A solar stepping stone comprises a housing having an upper surface, side walls and a lower surface, the housing defining a chamber. The input is a digital signal which is a modulated signal. The method also includes steps of enabling a bias current to a transconductance operational amplifier and changing an operating state of the drive circuit from the pulse width modulation mode to a linear mode. The force transfer device is connected to an exterior of the waste receptacle to transfer a force applied to the foot pedal to the door position adjustment device to move the door from the closed position to the open position. A neutralization reaction between the acid and alkali is caused by emitting the alkaline liquid agent to the surface of the to-be-cleaned substrate so as to efficiently remove the acid liquid agent remaining on the surface of the to-be-cleaned substrate.