The phenomenon of voltage generated from a soft sensor using polypyrrole in response to mechanical deformation is described and investigated. The sensor consists of two polypyrrole layers in contact with an electrolyte and operates in bending mode in air. The magnitude and sign of the induced voltage was found to depend on the type of dopant counter-ions and the nature of the surrounding electrolyte. The mechanical sensor response is shown to be a "reverse actuation", generating millivolt signals for millimeter sized deflections or similar to 1000 C m(-3) charge for 1 % strain in the polypyrrole layer. A model based on 'Deformation Induced Ion Flux' has been proposed whereby the strain induced volume change in the polymer produces a shift in the Donnan equilibrium between mobile dopant ions inside the polymer and in the external electrolyte. A simple thermodynamic model provides reasonable estimates of the size of the voltage and charge produced.