The circuit works with the IRF510. I am currently using a 24V dc relay but the circuit works fine. I do not know what happened the first time I tried the circuit. thank you all. I might come back if I face any other trouble later on.It works fine with the heating element too....

 

The circuit works with the IRF510. I am currently using a 24V dc relay but the circuit works fine. I do not know what happened the first time I tried the circuit. thank you all. I might come back if I face any other trouble later on.It works fine with the heating element too....

I understood that, the first time, the relay was powered from your 8v rail, and that now it is powered directly from the (was it 15v?) power supply, with the MOSFET connecting it to ground in both cases. That voltage change would make the difference. Other than that, it's very easy to get the pins of the MOSFET confused.