3 volt signal to trigger 7 volt?

Ok,so I get a 3 volt signal from a detector and use this 3 volt to trigger an alarm which atleast need a decent 7 volts to function.The detector could take a max. of 5 volts input which I got from the 7 volts DC supply.This 5 volts fed,gets transformed into 3 volts after the detection.So let me make this clear,I have a 3 volt and a 7 volt supply.I need to use this 3 volt to trigger the 7 volt circuit.I know what you are thinking-a relay !right?but I couldn't get  a 3 volt relay anywhere(though I got a 5 volt relay).I can tackle this situation in two ways.1)Either by stepping this 3 volt to 5 volts and making the relay do the job.but how? 2)Or some circuit to use this 3 volt to trigger the 7 volts(like a relay but maybe by using ic's).This is the final part of my project and so please help me.

What current do you need to switch ? You use a transistor to amplify your 3V effectively.
Adarsh_tronix (author)  steveastrouk4 years ago
hey steveastrouk!I think atleast 0.15 amperes is needed for the relay.But please do tell me which transistor to use...
Try a TIP 121, or a TIP32. They're darlington transistors and can switch quite a good load.
quatch4 years ago
if it isn't high amps, use an optocoupler? A transistor could probably also do the job.