Simple LED Chaser Circuit using Transistors

Visualize a line of flashing lights, but instead of bulbs, you use LEDs!

This LED chaser circuit makes them light up one by one, like a running chase.

It uses transistors as tiny switches to control the LEDs.

This is a great project for electronics beginners because it shows how transistors work in a fun way, making lights chase each other.

Circuit Working:

Parts List:

The article is a basic LED chaser circuit that uses five NPN bipolar junction transistors BJTs.

Below are the working process of the circuit.

The circuit is powered by a 6V source.

Each transistor Q1to Q5 has a capacitor C1 to C5 and resistor R1 to R10 connected to its base and collector in a voltage divider configuration.

When the circuit is switched on these capacitors start to charge through their respective resistors.

As the capacitor charges the voltage at the base of the transistor increases.

Once the voltage at the base of a transistor reaches a certain threshold around 0.7V for a silicon BJT the transistor turns on, allowing current to flow from the collector to the emitter.

This illuminates the LED connected to the collector LED1 to LED5 through its corresponding current limiting resistor R1, R3, R5, R7, R9.

When a transistor turns on the capacitor connected to its base starts to discharge through the collector emitter path of the transistor and the resistor R7 and C3 combination for example.

As the capacitor discharges the voltage at the base of the transistor starts to decrease.

Once the voltage falls below the threshold level the transistor turns off turning off the LED connected to its collector.

The capacitors C1 to C5 have different values which means they will take different times to charge and discharge.

This creates a sequential charging and discharging pattern across the transistors which in turn causes the LEDs to illuminate in sequence creating a chasing effect.

Formulas:

Configuring several levels of transistor driven LED sequences is a common step in the design of a cascading LED chaser circuit.

The following is a condensed formula that takes such a circuit into account:

LED Chaser Speed (Chasing Rate):

The charging and discharging time constants of the capacitors in the circuit may be used to roughly calculate the chasing rate (frequency) of LEDs.

τ = R * C

where,

• The time constant τ for RC circuits (where R is a resistor and C is a capacitor)

f = 1​ / 2τ

• The time constant τ can be used to determine the frequency f of LED chasing.

The chase speed of LEDs may be varied by adjusting the values of R and C.

The basis for creating a cascading LED chaser circuit is provided by this fundamental formula.

To maximize performance and provide the intended LED chasing effect, meticulous circuit design and testing are necessary.

How to Build:

To build a Simple LED Chaser Circuit using Transistors follow the below mentioned connections steps:

• Connect transistor Q1 base to R1 22k resistor, collector is connected to R2 470Ω resistor and LED1, emitter is connected to ground.
• Connect transistor Q2 base to R3 22k resistor, collector is connected to R4 470Ω resistor and LED2, emitter is connected to ground.
• Connect transistor Q3 base to R5 22k resistor, collector is connected to R6 470Ω resistor and LED3, emitter is connected to ground.
• Connect transistor Q4 base to R7 22k resistor, collector is connected to R8 470Ω resistor and LED4, emitter is connected to ground.
• Connect transistor Q5 base to R9 22k resistor, collector is connected to R10 470Ω resistor and LED5, emitter is connected to ground.
• Connect one end of capacitor C1 to collector of Q1 transistor and other end to base Q2 transistor.
• Connect one end of capacitor C2 to collector of Q2 transistor and other end to base Q3 transistor.
• Connect one end of capacitor C3 to collector of Q3 transistor and other end to base Q4 transistor.
• Connect one end of capacitor C4 to collector of Q4 transistor and other end to base Q5 transistor.
• Connect one end of capacitor C5 to collector of Q5 transistor and other end to ground.
• Connect one end of capacitor C6 to base of Q1 transistor and other end to collector of Q5 transistor.

Note:

• There are more advanced LED chaser circuits that can offer features like adjustable speed control multiple LED color or different chasing patterns.
• However, this basic transistor circuit provides a good starting point for understanding the fundamentals.

Conclusion:

To conclude, this LED chaser circuit offers a simple and effective way to create a chasing light effect using transistors.

With its minimal components and simple design, it is a great introduction to how transistors function and serves as a foundational project for beginners in electronics.

References:

Help needed in understanding this LED chaser circuit