may want to invest in one. Be forewarned that these power supplies can be expensive—over US$150. A cheap alternative is to build your own. A circuit based on the LM317AT is a good starting point. You can download the LM317AT data sheet from National Semiconductor's Web site. Three 6 VDC lantern batteries connected in series make a good "raw" power source for the LM317AT. Avoid flooded lead-acid batteries on your test bench. A spark could cause the battery to explode.

A 10 ohm, 1/2 watt resistor should always be placed in series with the output of the variable power supply. The resistor will limit current if a mistake is made in circuit layout (short or incorrect polarity). This will limit the damage to the circuit, the breadboard, and you! Parts can get hot—hot enough to melt the breadboard and give you a nasty blister if you mistakenly touch one of them.

Don't be surprised if your 10 ohm resistor bursts into flame while you are prototyping. This is actually a good thing—the resistor saved your circuit. A fuse could be used to limit the current, but fuses get to be expensive after a few mistakes.

Final Tips

Take your time! Breadboarding, like any other skill, takes time to master. Purchase more parts than you will need. This is especially true of the op-amps. Shop around; surplus electronics dealers can save you lots of money.

This project reinforces learning to build by using functional blocks. Circuit complexity forces you to build and then test each block. You will be glad to know that there are simpler ways to design this project. But I encourage you to build it as described here. Build a small simple block, and test it. Build another and test it. It's the best way to learn electronics—building, testing, and finally understanding.

Build other projects and apply these ideas. With practice, understanding will come before building. Then you

Clean soldering on the back side of the circuit card.

will see your ideas become reality. Construction electronics can be a rewarding experience. I hope this article inspires you to try!


Aaron Dahlen, 8285B Valley View Dr., Sebastopol, CA 95472 • [email protected]

Radio Shack, 100 Throckmorton St., Fort Worth, TX 76102 • 800-843-7422 or 817-415-3011 Fax: 817-415-3240 • [email protected]

Digi-Key, PO Box 677, Thief River Falls, MN 56701 800-DIGIKEY or 218-681-6674 • Fax: 218-681-3380 [email protected]

National Semiconductor Corporation, 2900 Semiconductor Dr., Santa Clara, CA 95052 408-721-5000 • • Data sheets and application notes for numerous semiconductors

CadSoft Computer, Inc., 801 South Federal Hwy., Ste. 201, Delray Beach, FL 33483 • 800-858-8355 or 561-274-8355 • Fax: 561-274-8218 [email protected] Freeware (limited) for drawing schematics and routing circuit cards

Data sheet for the LM324N op-amp is available at

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