referring to fig1 a vehicle windscreen wiper includes a wiper motor 1 and an associated self - park switch 2 which is closed during the sweep of the wiper blade but opens at the parked position at one extreme of the sweep . an electronic control circuit for controlling the operation of the motor 1 comprises a microprocessor 9 which is provided with a conventional crystal 8 . an integrated circuit voltage regulator 10 and capacitor 18 provide a regulated 5 volts supply for the microprocessor 9 , which supply also defines a logical high level for some of the microprocessor inputs . specifically , microprocessor input 19 is supplied with a logical high level via resistor 20 until a spring - loaded push - to - start switch 21 is actuated . initiation of the microprocessor program in this manner causes the output of a pulse to transistor 4 which is thereby rendered conductive causing the motor 1 to begin its operation . the pulse provided by the microprocessor is of sufficient length , e . g . 100 ms , to maintain the motion of the motor until the switch 2 closes to latch the motor in operation . if desired , the transistor 4 could be arranged to drive a relay having contacts connected in parallel with switch 2 . input terminal 22 of microprocessor 9 receives a signal from the self - park switch 2 which is at a low level while the motor is running but reverts to the high level in the parked position of the motor . the microprocessor calculates the time of the sweep of the wiper and thence calculates an appropriate delay before a second pulse is applied to transistor 4 . diodes 3 , 6 and 7 are protection diodes . at some point during each sweep of the wiper the microprocessor outputs a brief pulse to transistor 17 which discharges capacitor 16 . the capacitor 16 then begins to charge from the battery supply via resistor 15 and comparators 13 and 14 provide respective inputs to the microprocessor when two different voltage levels on the capacitor 16 are reached . comparator 13 provides an input when the voltage of capacitor 16 reaches 5 volts and comparator 14 provides an input when the voltage reaches a higher level determined by zener diode 12 which is supplied by resistor 11 . the times taken to reach these two voltages are determined by the microprocessor and thence the battery supply voltage is calculated . referring now to fig2 a flow chart of the software in microprocessor 9 is shown . firstly , in step 30 , various registers are initialized . the program remains in a loop around decision step 31 until the start switch 21 is actuated . when the start switch is actuated the microprocessor sends a pulse to transistor 4 to start the motor and simultaneously a timer or timing routine is started in step 32 . during the operation of the motor 1 a pulse is output to transistor 17 and the battery supply voltage is calculated in step 33 as outlined above . the program then remains in a loop around decision step 34 until the end of the sweep of the wiper when the self park switch 2 opens again . at this point the timer is read to determine the sweep time and the corresponding required delay is calculated in step 35 . the delay calculation may use an open - loop or closed loop method . the open loop control method assumes a fixed sweep time / delay profile and incorporates this profile between the maximum and minimum sweep times . the next delay , t d for a previous sweep time t s is given by ## equ1 ## where t smax and t smin are expected maximum and minimum sweep times established in the microprocessor . the profile f is chosen empirically to give satisfactory results , and may provide delay values in the range 0 to 3 s , for example . the closed loop method uses a single parameter , demanded wetness , which is held as the equivalent desired sweep time t sd . the delay time is calculated from the ` error ` between the sweep time t s and the desired sweep time , i . e . as ## str1 ## where k is a suitable gain factor . the calculated delay is applied by the microprocessor such that a following pulse is not issued to the motor until the end of the calculated delay . the program next proceeds to a decision step 36 at which it is determined if an update of the parameters t smax or t smin is required . a variety of tests could be employed for this requirement . for example , a test could be performed to see if the sweep time t s is less than the current minimum t smin or greater than t smax . if so the parameter t smin or t smax , as the case may be , is updated in step 37 to the current sweep time t s . the calculated battery supply voltage could be applied to scale the values of t smin and t smax but in a simpler procedure the sweep time is ignored , i . e . not applied to update these values , if the battery voltage is outside of a predetermined range . alternatively , an adaptive updating technique could be employed which does not require measurement of battery voltage and so step 33 and components 11 - 17 could be omitted . as an example of this technique , an estimated minimum sweep time t smin could initially be determined on the basis of the first measured sweep time , preferably equal to the first measured sweep time , and the estimated maximum sweep time set at the minimum plus a predetermined amount . the expected minimum could then be updated to a newly measured value when one or more newly measured time values occur which are lower than the currently stored expected minimum value . on updating the expected minimum value the expected maximum value could be updated by adding the predetermined amount . the predetermined amount could be a fixed amount , e . g . 70 ms , or could be variable to some extent in dependence on the width of the range of values of sweep time encountered in a session of operation . the above updating techniques allow the system automatically to adapt to mechanical wear , etc . a test is then performed in step 38 for a dry screen , e . g . if the sweep time has remained at its maximum value . if not , the program returns to step 32 to cause another sweep . if a dry screen is detected the program returns to step 31 to await another start signal from the driver - operated switch 21 . the screen could be considered dry and operation halted if a sweep time above the expected maximum is encountered on two successive sweeps . after operation is halted there could be a relatively long delay , e . g . 12 s , and a further sweep of the screen , which would serve to remove any residual moisture and confirm that dry conditions prevail . wiping would automatically restart if the sweep time of the final sweep is not indicative of the desired dryness .