Hello Friends,
I would like to switch a a relay when no signal coming, for that i used a PNP transistor switching circuit. In this circuit i am using same same supply voltage to the coil of relay as well as the base of transistor. Unfortunately there is some problems in the circuit. Any one can modify my circuit of suggest a new circuit for switching the relay. I am using only a DC 5 voltage supply in this circuit, the relay also can swithci n 5v. Please help me.
The two circuit i used given below

 

I would like to switch a a relay when no signal coming

What provides this mystery 'signal'?
What are its voltage/timing characteristics?
Is SW1 a normally volts-free switch?

 

In both circuits you are not providing any base current to the transistor to cause it to conduct (Between collector and emitter.) You need to connect the left hand side of the switch to the battery negative. With that done both circuits should work but the left hand one is the best. It is not good practice to rely on a 6 volt relay to pull in on less than 5 volts. (There will be some voltage drop between the collector and emitter of the transistor.)

Les.