Question about npn transistor gain and voltage output

[little_carlos]

Hey guys, i have a question with this thing thats been in my head for over a week
im using a pic 16f628A and im conecting a output to a bc547 npn transistor, the thing is that the pic output gives about 4V, and i need to amplify so the transistor swtich a relay, wich is for 5v, but i only get 3.5V of output from the transistor
is there any other npn transistor that would give me the 5V of output with only 4V in the base? or any configuration to increase voltage gain in the bc547? thanks for your time

[TKt48]

TTL outputs can sink relatively high current in low state, but output current in high state is minimal. You can use pnp transistir connected by emitter to power rail and base via resistor to pic output. Use then negative logic to drive relay. Or connect relay directly to the output and it will switch on when state is low.

[Paul Price]

You can use a NPN transistor, just activate the relay coil with the relay in the collector circuit to +5V, do not drive the relay using an emitter-follower, drive the base of the NPN relay driver with (for instance) a 470 ohm to 10k in-series base resistor, emitter is grounded. Put a 1N4001 diode or equiv. (cathode to +5V) across the relay coil.

[Zero999]

It sounds like you're not providing enough base drive current.

How much current does the relay need? If you don't know that, what's the coil resistance (measure with a meter) and calculate the current using Ohm's law.

It's actually impossible to get 5V out with a 5V power supply because there will always be some loss in the transistor. It's possible to get very close but the voltage across the relay will always be slightly less than the power supply voltage.

[kaindub]

I think you have the transistor connected as a voltage follower. The clue is that the emitter voltage is 0.7v less than the input.
You need to connect the output of the pic via a resistor to the base of the transistor.
Groung the emitter.
Connect the relay between 5v and the collector