Simplest LED blinker

Hello friends,

So, I have found a circuit designed by Dick Cappels for a simple LED blinker (no Arduino involved). I have tested the circuit and gotten it to work with a 9V power supply.

"In this implementation, a common NPN transistor is used. In the circuit, a 1k resistor charged the 330 uf capacitor until the voltage became large enough to get the emitter-base junction to avelanche. In the oscilloscope image, it can be seen that the peak voltage (yellow trace) was a little bit less than 9 volts. At this point transistor turned on quickly and partially discharged the 330 uf capacitor through the LED and the 100 Ohm current limiting resistor."

Now I am hoping to scale down the power supply to a 3.5V battery or lower, which seems not to work with the circuit as it currently stands (the LED does not turn on at all). Can someone help suggest what parts of the circuit I should look at changing to allow it to function off a lower voltage? Smaller capacitor? Different transistor?

Thanks in advance,


That is really cool!! I always like it when I learn something unusual.

Sadly, I don't think you can lower the voltage. The entire process depends upon avalanching the emitter-base junction, and that takes voltage. You may be able to find a very-low-voltage transistor with a very low Vebo (the 2N2222, for example, has a Vebo of ~6V which is why this works at 9V). Maybe a super-high-frequency transistor like the 2SC3707 with a Vebo of only 2V will work (though I see that transistor is on the path to being "discontinued").

Still, Digi-Key has 'em in stock (surface-mount only) so get 'em while you can.

The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons

Nominal voltage is 2.25v (2.6v max, 90mA max) This should work at 3.5v with a current limiting resistor.

It's the transistor that needs the unusual voltage spec, not the LED...
The circuit makes use of an unusual property/behavior of the transistor (the BE "avalanche" that the web page talks about), and that's pretty much a fixed value for a particular transistor (and not a particularly tightly controlled one, either.)

An LED blinker with two transistors is pretty easy...

Where'd the OP's original circuit go?

It's the first circuit shown at this web site:

The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected

Thanks for the replies all! Looks like this circuit won't work for me, as I am hoping to install the LEDs into several balloons and therefor hoping to keep the cost/size per circuit as small as possible (and also why I am looking for independent units opposed to several connected and controlled by Arduino). "Blinking LEDs" are an option, but I am hoping to get a slower blink rate (maybe around 1 second on, 1 second off). Is it time to look at control chips like the 555?? Is there a cheap method?

westfw, could you write more about your 2 transistor blinker if you think it might function for my balloon scenario?

thanks again,


You might be able to use a transistor flip flop circuit like below.

I am hoping to install the LEDs into several balloons and therefor hoping to keep the cost/size per circuit as small as possible

Ah. "Smallest and cheapest" may not be the same as "simplest."

Anyway, the usual two-transistor flasher is known as an "astable multivibrator", and there are lots of examples and explanations of how it works on the net. I used it for an EAGLE tutorial here: Make Hobbyist PCBs With Professional CAD Tools by Modifying "Design Rules" : 15 Steps (with Pictures) - Instructables I actually used this circuit during some of my early days learning eagle, and have a bunch of PCB layouts of various sizes, including an double-sided SMT version that's about 12x9 mm.

To only blink one LED, just replace the one you don't want with a piece of wire.
The circuit has two transistors, two capacitors, and four resistors in addition to the LED(s)

Other possibilities for "small and cheap" include:

Self-blinking LED.
Advantages: No other components.
Disadvantages: May not work from 3V battery (typically 5V?), No control over timing, poor range of colors/brightness/etc.

Logic gate based flasher. Usually 1 IC, 1 cap, 1 resistor (some wide variation, alas.)

Advantages: IC may be smaller/lighter, and sometimes cheaper, than 2 transistors! IC body may provide platform for freeform assembly. Operates well below 3V, depending on logic family.
Disadvantages: IC should be more expensive than transistors.

555 flasher: 1 IC, 1 cap, three resistors.
(many examples)
Advantages: relatively high current drive. Very standard. 555s are cool. large range of timings possible. IC may be smaller/lighter, and sometimes cheaper, than 2 transistors!
Disadvantages: relatively expensive. 3V operation will require a low-power 555 chip.

Microcontroller: 1 IC, one resistor.
Advantages: microcontrollers are very cool. complex behaviors are possible (patterns, darkness sensing, morse code.) Circuitry potentially very simple. IC may be smaller/lighter, and sometimes cheaper, than 2 transistors!
Disadvantages: probably more expensive (I see at least two choices for 8-pin microcontrollers less than $0.50 each in 100s.) Requires learning microcontroller programming (neither of them is arduino compatible.) Requires a device to program the microcontrollers (an Arduino probably works, if you have the correct software.)

Off-the-shelf: pre-manufactured LED blinkies

Advantages: perhaps fantastically cheap (includes batteries!) No building required. interesting colors, sometimes.
Disadvantages: no control over timing. "No building?! Where's the fun in that?"

Hooray! Thats a wonderful list of options to explore! Thanks so much! I'll go through all of your suggestions and see what I can suit to my project... very excited.

I'll post the results once I get them.

Thanks again all!

Do a search for a '555 relaxation oscillator' or '555 LED flasher' etc. The 555 is simple, reliable, and inexpensive. Don't worry about the apparent myriad of available circuits, when you redraw them they are all essentially the same.