First the diode. An inductor resists to changes in current. So if you switch off the transistor the relay coil will try to keep the current flowing, it becomes a current source. If there would be a low resistance path that would only create a low voltage, according to Ohm's Law. But if there's no way out for the current the voltage will rise to several tens of volts, and destroy the transistor. The diode provides a low resistance path where the current is drained to the positive power supply. BTW, a Schottky diode is a better choice here.

R1 and Q1 look OK. The BC546B has an \$\mathrm{H_{FE}}\$ of 200 minimum, and with a 5V input you have 1 mA base current, so the 200 mA collector current is more than sufficient.

Understanding of Relays and associated circuitry
It may seem as a kinda dumb question, and I'm totally aware of this. But, what I'm going to ask may be specific enough to not get closed:

I've been playing guitar and also a few other instruments for quite a while now, but I just can't sing. I can hear a melody in my head, but even if I try to hit the first note, I will fail very often. Now what I want to ask is, can anyone learn to sing in a manner that "won't hurt the ear of a non-musician"? I'm not asking if anyone could be the next Freddie Mercury, but does anyone have the ability to sing some backing vocals if needed, without just sounding terrible?

I always find it hard to get question on music get to the point, so i'll try it with this: Can anyone achieve semi-professional level of singing, even if one doesn't have a "good" voice?
