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# How can I modify this circuit to sequentially turn on LEDs in parallel, while avoiding voltage drops? Answered

I have a single pole, 12 throw rotary switch which I want to solder to a set of LEDs so that rotating the switch clockwise lights more LEDs as follows:

1 = 1 Red LED
2 = 2 Red LEDs
3 = 3 Red LEDs
4 = 4 Red LEDs
5 = 5 Yellow LEDs
6 = 6 Yellow LEDs
7 = 7 Yellow LEDs
8 = 8 Yellow LEDs
9 = 9 Blue LEDs
10 = 10 Blue LEDs
11 = 11 Blue LEDs
12 = 12 Blue LEDs

Since there will be a voltage drop across the diodes, the LEDs at the start of the chain will get progressively dimmer, and increasing the voltage would blow the LEDs in shorter chains.

One modification I know I could already make would be to not use the first 4 diodes on the yellow LEDs, and the first 8 diodes on the red LEDs, as a minimum of 5 yellows and 9 blue will be active for each position. This still leaves a maximum of 3 diodes in each chain, which I still think would cause the lights to be much dimmer.

Another solution I can think of is to have multiple wires coming from each terminal of the switch, each with their own diode, which would ensure all active LEDs had the same brightness (I'm not sure if this would cause the whole chain to be dimmer than shorter chains or not). The downside is this would need  31 wires rather than 13, and that's only if I bridged together all the shortest LED chains of each sequence: individual wires for absolutely every position would mean 79 wires+diodes).

To put this in context, I want to create the health bar from Dead Space 3:
If you can suggest a way of improving the circuit, or totally re-designing it, it'd help me loads!
Thanks!

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## Discussions

Drive the chain from a constant current source - and put everything in series.

I was planning to use a 4x AA battery holder to provide the 5v, but having them all in series would conveniently eliminate the need for diodes (Separate from the LEDs).

How does a constant current source work? Does it increase the voltage according to the total resistance of the circuit (I.e. 5v for 1 LED, 20v for 4 LEDs). The LEDs are 5v, 100mA, so should I look for a constant current source that can provide 100mA, up to 6 watts and 60V?

It actively measures the current being drawn, and adjusts the voltage to keep the current constant, its completely automatic, and has the effect you want. A better way might be to use the switch and some logic with transistors to switch on the LED strips.

Thanks, I'll look into that. I did just read up on a tutorial regarding the use of P channel MOSFETS instead of diodes, so I may not have to alter my original design too much after all.