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1  Using Arduino / LEDs and Multiplexing / Re: quiet start-up for shift register? on: January 29, 2013, 01:38:59 am
Thank you the quick and helpful answers.  I had tried controlling OE; what I was missing was the pull-up resistors.
2  Using Arduino / LEDs and Multiplexing / quiet start-up for shift register? on: January 29, 2013, 12:37:25 am
I have an arduino connected to a 74HC595 shift register, similar to the set-up in this tutorial (http://bildr.org/2011/02/74hc595/).  The output pins of the shift register are connected to some relays.  Output pin high sets relay off; output pin low sets relay on.

My problem is that when I start or reset the arduino, the relays chatter rapidly for a second or so.  I would like to avoid this.

What is the best way avoid fluctuations in the shift register output pins when the arduino is starting up?  I've tried writing to the shift register in the arduino setup() function, but that doesn't seem to be soon enough.  I'm using digital output pins 8, 9 and 10 on the arduino to control the shift register.  Would I have better luck with different pins?

3  Using Arduino / General Electronics / Re: Driving a shift register over a medium length wire on: January 09, 2013, 05:04:58 pm
Thank you, everyone, for the helpful answers.

LarryD: I'm using the relays to switch 24 volts AC (thermostat inputs to a radiant floor heat valve controller).  I assumed that for AC I needed to use mechanical relays rather than some sort of solid state alternative, but if I'm wrong about that please let me know -- I'll be pleasantly surprised.  I'm relatively new to this stuff.
4  Using Arduino / General Electronics / Re: Driving a shift register over a medium length wire on: January 09, 2013, 02:44:25 pm
Thanks for the suggestion.  I downloaded to source code for ShiftPWM (from http://www.elcojacobs.com/shiftpwm/ ), but I couldn't locate the discussion of push-pull line drivers.  Am I looking in the wrong place?
 
5  Using Arduino / General Electronics / Driving a shift register over a medium length wire on: January 09, 2013, 02:19:23 pm
I would like to control a bank of six relays with my Arduino Uno.  The relays are less than 5 meters from the microcontroller, so I'm hoping I can connect things with some medium length (less than 5 meters) wires.  In order to preserve output pins on the Arduino and (slightly) reduce the number of longish wires, I'm considering locating a shift register next to the relays and driving the shift register from moderately far away.

I know how to calculate the voltage drop and if I'm not mistaken it will be negligible.

What I don't know is what effect (if any) the the distributed inductance/capacitance of the wires will have.  I suppose this depends on the frequency of the digital signals sent to the shift register.  The sample Arduino code for shift registers that I've seen sets the pins high and low without any intervening delay()'s.  For 5 meter, unshielded wires (telephone cable, say), will I need to slow down communication with the shift register in order to compensate for slow transitions between 0 and 5 volts on the wires?
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