Discrete Components Enable Random Neon Display

Jul 1, 2007 12:00 PM
By Richard Dunipace, Principal Technical Marketing Engineer, Fairchild Semiconductor, Irving, Texas

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Toys and novelty items sometimes require random light displays to create a desired visual effect. However, creating a battery-powered random light display that consumes minimal power is a significant challenge. The simplest approach is to use LEDs with either unregulated or linearly regulated power. This would be visually pleasing but pose a significant drain on the system's battery. Still, the efficiency could be improved using a high-efficiency power supply.

LEDs are typically the first choice for power-efficient portable lights. However, I decided to use neon bulbs for their low-power consumption and the ease with which they could be made into a relaxation oscillator — just one resistor and one capacitor. This way each bulb could blink independently of any other and truly produce a random display. This also would simplify the overall design.

Neon bulbs require 65 V to 135 V to fire, depending on the bulb selected. This would require an efficient step-up power supply. Additional features such as low-cost construction with no special parts, low-standby power consumption and a regulated output voltage would ensure the display operated consistently throughout battery life. The final specifications for the supply would be 120-V regulated output at 68 A and regulated operation from 4 V to 9 V (6 AAA cells) at better than 88% efficiency (6-V input).

The power supply is a fixed on-time design that uses the off time to regulate how often the inductor charges. This approach avoids any inductor saturation issues and limits the maximum current drawn from the battery. R1, C1 sets the on time. The off time is set by R2, C1 and the conduction of Q1. At start-up, Q1 is biased by R3 to maximum conduction. As the voltage on the output capacitor C2 rises toward 120 V, Q1's conduction decreases as a result of the breakdown of Zener diode D3. The net effect is to increase the amount of time it takes to recharge C1 and complete the power cycle.

There are 48 instances of the neon bulb relaxation oscillator circuit, each powered from the same 120-V output of the power supply. With the 22-MΩ resistor and 0.1-μF capacitor, each bulb produces a single strobe burst once about every two seconds. Due to variations in the bulbs, capacitors and resistors among all the instances of the circuit, the fl ashing will be unsynchronized, and therefore the general pattern will appear random.