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Topic: LED's & Relay Boards (Read 4651 times) previous topic - next topic


Old School Programmer - ARGH ( COBOL ) - Yes I still pump blood, lol.

New to arduino & electronics and don't want to burn anything up...

Arduino: Mega 2560 - RadioShack
Relay: 12V 8 Channel Relay Module Expansion Board For Arduino, PIC, AVR, DSP, ARM, SCM

Relay Link: http://www.ebay.com/itm/12V-8-Channel-Relay-Module-Expansion-Board-For-Arduino-PIC-AVR-DSP-ARM-SCM-/221028171239?pt=LH_DefaultDomain_0&hash=item33764e3de7

Q: Can I add an LED between the arduino pin and the input pin on the relay board?

    IF SO: Do I need to add a resistor?

Q: Do TTL relay boards consume current from the arduino as do the LED's?

     IF SO:   How do I determine the amount of current required by each relay?

     AND:    How do I find the maximum available current for (Each Pin) and (Entire Board)

I would like to run 6 of those 8 channel boards with my arduino and would like status LED's


Oct 11, 2012, 06:17 am Last Edit: Oct 11, 2012, 06:54 am by johnwasser Reason: 1

Q: Can I add an LED between the arduino pin and the input pin on the relay board?

   IF SO: Do I need to add a resistor?

Not 'between' so much as 'along side'.  You do need a resistor to limit the current going through the LED.  Pick your LED's and use the highest resistance that gives you adequate brightness.  Something around 440 ohms should work.  Don't go below 100 ohms.  Connect the Anode (+) side of the LED to the Arduino output pin.  Connect the Cathode (-) side of the LED to the resistor.  Connect the other side of the resistor to Ground.  Since the LED will typically need 20 mA or less there will be no problem connecting the relay drivers (5 mA) to the same output pins.  The Arduino pins can handle 25 mA easily.
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Hello, I'm not sure because I'm also beginner, but I did some research because I have a similar project involving a 16 relays board and an arduino mega:

The relay board must be powered by external 12V power supply

On that relay board are opto isolators, one for each relay, it's basically a small LED, so the Arduino must provide a small current for just lighting those LEDs, about 5mA for each opto isolator. That's all the Arduino have to do, lighting a LED to turn ON a relay :)

You want 48 relays: do you have enough pins, as each relay will use one pin? If you don't, there is still a solution which is, using shift registers (search google "74HC595 arduino")

And here you can find useful infos about relays board: http://arduino-info.wikispaces.com/ArduinoPower

Good luck :)


Old School Programmer - ARGH ( COBOL ) - Yes I still pump blood, lol.

New to arduino & electronics and don't want to burn anything up...
:D I want to write a program, but I don't want to create any bugs... :D

Hardware errors aren't as common as software errors, which is an indication of how difficult programming is...    But sooner or later, you are going to hook-up something wrong, or backwards, and something bad is going to happen.  I'm an old-time hardware guy and it still happens to me once in awhile.

Be careful, and hopefully you won't burn-out your Arduino or your relay board.    When it comes to LEDs, resistors, capacitors, and cheap IC's or other cheap parts...  Buy extras! ;) 


Thank you so much for the responses.

I thought running a led in parallel with the relay board would work, thanks

I look forward to being a member of this community :)


I have been using a relay card from another manufacturer with almost the exact same design as that for the past month (except my input is active low), and I have not had any problems with the GPIO 5V output driving the coils or drawing and/or feeding anything bad back into the pi, although I don't dare turn all 8 relays at the same time (480ma through all the coils). I did discover on mine that between the active low problem and the fact that the 3.3V output of the GPIO pins was not enough to drive the optoisolator, I needed to use transistors to convert the 3.3V input to 5V and everything works fine now.


Oct 12, 2012, 07:08 pm Last Edit: Oct 12, 2012, 07:10 pm by NobleNoob Reason: 1
In further research of my relay options, I came across these relay boards available in 2, 4, 8, 16 relay's.


Since I use a converted ATX power supply for my projects 12V requirements, It seems I have a spare 36 Amps on the 5V rail :)

With the above relay board, do I remove the jumper between JD-VCC ^ VCC on the three pin header and connect my atx supply to the VCC & GND pins on the 3 Pin header?

Then Connect the 4-pin header GND to arduino GND and IN1, IN2 TO D5, D6 and leave the VCC Pin empty?

The current older boards I have use 12V coils on relay and only have TTL pins for the connection, ie GND + 8 TTL, The 12V is on screw terminals.

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