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Driver 8 LEDs with 74HC595 shift register


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#1 Paolo Patierno

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Posted 02 October 2011 - 01:04 PM

Hello, I know that If I want to drive a LED with a Netduino output pin, I need to use a transistor because Netduino pin has 8 mA max current and a led tipically works with 20 mA. Now I want to drive 8 led using the 74HC595 shift register. I connect a Netduino output pin to serial input of shift register. To connect the LEDs to the 8 output pin of shift register, I need to use some precautions to avoid burn Netduino ?? Can I connect each pin directly to each shift register output pin ? Thanks, Paolo

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#2 Nevyn

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Posted 02 October 2011 - 03:08 PM

Now I want to drive 8 led using the 74HC595 shift register.
I connect a Netduino output pin to serial input of shift register.
To connect the LEDs to the 8 output pin of shift register, I need to use some precautions to avoid burn Netduino ??

The Netduino should be alright if wired correctly. You can connect the SPI bus the the shift register directly.

The LEDs can then be connected to the shift register withount endangering the Netduino - assuming that you are powering the shift register with a suitable supply. I used an independent supply for this. If you are using the output from the Netduino then you need to ensure that the power drawn by the Netduino, Shift Register and the LEDs. For 8 LEDs I'd expect you to be alright as the total current drawn for the LEDs would only be 160mA will all of them on (working on 20mA each).

You may need to put a current limiting resistor between the output pin of the shift register and the LED.

Regards,
Mark

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#3 Stefan W.

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Posted 02 October 2011 - 03:31 PM

The Netduino should be alright if wired correctly. You can connect the SPI bus the the shift register directly.

The LEDs can then be connected to the shift register withount endangering the Netduino - assuming that you are powering the shift register with a suitable supply. I used an independent supply for this. If you are using the output from the Netduino then you need to ensure that the power drawn by the Netduino, Shift Register and the LEDs. For 8 LEDs I'd expect you to be alright as the total current drawn for the LEDs would only be 160mA will all of them on (working on 20mA each).

You may need to put a current limiting resistor between the output pin of the shift register and the LED.

Regards,
Mark


Careful, 74HC595s have low current ratings - http://www.nxp.com/d...74HC_HCT595.pdf says 35ma per pin, but only 70ma total sunk/sourced, so you'd need low-current LEDs (or have them at 8mA each), or add a driver stage after the shift register.
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#4 CW2

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Posted 02 October 2011 - 04:18 PM

or add a driver stage after the shift register.

Alternatively, you may consider TI's power registers TPIC6A/B/C595 or xxx596.

#5 Mario Vernari

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Posted 02 October 2011 - 04:49 PM

Alternatively, you may consider TI's power registers TPIC6A/B/C595 or xxx596.

WOW!...Very nice indeed!
Is it relatively easy to find? Any idea about the cost?
It seems designed also for inductive loads (coils, motors), and the G input may be connected to a PWM output so that the output power adjusted!
Much better than the classic 595...
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#6 Nevyn

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Posted 02 October 2011 - 04:58 PM

WOW!...Very nice indeed!
Is it relatively easy to find? Any idea about the cost?
It seems designed also for inductive loads (coils, motors), and the G input may be connected to a PWM output so that the output power adjusted!

You can get it in the UK from Proto-Pic - the component page is here. This means it's also probably on SparkFun as well as Proto-Pic look to carry a large amount of SparkFun stock. In terms of cost, the 595's are £0.45 each if bought in 10's, £0.55 each when purchased as singles. The high power version are £1.87 each.

Cheers,
Mark

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#7 smarcus3

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Posted 26 April 2012 - 05:06 PM

Can you add one of the TI's power registers TPIC6A/B/C595 or xxx596 to a daisy chain of 595s? If so how would it be hooked up?
Steve


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