Good Morning,
I've this scenario: During that ArduinoMega is "booting Up" the relay (switch when is low) is stay low then turned on and after when entry into loop go into the correct state ... that means that my load (for example a lamp) change state without to be managed during this time and this is not correct ...
I've checked the code and it's good and I've tried several pin...
COuld you help me kindly ?
thanks
in advance,
gnux
You have an Arduino Mega 2560 board ?
And your relays don't have a pre-defined state during power up ?
http://arduino.cc/en/Main/arduinoBoardMega2560
First of all, you should not use pin 13. A system led is connected to pin 13, and it might blink during booting.
Also avoid pin 0 and 1, they are for RX and TX to upload a sketch.
To set the pre-defined state of the relays, we have to know what kind of relays and how it is connected.
It is a module with relays ? Can you give a link to that module ? Or a schematic ?
Good Morning,
the board 2560
the relays is power on with logic state 0.
so is connected with pin 14 (I've tried to change pin for example use 8 or 9 but the reult is the same).
and i didn't use 13 or 1 ...
for the relay there is only a transistor that manage a relays ... in attchment the photo 
thanks for support me ...
is happen the same if i put a led instead of relay ... during the setup I'm attaching and then detaching interrupt
regards,
gnux
I can't find the brand and type of that relay module.
That transistor is normally a NPN transistor and a high level at 'IN' activates the Relay.
So I don't understand why a '0' activates the relay.
Did you connect it correctly ?
VCC to Arduino 5V pin.
GND to Arduino GND pin.
IN to Arduino pin 14.
Can you see the type of the transistor ? I'm 99% certain that it must be a NPN transistor that is activated with a logical '1'.
What is going on in your sketch ?
Can you upload your sketch between code tags (use the '#'-button above the text field).
Hi,
did you do this on setup? ->
void setup() {
pinMode(relayPin, OUTPUT);
digitalWrite(relayPin, LOW); // this depends on whether you have connected the load to the relay on the NC or NO position...
}
HTH,
Pedro.
hi,
Iam also having the same problem, I have included the said in my sketch, but not working,
I think thr arduino bord, when powerd on, it makes all the pins HIGH for a moment and then only turns to the instructed state,
Iam using a uno r3 board,
regards,
adal.
Not high, but high impedance.
During power-up the pins of the Arduino are all input pins.
So the voltage level is high impedance or floating.
That is not a problem with an opto-coupler or transistor. They need some current to be activated. Since there is no current, they are not activated during power up.
I don't know what is going on. Show us how it is wired, which relay module, the sketch, photos and so on. Perhaps we can detect a problem somewhere.
Ok...
I know for a fact that upon boot all IO pins are set to "input" so anything connected to them might see a floating voltage hence the somewhat "unreliable" behavior.
I would, in this case, put a pull down resistor of, say, 10K between the pin and ground ensuring a "LOW" during the startup phase.
HTH,
Pedro
Ok I will try with "opto-coupler" and will see if the problem will be fixed 
i hope thanks you for your suggestion
I have already tried with 470 ohm resistor but nothing change ,
thanks
Gnux
Caltoa:
That transistor is normally a NPN transistor and a high level at 'IN' activates the Relay.
So I don't understand why a '0' activates the relay.
Actually, on a lot of these el-cheapo chinese units, the circuit consists of a high-side PNP driver, rather than the low-side NPN driver (which you would expect).
Thus - the logic is reversed, and a logic-level LOW turns on the relay, while a logic-level HIGH turns it off.
You can fix this in your code, but this doesn't fix what happens at startup.
You could probably put a TTL hex inverter/buffer after the pin to invert the output (and leave your code alone), and this would fix the problem; its a bit of a hack - but so is the cheap relay unit.
/I honestly don't know why they continue to use this circuit; one would think an NPN system would be better, make logic sense, and possible be cheaper...unless there's a glut of PNP transistors in China?
Okay, now I'm no longer 99% certain it is a NPN, but 98%.
I tried looking through the pcb to the copper traces in the photo.
Test the relay module independent of the Arduino. Then use a pullup or pulldown resistor as needed. Don't use pin 13 when you hookup to the Arduino, the bootloader usually pulses that pin.
Caltoa:
Okay, now I'm no longer 99% certain it is a NPN, but 98%.
What are the markings on the transistor? If you look those up to get a datasheet, that will tell you...
thanks pdoriam,
making relay pin low solved my problem,
regards,
adal.