Simple Blinking LED Circuit

Intro: Simple Blinking LED Circuit

A very simple circuit that you can build to blink or flash LEDs. The circuit is built using transistors, resistors, capacitors, and LEDs. Of course you will need a breadboard, wire jumpers, and a power source. The parts list includes:

1. PNP Transistor, P/N 2907A, qty: 2
2. Resistor, value 470 Ohms, qty: 2
3. Resistor, value 100k Ohms, qty: 2
4. Capacitor, 10 uF, qty: 2
5. LED, Qty: 2
6. Breadboard
7. Jumper wires

Let's get started:

STEP 1: Add the Transistors

Add the two PNP transistors and the jumper wires from the power BUS to the emitter of each transistor. Because of the way I inserted the two transistors the emitter is on the left side of both transistors.

STEP 2: Add the Capacitors

Connect the two capacitors to the circuit. Connect the positive lead of the first capacitor to the collector of transistor 2. Next connect the negative lead of the same capacitor to the base of transistor 1.

Repeat the above process for the second capacitor. Connect the positive lead of the second capacitor to the collector of transistor 1. Connect the negative lead of the same capacitor to the base of transistor 2.

STEP 3: Add the 100K Resistors

Next connect the 100k resistors to the transistors. One lead of the resistor connects to the Base of the transistor, the other lead connects to ground. Do this for both transistors.

STEP 4: Add the LEDs

Finally add the two 470 Ohm resistors along with the two LEDs. I added a picture of a transistor to identify the Emitter, Base, and Collector.

Connect one wire of the first resistor to the collector of transistor 1. The the other resistor wire then connects to the positive wire of the first LED. The negative wire of the LED is then connected to ground.

Follow the same steps for the other resistor and LED. Connect one wire of the second resistor to the collector of transistor 2. The the other resistor wire then connects to the positive wire of the second LED. The negative wire of the LED is then connected to ground.

STEP 5: Supply Power and Watch the LEDs Blink

The last step is to supply power and watch the LEDs blink. I use a 9 volt battery and it worked fine.

For fun you can try other capacitor values to change the rate at which the LEDs blink.

Comments

[Pipa String]

A brilliant little circuit - it worked first time with a 9V battery. I used ancient CV9543 pnp transistors found in my junk-box and 6.8 muF electrolytic capacitors (I didn't have any 10 muFs). Will now explain the whole thing to my grandson and he can experiment with different value caps and other pnp transistors.

[tmichlovitch]

That's great, glad you were able to build it with your grandson and share that time together.

[Yodi]

I have tried so hard to build a blinking light with npn transistors and this way it does not work. The best I had was both lights stay on.

[Yodi]

The npns are c945

[prathameshchandekar7081]

Can you tell that from where the current is flow and it is flow from which terminal to which

[stoloui]

Does anyone know what size capacitor to use for 3 second interval blinking?

[DavidM681]

Anyone out there clever enough to produce the same circuit but for 3.3v?

[ChrisRC]

Would this work on 5V or if it need some adjustment to work what would it take?
Thank you.

[keerthan r]

why are capacitors used in this circuit
?

[tmichlovitch]

The capacitors are designed to charge and then discharge, which allows the LEDs to flash.

[anivia2409]

I dont know why my board did not blink. I used:
2 x 50V 10uF capacitors
2 x PN2222A transistors
2 x 330 Ohm resistors (I dont have 470 ohm so I used 330 ohm)
2 x 100k Ohm resistors
1 x 9V battery

Please help with diangosing it :(

[tmichlovitch]

Using a 330 ohm instead of the 470 should be fine, that's not your problem. Is either LED turning on? Check the placement of the capacitors, there should be a (+) and (-) marking on them. Make sure the capacitor leads are correct. Also double check the pins on your transistors to make sure that they were placed correctly

[phranki]

would somebody have an idea how this could be altered to solar
power, so it could come on at night ?

[phoenix98604]

I would like to be able to use several of these circuits each to make random single blinks recycling over 1-2 minutes. Can the LED's be made to turn on softly and turn off softly to extend the blink time (1-2 seconds)/

[tmichlovitch]

I wouldn't know how to accomplish that with this current circuit. Not saying it can't be done, I just don't know how to do it. Look into using a micro-controller like Arduino, you will easily be able to have the type of control you want over the LEDs

[Gina_ex]

Just a few questions:
1). Can this cct be expanded to make a design like a mini ( 6 - 8 ) multi-color LED Christmas tree ?
2). Would this - or a modified version - be programmable for random interval flashing of the LED's ,and if so what added components are required ?
What alternatives for power source are recommended, if any ?

[tmichlovitch]

I'm pretty certain you should be able to expanded the project to add more LEDs, but only by adding them in series to the existing circuit LEDs. So, if you added an additional LED to each circuit leg with an existing LED, then you could have 2 on and 2 off, flashing back and forth (instead of 1 on and 1 off flashing back and forth).

This is not programmable. If you're interested in programmable circuits definitely check out Arduino, Raspberry Pi, and others.

[Quoc Quyen]

Transistors have 3 poles E, B, C and its structure as 2 diodes in reverse. Why is the power source going from C to E without stimulating pole B? Please explain to me

[Vivian N]

I think it's because the circuit uses PNP transistors, which stop conducting when current is applied to the base, hence why the lights blink when the capacitor discharges. If it used NPN transistors, there would need to be current going to the base for it to conduct.

[tomatoskins]

These look awesome! Electronics are great! I've never thought to use this type of circuit before.