need to invert teh signal

[sherazi]

hi .. i am having this circuit, and i want that the o/p of PNP BJT should now be inverted... i tried but failed , plz help me , how can i invert the signal so that if the relay was supposed to be off by the current schm now it should be On and viceversa...

 

[john blue]

just add an inverter?

 

[ronv]

T1 is an emitter follower so it doesn't invert. So you should be able to move your relay back to T2 and get what you want.
PS. D1 is backwards.

 

[crutschow]

T1 is an emitter follower so it doesn't invert. So you should be able to move your relay back to T2 and get what you want.
PS. D1 is backwards.

T1 and T2 are both on at the same time so moving the relay to T2 won't change the signal polarity. You need to change T1 to an NPN with the emitter grounded and put the relay in the collector to invert the signal.

As ronv noted the diode across the relay is backwards. The diode cathode must be connected to the positive side of the relay coil voltage when the relay is on. Otherwise it will short.

 

[alec_t]

I think what ronv was saying is discard T1 (it's not needed) and connect the relay to T2 collector in place of resistor R7 ?

Alec

 

[colin mac]

I think what ronv was saying is discard T1 (it's not needed) and connect the relay to T2 collector in place of resistor R7 ?

Alec

Then the relay would still activate at the same time as it is already.

 

[sherazi]

sorry the diode placement is a mistake in the sch.
the purpose of doing this is to make sure that the o/p of relay should be NC normally but it should open if there is power failure,

i can use NC of the relay with some device to act as NC if there is power, but i thinkif i use a NPN as a switch connecteing base to vcc and let the C and NC of relay at Collector and emmiter, the problem i would face is that i should use GNd... but C can be from any other circuit.. is there another solution?