555 Thobbing Led Issues

Thread Starter


Joined Sep 22, 2009
Hi All,

I wonder if somebody can help me see where i've gone wrong.

I've followed a few guides on the web as to make a 555 LED "Throb" and this is what i've got so far.

BC141-10 - Low Power LF NPN Transistor in TO39 Case
555 Timer
100v 100 uF
33K Resistor

Pin 1 - Ground
Pin 2 - Connected to pin 6 & Base of BC141-10 NPN
Pin 3 - 33K Resistor to base of NPN
Pin 4 - Connected to pin 8 and 9V +
Pin 5 - Not in use
Pin 6 - Connected to pin 2 & Base of BC141-10 NPN
Pin 7 - Not in use
Pin 8 - Connected to pin 4 and 9V +

100v 100uF connected + to base of NPN and - to negative rail.

LED + from Emitter of NPN, and - from the negative rail.

With no load on I can see 2v - 5v ramping up and down on my multimeter, but when a load is connected the voltage stays constant, for example with a blue led it stays on 3.3V, and a yellow led 3v.

It fades up when turned on but then just holds.

Can anybody suggest any ideas? I've used the bigger NPN as I wanted it to be capable of "throbbing" 10 Yellow LED's.

I know not all the guides say to connect pin 2, to the base of the NPN as well, but without doing so I just get a constant voltage of 8V at the emitter of the NPN.

Thanks very much.
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Thread Starter


Joined Sep 22, 2009

With connecting the LED's directly to pin 8, what kind of current am I going to be limited too?

10 Yellow LED's, are around 400mA.


Joined Feb 4, 2008
If you find its datasheet in google you will read that it can supply 200mA on its output (pin 3).


Joined Mar 24, 2008
We've got several other people with similar projects. Last I heard you wanted a comet tail, which has been done and I responded to. You didn't show a schematic on your first post on this thread, which leaves us all guessing as to what you're doing.

If you want a throbbing LED (as in singular) it is much simplier.

I did not include small resistors since these circuits will operate in the high side of the pot settings, so don't let the pots reach 0Ω.

You can increase power savings by upping R1 and R2 to 100KΩ, and C1 and C2 to 22µF, and using a CMOS 555. R3 was selected for a green LED. Any general purpose PNP transistors will work, but don't use prepackaged Darlingtons, they need to be separate. Define your specific needs and we can show you other ways.

Have you read this?

LEDs, 555s, Flashers, and Light Chasers

Bill's Index

You will find another independent thread with vertually the same name in the char forum, it is one of several that have brought this up.
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Thread Starter


Joined Sep 22, 2009

Thanks to everybody for their replies so far, they’ve been really helpful.

Apologies for posts in a few new topics in the past 48 hours, have a new project on that I need help in a few areas, but I’m really glad I found this forum!