Brantham Electronics was approached to design an audio warning device to notify the operator when a grain drier is full.

As the drier fills up with grain, the grain closes a switch covered with a rubber membrane. An audio siren sounds a warning for a preset time to notify the operator. When the drier is emptied and refilled the siren will again sound when the grain covers the switch.

The ingenious aspect of this design is that there is no on/off switch (apart from the switch behind the membrane).  The circuit does not draw any current from the battery except when the siren is sounding, thus prolonging the battery life. This is an important consideration as the unit is likely to be four metres above the ground and may be not be easily accessible.