Variable speed DC motor controller apparatus particularly adapted for control of portable-power tools

In a portable electric tool having a DC motor for driving a tool bit and for controlling the speed and torque at which said tool bit rotates, the improvement comprising:

• a zero displacement switch means coupled to said tool and operative to provide an output voltage proportional to the pressure applied directly to said switch via the hand of the user,
• control means coupled to said motor and responsive to said output voltage to variably control the speed of said motor according to the pressure applied to said displacement switch means,
• said control means including a pulse modulator for converting said output voltage to a pulse width modulated signal at an output of said modulator and means for applying said pulse width modulated signal to said motor for varying the speed of said motor according to said applied pressure, and
• switching means having first and second output electrodes and a control electrode, with said first output electrode coupled to one terminal of said motor and with said second output electrode coupled to a point of reference potential with said control electrode operative to switch said first and second output electrodes according to a control voltage applied thereto and with said control electrode coupled to said output of said pulse width modulator to operate said switch according to said pulse width modulated signal to therefore control the current through said DC motor and therefore the speed of said motor according to said pulse width modulated signal.