The ubiquitous LED continues to expand frontiers and applications with a plethora of sizes, colors, and output lumens. Flashing LEDs that incorporate internal electronics for oscillation offer embedded applications the advantage of size. However, designs that require multiple flashing LEDs are impeded by the reality that individual flashing LEDs are automatons and will blink asynchronously. In addition, these flashers can be an order of magnitude more expensive than their nonflashing brethren.
Here's a way to economically synchronize an array of standard LEDs using only two additional components (see the figure). A flashing LED is required to "set the pace" of the standard "slave" LEDs. When the flashing LED turns on, its corresponding current is sensed by the transistor via the 47-(omega) resistor. The transistor's collector current then drives the standard LEDs.
This design is scalable as long as the collector-current rating of the transistor isn't exceeded. The test circuit used 17 "slave" LEDs plus the flashing LED. Peak collector current measured 155 mA using the 3-V rail. A final version of the circuit incorporated a surface-mount transistor and resistor, which occupied less space than a standard LED. Cost and size restraints make this an ideal circuit for embedded applications such as toys or indicators.
I hate all you Indians trying to graduate from univertisty for an Electrical Engineering degree ask ing for things so ******* rudely.
Anonymous -September 08, 2006
Joseph Balinda: This site is a "think tank" not a engineering samples shopping center.
Anonymous -September 06, 2005 (Article Rating: )
I like what you people do with electronics. Send me some samples to try out. Thank you.
Joseph Balinda -January 13, 2005
The circuit in the Design Brief, "Resistor and Transistor Synchronize Flashing LEDs" has a serious problem. The current in the standard LEDs is very poorly controlled and will vary dramatically due to normal component variation, temperature changes, number of LEDs driven, etc. and can easily be high enough to physically damage the components. Perhaps the diagram was miscopied - simply adding some current limiting resistors would solve the problem. Also, you should give the model number for the flashing LED since the sense resistor must be chosen accordingly. -- paul
The author's response:
Paul, the VCE of the 3904 is about 1V, and with a 3V rail, you're left with 2V, which is perfectly fine for driving most LEDs, without the need for a resistor. Most current limiting resitor applications happen at the full supply rail, or at 5V. If, by chance, a user is trying to apply a special case LED, then that engineer would know enough to put a resistor to limit the current. However, the vast majority of LEDs will work with this circuit as-is. Let me know if you require any further data. Thanks. --Robert Most
Paul -November 04, 2004
Your Comments:
Enter the text from the image below
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
Electronic Design UPDATE provides readers with late-breaking news, opinions from industry experts, and timely technology stories. It's a unique opportunity to get your product message in front of engineers, engineering managers, and corporate managers while they're reading about critical information online.
Reprints
)
