Liquid-level detector

A liquid level meter comprising:

• a light-emitting diode and a phototransistor pair connected in a Darlington configuration mounted on the same surface of a common substrate;
• a U-shaped optical guide for conducting light emitted from said light-emitting diode to said phototransistor pair, the amount of light reaching the light-responsive element being proportional to the index of refraction of the material surrounding the optical guide, said optical guide being supported on said substrate so that the ends of said U-shaped optical guide face the light-emitting element and the light-responsive element, respectively, said U-shaped optical guide defining a plane disposed parallel to the plane of the top surface of the liquid, the level of which is being detected;
• a ceramic cover extending from said substrate and surrounding a portion of said U-shaped optical guide, said cover optically shielding said light-emitting photodiode and phototransistor optical pair from each other across said substrate surface and from ambient light, said ceramic cover also shielding said photodiode and phototransistor Darlington pair from ambient heat;
• a power source for supplying a constant current to said light-emitting photodiode;
• and
• means for detecting a variation of an output signal of said phototransistor optical pair.