google - ir thermometer circuit
https://www.google.ca/search?q=ir+thermometer+circuit&rlz=1C1NCHB_en&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiPxdD5zpLbAhVJ_IMKHfJDAd8Q_AUICigB&biw=1440&bih=784