[General] Relay activation if ULN 2003 is disconnected

[nauman_mrd]

Hi friend

i am a little confused in designing a circuit, i have attached my idea here in attachment.


under the normal condition the ULN2003 should work as normal and able to activate/deactivate relay coil through micro controller by setting it to 0 and 1 through programming,

but what i want is
when somebody cut down/disconnect the output of ULN2003 to relay, and relay don't get the GND (0V) from ULN 2003 to relay, i want to activate the relay in this condition as well. and when the wire is rejoined with the ULN2003 output, the relay should work under normal condition.

Is there any way to make it possible ?

 

[betwixt]

If you can get a relay with normally closed contacts and turn it OFF with the ULN2003 it will work. The asbsence of power will leave the contacts closed.

Otherwise you need an alternative ground connection for the relay to operate and it has to be wired in such a way that it stays on the relay side of the disconnection. Whether this is possible depends on your exact application and resources.

Brian.

 

[nauman_mrd]

Thanks betwixt
i am using 3 contacts relay (Common, NC, NO) and i am using common and NC contact under normal condition.

but if ULN2003 is disconnected i want to activate relay coil to provide GND to relay externally but i am not getting any idea how to do it,
while when the ULN2003 is connected again it should work as normal.

one thing i got in my mind is to use pull down resistor at relay coil's pin but i don't know whether it will work or not

 

[Venkadesh_M]

Best way is inverting NC and NO contacts with the neagative logic, it will be automatically work in that way but that depends your application. But this is not complete.

 

[betwixt]

That's what I meant. If you reverse the NC and NO connections the relay will close with no voltage across the coil, as it would be if it was disconnected. The problem then is the relay works the wrong way around to the way you want it, closed becomes open and vice versa. To fix that, invert the signals going IN to the ULN2003 so what would have turned the relay on now turns it off. You might be able to do the signal inversion in software if a micro is driving the ULN203 or you can wire a logic inverter (NOT gate) in series with the signal feeding the ULN2003.

Brian.

 

[nauman_mrd]

Best way is inverting NC and NO contacts with the neagative logic, it will be automatically work in that way but that depends your application. But this is not complete.

You are right but NC contact is recommended for normal operation in my application