LED Driving

Nov 18, 2009 5 Replies
J

Hi,



I have a tech. question about the Nrf24z1. I have one chip on the transmitter board and other chip on the receiver side. I am planning to toggle the transmitter side chips pin 7 and pin 8 (DD[1] and DD[0]) bits. Now, in response the receiver side chip's DD[1] and DD[0] will toggle too.



I need to drive one LED from DD[0] and the other LED from DD[1]. I am not a hardware guy, So, I thinking that may be somebody can advise me a LED driving circuitry for this problem. I do not know the exact current driving capability of these pins.



I am planning to use panel mount LEDS. But I do not have their part numbers yet. Please advice.



Thanks Regards, Raza



THE DATA SHEET OF THE CHIP IS AS FOLLOWS



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J

The way I'm reading it, DO[1] and DO[0] will toggle.

These can drive 10mA, it appears, which is more than enough to drive an LED directly. Just hook up, e.g., DO[0] to a 330ohm resistor and then the LED -- you'll probably get some 3-4mA through the LED (it depends on the voltage drop of the LED, which these days varies a lot depending on the color and efficiency of the LED you choose), which should be a good start.

---Joel

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J

Thanks. Will it be good idea to use a buffer IC or transistor to drive the LED? If yes that can you suggest some parts.

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J

It's not really necessary, but if it makes you feel better, sure, it's a fine idea.

If you're doing this as a prototype: Almost anything would work... if you want more current drive than you'll know what to do with, a 74AC244 (or 74AC125 or most others) can drive 24mA -- oodles or current. If you want something a little tamer, a 74HC244 (or many others) can drive 6mA.

If you're doing this as a product but don't have to shave off every last penny: NC7SZ57/58

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are great... they're "universal" 2-input, 1-output logic gates -- keep a bunch in a drawer and you'll never need to stock a plethora of different logic functions. They can also drive far more current than you'll need for an LED. (The only downsides are that these are rather small surface-mount parts and cost around a quarter each in small quantities.)

---Joel

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T

Use a 2n7000 fet, with a suitable resistor in the drain, but there's no reason not to drive the led directly, via a small resistor of 100 ohms or so. the value will depend on the Vcc of the RF chip, and the type of led. eg, 3V3 and a blue led won't work as the led needs ~4V. A red led and 100 ohms would give you about 10mA @3V3

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J

.

LED=20

LED -- =20

voltage drop=20

Joel, that IC runs from a 1.5 V supply. It often takes more voltage to get the LED to emit (visible) light. Some kind of buffer is likely necessary to get LED drive to high enough voltage. Though 2 equal channels at an combined 2 Mb/s is cool.

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