Concerns

Okay so how unsafe are fetal heartbeat dopplers?! Some people are saying they aren't, some people are saying they're completely unsafe. I bought one yesterday, found my friends babies heartbeat and my cousins babies heartbeat. They're both further along than I am...I'm on 11weeks but I can't seem to find my babies. I'm scared that I've used it so much since I've got it, that I've exposed my baby to too much radiation. I just had an ultrasound yesterday and everything was good and heartbeat was a strong 170...I'm now freaking out again...