Simple UJT LED Flasher Circuit

A UJT LED flasher circuit is a super simple way to get an LED to blink on and off.

It uses a special transistor called a UJT, which acts like a tiny flasher switch.

This UJT creates a regular pulse of electricity turning the LED on and off at just the right time.

These circuits are perfect for fun DIY projects, adding some blinking personality to your decorations or even as a cool way to learn about electronics.

Circuit Working:

Parts List:

This simple Mock Flasher LED emulates the indicator of a complex alarm system offering a practical solution for enhancing security.

It can be installed on doors, gates and vehicles to perplex potential intruders.

The circuit is quite simple employing a unijunction transistor as an oscillator.

A unijunction transistor UJT is an electronic semiconductor device with a single junction.

It features three terminals: an emitter E and two bases B1 and B2.

The base consists of a lightly doped n type silicon bar, with ohmic contacts B1 and B2 attached at its ends.

The emitter is p type and heavily doped.

The resistance between B1 and B2 when the emitter is open circuited is known as inter base resistance. The 2N2646 is a commonly used type of UJT.

The UJT is biased with a positive voltage between the two bases creating a potential drop along its length.

When the emitter voltage exceeds approximately one diode voltage above the point where the P diffusion emitter is located current begins to flow from the emitter into the base region.

The lightly doped base region experiences conductivity modulation due to the additional current, reducing the resistance of the base portion between the emitter junction and the B2 terminal.

This reduction in resistance results in the emitter junction being more forward biased leading to even more current injection.

Overall, this process creates a negative resistance at the emitter terminal, making the UJT particularly useful in simple oscillator circuits.

In the mock flasher LED circuit, the UJT 2N2646 functions as a simple oscillator with the oscillating components R1 and C2.

The LED is connected between the base2 B2 of the UJT and the ground.

Resistor R2 limits the current through the UJT and the LED.

Upon power on capacitor C2 charges through R1.

When fully charged the emitter of the UJT becomes forward biased, allowing current to flow into the base and light up the LED.

The discharge of C2 is followed by its recharging creating an oscillating cycle that causes the LED to flash.

Formulas:

Formula for UJT LED Flasher Circuit is mentioned below:

Charging Time T:

The frequency of oscillation is determined by the charging time of capacitor C1, which may be computed using:

T = 0.7 * RB1 * C1

where:

• RB1: Resistance connected between B1 and the positive supply.
• C1: Capacitance connected between B2 and ground.

Frequency (f):

The charging time T is inversely related to the oscillation frequency f:

f = 1 / T​ = 1​ / 0.7 * RB1 * C1

LED Resistor RL:

Determine the resistor value RL for the LED by taking into account the supply voltage and the LED parameters, which are usually the forward voltage V_LED and the required current I_LED respectively.

RL = V_CC​ − V_LED / ​​I_LED​

where:

• V_CC​ is the supply voltage for 9V
• V_LED​ is the forward voltage drop across the LED.
• I_LED​ is the desired current through the LED i.e 20mA.

You may efficiently build and construct a UJT LED flasher circuit that flashes the LED at a frequency set by the timing components RB1 and C1 by following these calculations and instructions.

Make any adjustments to component values and thoroughly test the circuit to ensure that it operates as intended.

How to Build:

Below are the steps to build a Simple UJT LED Flasher Circuit:

Connect the UJT:

• Place the 2N2646 UJT on the breadboard.
• Connect a LED between B1 terminal of the UJT and ground.
• Connect a 470Ω R2 resistor between B2 terminal of UJT and a positive supply.
• Connect a 10k R1 resistor between emitter of UJT and the positive supply.
• Connect capacitor C2 100µF between the emitter terminal of the UJT and ground.

Power Supply:

• Connect the positive terminal of your power source to the positive supply of the breadboard.
• Connect the negative terminal of your power source to the negative of the breadboard.

Testing:

• Ensure all connections are secure and correct.
• Power on the circuit.
• If everything is connected properly, the LED should flash intermittently simulating the indicator of an alarm system.

Important Notes:

• Double check all connections and component values before powering on the circuit to avoid damaging the components.
• Use a suitable power source and ensure it is connected with the correct polarity.
• Adjust the values of R1, R2 and C2 to modify the flashing rate of the LED if desired.

Conclusion:

In conclusion, a UJT LED flasher circuit is a simple electronic circuit that uses a Unijunction Transistor to flash an LED on and off at a regular interval.

This circuit is simple, cost effective, and can be easily built for various applications such as DIY projects, decorative lighting and educational purposes.

References:

Datasheet 2N2646