The rfduino relay shield use
PDTC114YU transistor which R1=10k, R2=47k.
But the output of gpio from rfduino is 3.3V, that mean, (3.3v-0.7v)/10k = 0.25mA, this would be base current to the transistor.
hfe is normally 20 (on saturation situation), then the collector current would be 0.25mA x 20 = 0.5mA..am I right?? but 0.5mA would be very small current to operate relay. I measured the relay needs at least ~20mA. So it is very strange. Somebody can explain for me?