Control multiple leds using only 2 connection wires

A transistor is basically a switch that can disconnect ground or 5V. There are transistors of all sorts, and some can switch truly huge loads. 50 LEDs is no big deal.

So, the question then is this: Can a single 2N3904 switch 50 LEDs?

To answer that, we'd need to know first what kind of LEDs. But let's assume you are using the usual nothing-special variety. A reasonable estimate of the maximum current of these is \$20mA\$. If you have 50, then the maximum current is \$20mA \cdot 50 = 1000mA \$. Looking at the datasheet I see the maximum collector current for 2N3904 is \$ 200 mA \$. So the answer is no: you can not switch 50 LEDs with one 2N3904.

I suppose you have several options:

• use multiple 2N3904
• use fewer LEDs
• use the same LEDs, but drive them with less current
• use a bigger transistor (TIP121 is very easy to find)
• use some other switching device

Of these, I think reducing the LED current or using a larger transistor is probably the most likely solution. Other switching devices (like a relay) are probably more expensive and slower.

When using a red and green LED, you can utilize the difference in LED forward voltage as in the circuit below. Make sure to use high efficiency LED's. These have a low internal resistance and a nice and sharp bend in the forward voltage/current curve.

^{simulate this circuit – Schematic created using CircuitLab}

Normally the green LED lights, which has a forward voltage of approximately 2V. Then when the switch closes, the red LED light and due to its lower forward voltage, approx. 1.8V, the green LED will turn off from (voltage) starvation.

If the green LED does not entirely turn off, you can add a regular 1N4148 diode in series with it to make the effect stronger, but this is usually only necessary with cheap and relatively low efficiency LED's.