Electronics Forum

by **Veys** » Fri Jul 25, 2014 2:54 pm

Hello All,

I am currently designing [b]an LED array[/b]. It consists of 20 LEDs which will light up one at a time in sequence. The first design looked something like this with the switch array representing logic 0 outputs on micro-controller:

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: Buffer Resistor Design; IR On.png (151.7 KiB) Viewed 13231 times

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I need to ensure that the intensities are controlled however using a constant current method. I have decided upon this design which uses a shunt regulator to maintain required current through LED:

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: Shunt Regulator Design; TLV431 Single IR LED ON.png (81.85 KiB) Viewed 13231 times

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The problem I am having is that I cannot come up with a suitable way of replicating this design x20 times over, short of just using x20 of each components. Any ideas? MUX's are expensive no?

Regards,

VEYS

by **pebe** » Sat Jul 26, 2014 7:18 am

Why do you need a constant current controller?
I assume you are using a stab 5V source, in which case your simple resistor feed, R1, should be OK.

by **Veys** » Sat Jul 26, 2014 11:24 pm

LED intensities depend on current flow through them. The application requires the LEDs to operate at known wavelengths and so the typical buffer resistor design is not adequate.

by **pebe** » Sun Jul 27, 2014 8:54 am

The wavelength of a LED is determined during manufacture and stays the same whatever current is used to drive it. I have driven an IR LEDs with pulses of 4us at a current at over 1A to get a range of 30ft, and the wavelength did not change. I would be interested to know your application.

If your application is very fussy on drive current I can give you a simple constant current circuit to replace R1 in your first circuit.

by **Veys** » Sun Jul 27, 2014 9:23 am

It is the intensity that varies not the wavelength. I ensure uniform
brightness is controlled via current input.

by **pebe** » Sun Jul 27, 2014 9:45 am

[quote="Veys"]The application requires the LEDs to operate at known wavelengths and so the typical buffer resistor design is not adequate.[/quote]
I assumed that you thought the wavelength changed.
[quote]I ensure uniform brightness is controlled via current input.[/quote]
In your top circuit, R1 is common to all LEDs, so they should be equally bright.
However, do you want the circuit?

by **Veys** » Sun Jul 27, 2014 10:09 am

Apologies the wording was ambiguous. The purpose is to emit pre-defined wavelengths at equal intensities. Thus if the current isn't controlled I will have no control over the intensity emited. Any circuit revisions are welcome.