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Unread 31-07-2013, 22:07
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Re: Arduino + Ethernet Shield Help

Quote:
Originally Posted by jwallace15 View Post
What sort of thing are you talking about? Resistor? Transistor? 1000µf capacitor?

(ok the capacitor was a joke but still, do you mean a transistor?)

I did notice when I first started using it that strange things would happen.. Such as when I'd try to close the circuit for all 8 relays that the LED's would dim and only 4 relays would click. I also had trouble with the digital input pins (I have emergency stop buttons on the control panels, and was trying to get it so that if the buttons were pressed it would turn on a relay that closed the circuit for a very bright red LED matrix to simulate a warning light) when the relay was activated.

I got my R3 from a kit off of Sparkfun and it included a toy motor to switched on by a transistor (but run off of the Arduino +5v output). It also had a temperature sensor. One day I was trying to simulate a thermostat by entering in a selected temperature into the Serial Monitor, and then if the current temperature was above the entered temperature it would turn on the motor. I'm not sure what happened but the temperature sensor started giving me bogus readings of 500+ degrees (it was not a programming error as I tested it with a code I had written before that worked fine). I think possibly I burnt out the sensor?

/tangent

Anyways, are you suggesting adding a transistor between the Arduino and the relay?
Sorry I am quite tired.
I deleted my post above because it was going off on a tangent of it's own.

http://www.amazon.com/SainSmart-8-Ch.../dp/B0057OC5WK
The PCB layout image at your Amazon link says something like:

Code:
Input control signal LOW state current:
2.5V at  0.1mA
3.3V at 0.18mA
  5V at 0.35mA
The Arduino use Atmel AVR MCU and they can sink and source a maximum of 40mA per I/O and generally a chip total of 200mA.
Look at the specification sheets for the Atmel AVR chips in question and this link:
http://forums.adafruit.com/viewtopic.php?f=24&t=17802

Here's the thing....it seems unlikely that board uses so little control current as it appears that the input goes through a resistor into the opto-isolator if you look at the blurry schematic.

From the pictures that board appears to have 817C opto-isolators (the close up shows B1312 and 817C).
The datasheet for that is likely here:
http://www.futurlec.com/LED/PC817.shtml

That part lists 50mA maximum forward current and is setup for a nominal 20mA current.

I suspect having thought about this that the numbers as shown on the layout provided are wrong.
How does one operate a 20mA LED with 20x less current and expect that to work?
So either I am wrong and those transistors are the buffer or there is no buffer and those specifications are wrong.

1. Wire that thing up.
2. Disconnect the Arduino.
3. Take a DC ammeter or a suitably configured DMM and measure the current when you close the circuit between one control input pin and the system ground.

If that current exceeds 30mA you do not want to drive it with the Arduino directly.
I suspect the inputs source more than that which is why the reviewers are complaining they can only turn on 5 with the Raspberry Pi.

The safe bet here, given how easy it is to test, is to test this.
If the measured current exceeds say 30mA greatly then drive the 8 relays with a ULN2803 (8 Darlington transistors) or 8 discrete transistors. Just remember to account for the junction voltage drops.
If the measured current is between 30mA and 40mA look at the TTL 7407 or 7417 chips.
Just keep in mind that TTL 7417s are rarely available from any company except Texas Instruments.
7407, 74LS07 and even 74HC07 (open-drain CMOS equivalent) are a bit easier to find these days.
If the measured current is around 20mA or less then it should be fine but just remember 20mA x 8 circuits = 160mA.
If you make all 8 Arduino outputs low then that 160mA will consume a large portion of total maximum drive the chip can handle. So be careful about what, besides that board, you put on there while it's like that.
If the potential for 160mA concerns you, then again, look at the 7407 because that will dramatically reduce the current demand on the Arduino (roughly 1/60 the current for all 8 circuits).

Last edited by techhelpbb : 01-08-2013 at 01:03.