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I have a RPi2 to which is connected a 4 channel relay board and a LDR sensor module. The connections are as follows:

Relay Board VCC => Pin #2 (5V)

Relay Board Ground => Pin #6 (Ground)

Relay Board Input1 => Pin #11 (GPIO17)

LDR Sensor VCC => Pin #4 (5V)

LDR Sensor Ground => Pin #14 (Ground)

LDR Sensor Data => Pin #13 (GPIO27)

A python script monitors the LDR sensor and depending upon that the Relay is turned ON or OFF.

The problem is that when the relay is switched ON, the Red LED on the Pi starts blinking. I have tried various USB adapters rated 5V 2A and 5.1V 2.1A, but the problem persists. The relay is working fine but I am worried about the blinking LED.

  • We need to see your code and pictures of all the connections. What leads you to believe it is a power issue? How do you know the relay works as intended? Are you using a flashing or standard LED? How is the LED powered? – Steve Robillard Aug 16 '16 at 6:20
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    The LED I am mentioning about is the Red power LED on the RPi. Also, the relay is working fine as an AC lamp is working perfectly controlled by the relay. This may help raspberrypi.stackexchange.com/questions/47431/…. Note: the grounds are connected. – Aneesh Relan Aug 16 '16 at 6:25
  • OK, that makes it clearer what LED you are referring to. in that case I suggest using a separate 5V powersupply for the relay you could use a uln2803 darlington array to control the relay (assuming it is active low). This may help raspberrypi.stackexchange.com/questions/47431/…. Note: the frounds are connected. – Steve Robillard Aug 16 '16 at 6:31
  • Look at the link. – Steve Robillard Aug 16 '16 at 6:34
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If the Power LED goes off or flashes the power supply is inadequate - no amount of discussion will change this. NOTE that the Pi itself will run down to 4V (or possibly less) and the relays may also work, but this is asking for trouble if any peripherals are used.

I doubt the relay board draws enough current to affect any decent supply. The only way to check is to actually load the power supply and measure the voltage.

See Raspberry Pi Power Limitations

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