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Newcomer to electronics -- I built a circuit based on the first circuit in this post https://electronics.stackexchange.com/a/162032/103970, however it only works with green (3.2 VF) and red (1.8 VF) LEDs because of the different forward voltage drops. How can I modify this circuit to work with any color LED, ideally two of the same type?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Have you tried any of the other ones on that question? \$\endgroup\$
    – Passerby
    Commented Mar 18, 2016 at 2:06

2 Answers 2

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If you don't need the left-hand LED to be completely off, just much dimmer than the second, you can try something like

schematic

simulate this circuit – Schematic created using CircuitLab

Assume the LEDs have a Vf of 2 volts. When the switch is closed, D1 is on with about 8 mA current, and D2 is off. With the switch is open, D1 is still on, but its current is about 0.6 mA, while D2 is on with a current of about 10 mA.

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  • \$\begingroup\$ If you put 1K in parallel with D1 and increase R3 to 1.3K you'll get about 8mA on each and D1 will go off completely. \$\endgroup\$ Commented Mar 18, 2016 at 11:17
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This is my first time posting an answer so excuse my formatting etc.

As you said in your question, the circuit only works with LEDs that have a different forward voltage, that is why two identical LEDs will not work with that circuit.

We can't change the forward voltage of the LEDs, but what we can do is add a diode in series with the first LED (The green LED in your circuit) to make the voltage drop along that branch higher, a typical silicon diode will have around 0.6 to 0.7VF but always remember to check the datasheet

edit: If you wanted to control the current of the LEDs a bit better you could take the 1k resistor out and put individual resistors in each branch

schematic

simulate this circuit – Schematic created using CircuitLab

Here's a simulation of the circuit, I've made the LEDs have the same VF but you can edit the values of components by right clicking them.

Falstad circuit simulation

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