April 10, 2014 | Circuits |

Nostalgia: A Two Transistor LED Flasher Circuit

I am feeling a little nostalgic. So I rebuilt one of the first circuits I put together when I started messing around with circuits in eighth grade. It's a very simple beginner project, and might appeal to your child - real or inner!

Another reason I wanted to put this up is because most "blinky" circuits I see now are based on the venerable 555 IC. This is fine, but the two transistor circuit is simpler for a beginner to understand than the innards of the 555, I think.

Here is the circuit. It's a two transistor astable multivibrator. (Read the wikipedia link for the gory details of how it works. ;-) )

For supply, I used a 3V CR 2032 coin cell. The transistors are 2N2222, but I think any NPN transistor would work in this case. The LEDs are Red, which have low turn-on voltage, and this is important, since we're using a coin cell here. The circuit was assembled on a breadboard.

Even for this simple circuit, some calculations are involved to get it right. The total period of oscillation is given by:

$$ T = t_{1} + t_{2} = \ln 2 R_{2}C_{1} + \ln 2 R_{3}C_{2} \approx 0.693(R_{2}C_{1} + R_{3}C_{2})$$

Choosing $$ R_{2} = R_{3} = 47 K\Omega$$ and $$ C_{1} = C_{2} = 22 uF $$ gives us a total period of about 1.4 seconds - 0.7 seconds per LED. Good enough.

Now, to compute $$ R_{1} = R_{4}$$. Assuming a 0.6 V drop across the transistor, and a 20 mA current through the LED gives us $$ R = \frac{3 - 0.6}{20 mA} = 120 \Omega$$. So a 100 Ohm resistor, which is more common, will do just fine.

And here's the circuit in action:

Build it for your child! :-)


Need help with a hardware project or product? Drop us an email at We offer consulting services on AVR and Nordic nRF BLE - hardware design, firmware development, prototyping, PCB design/assembly, sourcing and manufacturing. We can help you bring your product to market!


We love hearing from our readers. Email us at for questions or comments on this article. If you found this article useful, please support us by buying some of our hardware products.

Please sign up for updates

Once in a while, we will send you an email update on the latest Electronut Labs projects and products. Your email address will never be shared or abused, ever.

2016 Electronut Labs. All rights reserved.