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How to build a blinking LEDs using Resistors, Capacitors, And Transistor in DCAClab

It is possible to blink or flash an LED using just a transistor. This interesting circuit makes use of a rarely used property of the transistor known as the reverse avalanche breakdown. Since this is a non-standard feature, this may not work on all NPN transistors. Also, note that you may need more than 9V voltage for this to get working on most transistors. I was able to get this working on the EBC transistor with a 9V supply.

In reverse avalanche mode, the collector to emitter circuit of the transistor is connected in reverse polarity. Hence positive terminal of the power source is connected to the emitter and the collector is connected to the negative terminal. In usual operation, very little current will flow from emitter to collector in this configuration. However, if we increase the voltage of the power source, beyond a specific value, the emitter to collector junction breaks down and a large current will flow through the circuit. The base of the transistor is not used at all in this configuration.

How Blinking LED with Transistor Works?

When the circuit is connected to the 9V battery, the current starts flowing through the 470 Ohms resistor charging the 470uF capacitor. This steadily increases the emitter to collector voltage in the transistor. When the voltage reaches the reverse avalanche breakdown region of the transistor, the emitter to collector junction breaks down and the capacitor is discharged through the 100ohm resistor and the LED. This lights up the LED. In the avalanche breakdown region, the transistor exhibits negative resistance, higher current leads to lower resistance!

Due to the discharge, capacitor charge and voltage drops until the avalanche mode cannot be any longer sustained. The transistor enters its normal operation and the emitter to collector high resistance is automatically restored. This switches off the LED triggering another cycle of capacitor charge and discharge!

Note: That we are using the base of the transistor in this 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. NPN Transistor, 100hFE,  Qty: 3
  2. Resistor, value 470 Ohms, qty: 6
  3. One terminal Vs, 9V Qty: 3
  4. Capacitor, 470 uF, Qty: 3
  5. LED, Qty: 3
  6. Breadboard
  7. Jumper wires

Add 3 Transistors in DCAClab’s Breadboard:

Add the three NPN transistors and the jumper wires from the power BUS to the emitter of each transistor. Because of the way I inserted the Three transistors the emitter is on the left side of 3 transistors.

Add the Capacitors:

Connect the Three capacitors to the circuit. Connect the positive lead of the first capacitor to the collector of transistor 1 Next connect the negative lead of the same capacitor to the emitter of transistor 1.

Repeat the above process for the second capacitor. Connect the positive lead of the second capacitor to the collector of transistor 2. Connect the negative lead of the same capacitor to the emitter of transistor 2 and repeat the above process for the Third capacitor.

Add the 470 Ohms Resistors & ground:

Next, connect the 470 Ohms Three resistors to the transistors. One lead of the resistor connects to the collector of the transistor, the emitter lead connects to the ground. Do this for both transistors.

Add the LEDs & Source:

Add all 470 Ohm resistors along with the 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 other resistor wire then connects to the positive wire of the first LED. Follow the same steps for the other resistor and LED. Connect one wire of the second resistor to the base of transistor 2 and repeat all resistors.

Supply Power and Watch the LEDs Blink:

Finally, 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.

To see live simulation: Click the link: Build A Blinking LEDs with-Transistor-and-Capacitor

Instructor

Md. Anisur Rahman Likhon


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