Single Transistor LED Flasher Circuit

It is possibly the smallest LED flasher to date, which is able to flash an LED ON/OFF infinitely using a single transistor, a resistor, and a capacitor.

Can you imagine making a great looking LED flasher or blinker with just a single transistor and a couple of other passive parts?

That's exactly what I have explained in this post! This is perhaps the world's simplest and the tiniest LED flasher you can get!

How it Works

I came across this phenomena some eight years ago (2006), accidentally, while trying to make a smallest possible motorcycle side indicator flasher, and was surprised by the phenomenon.

However, then I realized that the phenomenon was already discovered by Mr. Dick Cappels while investigating the negative resistance theory in BJTs by the Japanese researcher Mr. Reona Esaki (Aka Leo).

Reona Esaki's thesis work in the relevant field and on tunnel diodes ultimately won him the Nobel Prize in 1972.

That looks too good to be true, however the following diagram will simply prove that it's really possible to create a working LED flasher circuit using just one general purpose transistor as the main component.

At that time, I was unaware that this was occurring as a result of the transistor's negative resistance characteristics.

The circuit actually exploits the negative resistance factor in transistors to produce the blinking effect.

I'll be soon writing a comprehensive article on this and we'll see there how the concept can be modified in many different ways.

Parts List for the proposed single transistor LED flasher circuit

• R1 = 2K7,
• R2 = 100 Ohms,
• T1 = BC 547,
• C1 = 100 uF to 470 uF
• LED = Any Type, any color

The flashing rate could be varied either by changing the value of R1 or C1 or both together. But the supply voltage not be less than 9V otherwise the circuit might fail to work correctly.

You may also love to read this article: Blinking LED Circuit using LDR

Circuit Diagram

Calculating the LED Flashing Frequency

You can use the following formula for approximately calculating the LED ON/OFF blinking rate

Frequency (f) ≈ 1 / (2.1 * R * C)

Where:

• R is the resistance in ohms
• C is the capacitance in farads

This formula gives us a rough idea of what the flashing frequency might be. But the actual frequency can be affected by a hand full of different factors, for example like the specific components you are using in the circuit, the voltage of the power supply and also the temperature.

To adjust the flashing rate:

• Increase the resistance (R): If we make the resistance higher, it will take longer for the capacitor to charge up which means the flashing rate will be slowed down.
• Decrease the capacitance (C): On the opposite side, if we lower the capacitance vaalue, the capacitor will charge up faster causing the flashing rate to speed up.

Connecting an External Transistor for Higher Loads

Video Clip:

PCB Design

Circuit Basics

In this circuit we are using one NPN transistor which is BC547, one electrolytic capacitor of 330 uF, two resistors of 2.7 k and 100 ohm, and one LED, and supply voltage is +12 V.

Here the LED is connected in the collector path of transistor, while the base of the transistor is not connected directly to the supply, but instead it is controlled indirectly through the RC network made by the 2.7 k resistor and the capacitor, and this indirect base control is the main reason why oscillation becomes possible.

Start Condition When Power Is Applied

When power is applied then the capacitor is in a fully discharged condition at the start, and because the capacitor is empty at this moment, current immediately starts flowing from +12 V supply through the 2.7k resistor into the capacitor.

At the same time a small voltage slowly begins to appear across the capacitor and this rising voltage also starts appearing at the base of the 2N2222 transistor, but the transistor still remains in the OFF state because the base voltage is not yet high enough.

Capacitor Charging Action

As time passes, capacitor continues to charge slowly through the 2.7 k resistor and because this resistor limits the current, voltage rise at the capacitor and at the transistor base is gradual and not sudden

When the base voltage reaches around 0.6 V to 0.7 V then base-emitter junction of the transistor becomes forward biased, and at this point the transistor suddenly turns ON.

LED Turns ON And Rapid Discharge

When the transistor turns ON then collector current starts flowing through the LED and 100 ohm resistor, and because of this current flow the LED lights up.

Now the important thing happens because the transistor in ON state provides a very low resistance path for the capacitor, so capacitor discharges very quickly through the base-emitter junction and through the transistor itself, and this fast discharge is the key feature of the Esaki oscillator.

Transistor Turns OFF

As the capacitor discharges rapidly, then base voltage falls sharply.

When this base voltage drops below the required base-emitter threshold then the transistor immediately switches OFF.

When the transistor switches OFF then the collector current stops flowing and the LED also turns OFF, and circuit returns back to its initial condition.

Cycle Repeats Continuously

When the transistor is OFF again then capacitor once again starts charging slowly through the 2.7 k resistor, and this slow charging followed by sudden discharging keeps repeating again and again.

Because of this continuous repetition the LED keeps flashing ON and OFF as long as the supply voltage remains connected.

Why This Circuit Is Called Esaki Oscillator

This type of oscillator works because of non-linear behavior of a semiconductor junction, mainly the base-emitter junction of the transistor.

And the sudden ON OFF action caused by fast discharge of capacitor creates a behavior that looks similar to negative resistance effect observed in tunnel diodes, which were studied by Leo Esaki.

That is why this simple single-transistor relaxation oscillator is often referred to an Esaki oscillator.

Flashing Speed Control

The flashing speed of the LED mainly depends on the RC time constant formed by 2.7 k resistor and 330 uF capacitor, so when the resistor value is increased then the capacitor charges more slowly and LED flashes more slowly.

When the capacitor value is increased then OFF time becomes longer and flashing rate reduces, but the 100 ohm resistor does not affect oscillation frequency because it is only used to limit the LED current and protect the LED and the transistor from excess current.

Why This Esaki Oscillator Does Not Work At 5 V Or 6 V

Now we see this circuit works fine at 9 V or 12 V but when we try at 5 V or 6 V then the circuit stops working or works very weak.

The reason is simple, the voltage becomes too low for this type of oscillation.

When we give supply then the capacitor must charge and push voltage to the base of the transistor but the transistor base-emitter already needs around 0.6 V. And the LED in the collector also needs voltage to conduct, and the resistors also eat some voltage.

So now when supply is only 5 V or 6 V then very little voltage is left for proper action.

So the capacitor charges very slowly and also cannot reach a strong enough level and when the base voltage reaches near 0.6 V then the transistor only turns ON very weak, not strong.

Because of this weak ON condition the capacitor cannot discharge fast and without fast discharge the oscillation cannot happen properly.

When the transistor does not switch sharply then the circuit stays stuck in between ON and OFF and the LED either stays dim or does not blink at all. This is why at low voltage the circuit fails.