Submission, CMOS 555 Long Duration Red LED Flasher

Thread Starter


Joined Mar 24, 2008
Status: Ready for Proof Reading
Illustrations: Complete
Experiments: Schematic verified, duration test running since 2/15/09 7AM.
……………..5/16/09 - Flasher still going, has been dimming steadily for the last 2 months. Still usable, but less than ½ brightness.
……………..5/25/09 - Declared dead. It is still flashing, but about 10% brightness.



  • Two AAA Batteries
  • Battery Clip (Radio Shack catalog # 270-398B)
  • One CMOS TLC555 timer IC (Radio Shack catalog # 276-1718 or equivalent)
  • Q1 - 2N3906 PNP Transistor (Radio Shack catalog #276-1604 (15 pack) or equivalent)
  • Q2 - 2N2222 NPN Transistor (Radio Shack catalog #276-1617 (15 pack) or equivalent)
  • D1 - Red light-emitting diode (Radio Shack catalog # 276-041 or equivalent)
  • C1 - 1 µF Tantalum Capacitor (Radio Shack catalog 272-1025 or equivalent)
  • C2 - 100 µF Electrolytic Capacitor (Radio Shack catalog 272-1028 or equivalent)
  • R1 - 1.5 MΩ ¼W 5% Resistor
  • R2 - 47 KΩ ¼W 5% Resistor
  • R3 - 2.2 KΩ ¼W 5% Resistor
  • R4 - 27 Ω ¼W 5% Resistor (or test select a better value)
  • One DVM or VOM


Lessons In Electric Circuits, Volume 1, chapter 16: Voltage and current calculations
Lessons In Electric Circuits, Volume 1, chapter 16: Solving for unknown time
Lessons In Electric Circuits, Volume 3, chapter 4 : Bipolar Junction Transistors
Lessons In Electric Circuits, Volume 3, chapter 9 : ElectroStatic Discharge
Lessons In Electric Circuits, Volume 4, chapter 10: Multivibrators

  • Learn a practical application for a RC time constant
  • Learn one of several 555 timer Astable Multivibrator Configurations
  • Working knowledge of duty cycle
  • How to handle ESD sensitive parts
  • How to use transistors to improve current gain
  • How to calculate the correct resistor for a LED






NOTE! This project uses a static sensitive part, the CMOS 555. If you do not use protection as described in Volume 3, Chapter 9, ElectroStatic Discharge, you run the risk of destroying it.

The circuit shown in the previous experiment, CMOS 555 Long Duration Minimum Parts Red LED Flasher, has one big drawback, which is a lack of LED current control. This experiment uses the same basic 555 schematic and adds transistorized drivers to correct this.

The parts used for this transistor driver are non critical. It is designed to load the TLC555 to an absolute minimum and still turn on Q2 fully. This is important because as the battery voltage approaches 2V the drive from the TLC555 is reduced to its minimum values. Bipolar transistors can be good switches.

Since LEDs can have so much variation R4 should be tweaked to match the specific LED used. The current is limited to 18.5ma with 27Ω and a Vf (LED forward dropping voltage) of 2.5V, an LED Vf of 2.1V will draw 33ma, and a LED Vf of 1.5 will draw 56ma. The latter is too much current, not to mention what that would do for the battery life. To correct this use 47Ω if the Vf is 2.1V, and 75Ω if the Vf is 1.5V, assuming the target current is 20ma.

You can measure Vf by using the jumper shown in red in the illustration, which will turn the LED on full time. You can calculate the value of R4 by using the equation: R4 = (3V-Vf) / 0.02A

It was mentioned in the previous experiment that capacitor C2 extended the life of the batteries. An interesting experiment is to remove this part periodically and see what happens. At first you will notice a dimming of the LED, and after a week or two the circuit will die without it, and resume working in a couple of seconds when it is replaced. This flasher will work for over a month using fresh alkaline AAA batteries.


The CMOS 555 oscillator was explained fully in the previous experiment, so the transistor driver will be the focus of this explanation.

The transistor driver combines elements of a common collector configuration on Q1, along with common emitter configuration on Q2. This allows for very high input resistance while allowing Q2 to turn on fully. The input resistance of the transistor is the ß (gain) of the transistor times the emitter resistor. If Q1 has a gain of 50 (a minimum value) then the driver loads the TLC555 with more than 100KΩ. Transistors can have large variations in gain, even within the same family.

When Q1 turns on 1ma is sent to Q2. This is more than enough to turn Q2 fully, which is referred to as saturation. Q2 is used as a simple switch for the LED.
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Joined Jan 18, 2008
Don't you need a jumper between the two (+) rails? I suspect some beginners would not know the rails need that connection.

Since this forum is international and Radio Shack (RS) may not even be national shortly, regular part numbers or use of part numbers that can be referenced to regular part numbers might have greater utility. For example, the RS LED is simply an RS part number with very little description on RS's web site and no link to a datasheet. A table with RS's numbers and Mouser's or Digikey's numbers would provide that cross-referencing ability that RS's numbers alone lack.


Thread Starter


Joined Mar 24, 2008
Look at columns 12 and 13 on the protoboard illustration, and you will see the jumpers you're looking for. I've still got to update the resistor color codes.

I hear you about Radio Shack, but for the moment they are the standard. You will note in the parts and materials I give a complete description, I use the RS part number so people can get what they need if a Radio Shack happens to be near by. Speaking for myself, I have never been more than 20 miles away from one in my entire life, and at the moment can count 3 or more in a 5 mile radius.

I suspect the "or equivalent" is going to be a lot more common in the future.

I didn't invent these standards for the book, I'm just maintaining them. I've already pushed the envelope a bit with the Theory of Operation, which I felt was lacking in the other articles. To some extent I find the restrictions well, restrictive, but I want my articles to intigrate with the rest as good as I can.

BTW, thanks for the feedback!
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Joined Jan 18, 2008
Sorry about the first error. I saw those jumpers, but the didn't ring my bell.

As for RadioShack, I didn't realize it was a requirement. I also live close to 3 Radio Shacks and two fire hydrants. I haven't had the need to use either in the past year. :D


Thread Starter


Joined Mar 24, 2008
I already have 4 more articles in the back of my mind, two of which have actually been started.

CMOS 555 Long Duration Blue LED Flasher
CMOS 555 Flyback LED Flasher
555 VCO
555 Function Generator

I have heard some folks complain that this site (AAC, not just my articles) seems to use the 555 for everything. It's hard to fight a winner though, that little sucker really is a swiss army knife of electronics. I'm advocating a separate chapter in the Experiments volume myself, even if I have to write it.

BJMicro moved, they aren't in walking distance for me anymore. They are still within 10 miles though. They aren't meant for a walk in shop though, but they allow it (thank you!). Me and Ellie May, their fully grown puppy of a Great Dain with one blue eye and one brown eye, are good friends, which could have something to do with the dog treat and petting I give while I'm there. She likes to put her nose into my pockets (all of them, shirt included) to see what I brought. Good thing I like dogs.

Tanner's is pure walk in (no real internet business) and 30 miles away, but it is an old fashion electronics store, the kind that used to be common (and isn't). I outfitted my resistor kit for 2¢ each, because of the quantity. I think of it as a candy store, even among the old fashioned electronics stores they are bigger and better. Just hope they stay in business a long time.

I still shop at Radio Shack, I have to use their parts for the experiments. Their battery holder selection is actually very good, high quality (better than Tanner's or BJMicro), comprehensive, and not too much more expensive. Just wished they carried micro clips, which is what I actually use instead of what is drawn.

You like the latest battery illustration?
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Thread Starter


Joined Mar 24, 2008
This article is also done, baring any last minute edits. The LED is still bright, but dimmer. Less so than the minimum parts version, which is probably because the transistors turn on more predictably.

Any final comments?