I have a question. I want a bulb to glow if the supply is 14 volts and if the supply goes to 7v or lower, the bulb must not glow. Is there any way to do that.

Thanks

 

An op amp could accomplish this.

 

How much power does the bulb require? It must be a bulb, or a led would do?
You mean that the bulb must light when above 7v?

What do you want to do, exactly?

 

Wouldn't a Zener diode be the easiest method?

 

A normal bulb of 12v. I mean it must light when the supply is 12v to 14v, but when the supply drops from 12v to 6v or 7v, it must not light. Any kind of single point bulb which are used in cars. The exact watt is not known to me.

 

If you are trying to measure the battery voltage - you should be made aware that if an automotive battery is below 12.5v at 25°C, it is beginning sulphation. Sulphation will eventually cause the battery to not be able to accept or release a charge. If it drops down to 11.4v at the same temperature, it is considered 100% discharged. Discharging an automotive battery that deeply will cause permanent damage, and it will not last long after that.

What exactly is it that you wish to do? Find out when your car's battery is low? If so, 7v would mean that it is time to buy a new battery, as you will not be able to revive it.

 

A normal bulb of 12v. I mean it must light when the supply is 12v to 14v, but when the supply drops from 12v to 6v or 7v, it must not light. Any kind of single point bulb which are used in cars. The exact watt is not known to me.

A """"normal"""" 12v bulb in a car can be a 4w bulb or a 75w+75w bulb. Not a lot of difference, just that one will need a circuit that can handle 350mA, and the other 12.5A
/sarcasm

Just take the bulb and read what power it is. It's tipically written on the metal part of the lamp.

Also, what you mean is that it must light with 12v or more and don't light when it's below 12v?