This article shows you how you can make a simple LED Indicator.

You can see the circuit working in the video:

Step 1: Design the Circuit

I have drawn the circuit in the old PSpice software:

The old PSpice software did not have the LED component. Thus I used three diodes instead to model each of the three LEDs. The R1a and R1b can be converted to Thevenin's equivalent circuit to calculate the maximum LED1 current:

Vth1 = Vs * R1a / (R1a + R1b) = 4.5 V * 3,900 / (3,900 + 100) = 4.3875 V

Rth1 = R1a * R1b / (R1a + R1b) = 3,900 * 100 / (3,900 + 100) = 97.5 ohms

IledMax1 = (Vth1 - Vled1 - Vbe2 - Vbe1) / Rth1

= (4.5 V - 2 V - 0.7 V - 0.7 V) / 97.5 ohms = 0.01128205128 A = 11.28205128 mA

The same method can be applied to calculate the maximum LED current of the other two LEDs.

Vth2 = Vs * R2a / (R2a + R2b) = 4.5 V * 3,300 / (3,300 + 120) = 4.34210526316 V

Rth2 = R2a * R2b / (R2a + R2b) = 3,300 * 120 / (3,300 + 120) = 115.789473684 ohms

IledMax2 = (Vth2 - Vled2 - Vbe2 - Vbe1) / Rth2

= (4.5 V - 2 V - 0.7 V - 0.7 V) / 115.789473684 ohms = 0.0095 A = 9.5 mA

Vth3 = Vs * R3a / (R3a + R3b) = 4.5 V * 2,700 / (2,700 + 150) = 4.26315789474 V

Rth3 = R3a * R3b / (R3a + R3b) = 2,700 * 150 / (2,700 + 150) = 142.105263158 ohms

IledMax3 = (Vth3 - Vled3 - Vbe2 - Vbe1) / Rth3

= (4.5 V - 2 V - 0.7 V - 0.7 V) / 142.105263158 ohms = 0.00774074074 A = 7.74074074 mA

Calculate the equivalent emitter resistance:

Req = Re || Rth1 || Rth2 || Rth3 = 38.5624852748 ohms

The is very low resistance. Thus you will need an additional transistor connected in parallel with the Q2 transistor to reduce the power dissipation and the maximum current to prevent possible transistor failure.

The minimum input resistance will equal:

Rin = Req * (Beta + 1) * (Beta + 1)

= 38.5624852748 ohms * 21 * 21 = 17,006.0560062 ohms = 17.01 kohms

A typical transistor current gain, Beta is usually equal to 100. Some transistors can have a typical current gain of 500. Current gain is influenced by:

- temperature,

- production tolerances,

- aging,

- biasing current.

More information on the Thevenin’s Theorem can be found here:

https://www.electronics-tutorials.ws/dccircuits/dcp_7.html

Step 2: Simulations

Simulations show that the minimum current is above about 5 mA. This should be just about enough to keep the third LED on.

Step 3: Make the Circuit

I did not use a soldering iron. I twisted the wires together with pliers.

Step 4: Test the Circuit

I used 1 kohm potentiometer as a voltage reference. However, I believe that you can use a 10 kohm potentiometer if you do not have the 1 kohm variable resistor.

Conclusion

You can try adding additional LEDs to the circuit.