Abstract: An automatic blade angle controller system capable of maintaining a blade an earth-working tool level with the ground. A transparent, closed manometer partially filled with an opaque liquid is contained in an opaque enclosure. A light source is located within the perimeter of the manometer. Two solar cells are located on opposite sides of the enclosure in such manner that the light from the light source must pass through the manometer before impinging on the solar cells. The opaque liquid partially blocks the light from impinging on the solar cells. The voltages from the solar cells are subtracted resulting in an error signal. Of course, when the two voltages are of equal value, no error signal is produced, thereby establishing a level reference position for the enclosure. The enclosure is rigidly affixed to the blade, thereby establishing a reference position for the blade. A servo-hydraulic system returns the blade to its reference position upon receipt of the error signal.