Help with NPN transistor / relay circuit

[Gatzy118]

Hi, i'm working on a project at school and have a problem with using a sensor to trigger a transistor.

The circuit is shown below. Basically i want the relay to be on when the sensor gives an output of 5 volts, and off when it drops to 0.4 volts.

However, connected to the base of an NPN transistor, the BE voltage reads 0.43V?

Could someone suggest how to fix this problem or a different technique involving the 5volts output from sensor and a relay?

the 8.8kohm resistor is the resistance of the sensor so i assume it needs to be there :s

Thanks a lot, Nick

 

[Gatzy118]

Ahhh, i believe i have solved it, i needed a ground

 

[ssembo]

How??????????? Man, mine is; i want to trigger a relay using an npn transistor when there is a 5v on the comparator output. what type of relay do i have to use, transistor type because the relay will have to switch a high voltage transformer. thanks. Colin

 

[Reloadron]

You would use a circuit similar to the one posted above. First you need to know your relay coil current / voltage needs. Then with that known you can worry about selecting a transistor that will drive that load (relay coil). You really need to provide more information. Also define "high voltage" that the relay will be switching. Also, the current load the relay will be switching.

Ron