A measurement while drilling apparatus for transmitting information carrying signals by fluid pressure changes in a borehole fluid circulation system having a flow restriction causing a high pressure zone and a low pressure zone, the apparatus having a passageway interconnecting the zones and a solenoid actuated valve interposed in the passageway, a sensor for sensing the magnitude of downhole measurements and for generating a succession of electric voltage changes indicative of the magnitude, a capacitor and a source of electric current for charging the capacitor, a circuit responsive to the sensor electric voltage changes for initiating successive discharges of the capacitor, which discharges are applied to the valve solenoid to produce successive openings and closings of the passageway thereby generating pressure changes in the borehole fluid circulation system.