Nuts and Volts - July 2014

An oldie but goodie, the LM3914 is still being manufactured today and serves as the heart and soul of our desktop rocket.

You Can’t Hold a Roman Candle to This

Desktop fireworks is one of the simplest applications for the LM3914 as presented in the manufacturer’s technical datasheet (NOTE: The Texas Instruments LM3914 technical datasheet was used during the development of this project), with just one resistor used in the circuit. Adding to this solitary resistor are two blinking red LEDs which will be used as replacements for the final LED in the bar graph (LED 10). In other words, rather than the LM3914 lighting LED 10 in the bar graph, the two blinking LEDs will be triggered. Figuratively, these blinking LEDs serve as the explosion of the aerial shell that typically culminates the launch of a rocket.

In order to gain the maximum impact with desktop fireworks, the entire circuit is mounted on a piece of lightweight corrugated cardboard stock and is cut into the shape of a rocket. Here is where you can get creative and let either whimsy or realism guide your rocket design.

Regardless of your creative direction, however, remember to cut slots into the base of your rocket and on an extra set of fins. These two slots in the rocket and fins can then be slid together (forming a “+” base) for keeping your rocket on the “launchpad,” enabling you to ignite a desktop fireworks display at a moment’s notice. After you’ve mounted the circuit on your cardboard rocket base and installed a 3V lithium battery, operation is as easy as 3-2-1, FIRE! First, flick the SPDT power switch; you’ve just lit the fuse. Slowly rotate the potentiometer; the rocket streaks skyward. When the top two LEDs remain lit (remember, the last LED — LED 10 — is NOT connected to the LM3914; instead the two blinking LEDs will act as the “last” LED), rotate the pot two more “clicks.” The rocket explodes with bombs bursting in air. If you leave the final blinking LEDs powered, they will slowly ebb in and out of synchronization in a nice soothing visual effect.