Transistor LED Flasher

Source: http://www.josepino.com/?transistor_led_flasher   Tags: electronics circuits light diy

If you are familiar with LED flashers using transistor, you may know the basic one uses two transistor, one capacitor and three resistors. There is other kind of oscillator called "flip-flop". As I'm starting to develop some basic circuits, I need a simple LED flasher that can be used in many other applications.

After some hours thinking how to create a new simple LED flasher with the minimum components, I decided to use a simple RC. Here is the schematic of my two-transistor LED flasher:
Taken from http://www.josepino.com

The RC circuit is connected to the base of the NPN transistor (3904) and it gives the signal to the PNP transistor (3906) to turn on the LED. When the PNP transistor is ON, it discharges the capacitor and it turn off itself, so the cycle repeats again.

The resistor and the capacitor define the frequency. The circuit with the lightbulb may not work if the bulb works with 6 volts or its resistance is low.

12/26/2007 - Update:

While cleaning "the attic" a few weeks ago, I found this old project. This one uses the same circuit. It was made in 1996 or 1997. A 3 Volts lithium "coin" battery provides power up to 4 hours.

The idea of this one was to get a flashing light without bulky or heavy parts. Regardless there are some LED flashers on the market, A LED with integrated circuit inside the acrylic, the cost is higher than this circuit in some cases.

At least you can get an idea how to make a "coin-type" battery holder.

08/10/09 - 03:52:54 acmefixer from near Disneyland, So. California wrote: Please update schematic with the speaker impedance.  Also, for driving a light bulb, the 2N3906 or 2N3904 are very poor choices.  They are rated 100mA maximum, and lamps may take more current than that.  Use a 2N4401 or 2N4403, or BC337 or BC327.  Thanks for the great website.
08/10/09 - 04:01:04 José Pino wrote: Thank you for your advice! The original circuit that I made, uses the TIP115 PNP transistor.  For some reason [I didn't paid attention] I forgot to change it on the schematic.
08/28/09 - 06:10:09 Roman Vaughan from New Zealand wrote: You could increase the frequency to lets say 16.5 KHz and connect a speaker and use it to annoy everyone in the room :P
09/11/09 - 20:51:36 Ben Leveque from stevenage, UK wrote: is there a way to speed up th flashin so that its like a nightclub strobe light?
if so. how?
benleveue@hotmail.co.uk
email me, please
09/11/09 - 21:24:33 José Pino wrote: Changing the capacitor with a lower value will increase the frequency.
10/10/09 - 19:57:05 Paul from NJ wrote: I want to adapt this circuit to control a relay, I can do that.  I also need it to "flash" every 30-90 seconds.  Any idea on what values I need to change the resistor and capacitor to?  Thanks.
10/30/09 - 19:39:29 José Pino wrote: You can use bigger values for the capacitor, like 1,000 mF, but long delays (greater than one minute) may damage the 555.  You may need to use another IC for long delays, for example a Cmos 4017.

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