Mosfet switch circuit

Hi wondered if any of you tech wizards could help me with a small switching circuit. I want to piggy back off a flow sensor on a combo boiler to interface with a thermal store for the supply of hot water at the taps. The sensor voltage readings as follows earth 0V red Vcc 5V black with a yellow sensor wire reading 2.35 volts on activation. I need a high input resistance amplifier so as not to affect the existing circuitry ie minimal current drain which will switch state reliably with the 2.35 volt switching voltage which can be fed to a 12v relay to give 240v switching capacity. Low input voltage probably within noise region of TTL and probably below threshold for cmos or mosfet, with possible switching reliably issues hence some amplification may be required. Power supply available 12 volts DC. Look forward to any comments. Dave