Minilec [india] offers infrared optical sensors [coolant liquid level sensor frkp2]. The optical sensors operate on external 12 or 24v dc supply and provide for single output to operate the relay. These sensors can be directly interfaced with plcs or cnc controllers. The sensors can be mounted from the top or sidewall of the coolant tanks. Optical sensors make it reliable and very effective. Along with all these, relay module dlcd 1, which can be used with the sensors are also offered.
