what do you mean by port mode?

Not sure what Arduino you have or what kind of ports it has.

As an example, some ports on some processors can be configured in modes such as:

Inputs
Open drain
Push pull
Pull up resistor of various values
Pull down resistor of various values.

So, that's what I mean by mode. In the simplest definition it used to be input and output. Now it's much more complex than that. The output voltage may only go to 3.3 V for example.

 

I calculated the value of the base resistor and is coming around 5k ohms for 10mA of current

That calculation is incorrect. Start with the high output voltage of the microcontroller. Subtract from that the Vbe of the transistor. This is the voltage that will appear across the base resistor. Use that to calculate the base resistor.

ak

 

That calculation is incorrect. Start with the high output voltage of the microcontroller. Subtract from that the Vbe of the transistor. This is the voltage that will appear across the base resistor. Use that to calculate the base resistor.

ak

I tried the circuit again with the right ULN connections but the circuit works properly for 4 to 5 times and then again the relay is backeffecting the circuit. Is that mean that the diode and transistor are not protecting the circuit with the required efficiency? I have used BC548 with IN4007 and relay.

 

What do you mean by "backeffecting" the circuit?

ak

 

What do you mean by "backeffecting" the circuit?

ak

when I give command through the bluetooth for switching on the appliance it works for sometime but after that the relay does not operates and due to back EMF or current, the arduino is misbehaving i.e blinking fast and bluetooth module got disconnected straightaway.

 

Why do you have two threads going for this issue??
http://forum.allaboutcircuits.com/threads/relay-problems.129384/
Max.

 

You may want to monitor the 5V output from the 7805 with a voltmeter (better with a scope). It sounds lkke more than one of the relays is turning on causing too much current draw from the 7805. That will cause the 7805 voltage to drop lower. Which in turn causes the arduino to go crazy.

 

You may want to monitor the 5V output from the 7805 with a voltmeter (better with a scope). It sounds lkke more than one of the relays is turning on causing too much current draw from the 7805. That will cause the 7805 voltage to drop lower. Which in turn causes the arduino to go crazy.

right now i m controlling a single relay on arduino but still having the problem.

 

My friend made IOT based home automation and he used the same ULN and relay circuit with IN4007 and BC548 and his project is working fine but why is it misbehaving in my case?

 

My friend made IOT based home automation and he used the same ULN and relay circuit with IN4007 and BC548 and his project is working fine but why is it misbehaving in my case?

As per your other thread, the ULN has the diodes in already!!
Max.

 

As per your other thread, the ULN has the diodes in already!!
Max.

What should I do then? Is IN4007 not working properly or the current and voltage through 7805 is dropping and hence arduino is going crazy also i dont have time to replace the ULN from the circuit. In my other thread one member posted that this might be the inrush current which is responsible for this back EMF and suggested me to add a capacitor to the supply from where the bulb is getting current.

 

Thank you all for your valuable suggestions and solutions. The project is now working properly.

 

Thank you all for your valuable suggestions and solutions. The project is now working properly.

What was the problem and how did you fix?

 

Sorry for not replying, Actually the problem was with the darlington array IC which I was using for the amplification of current and I fixed it by replacing it with a simple NPN transistor circuit.