Wiper Delay

04 This two-key wiper delay circuit is remarkable for its simplicity and ease of use. The wipe is started by pressing the SET switch, which also serves to adjust the length of the wipe interval. The cir- cuit is turned off by pressing the RESET button. The wiper delay shown in Fig. 1 consists of three opamps and a monostable multivibrator (MMV). Opamp Ai is set up as a triangular wave generator, con- trolled by the output of the MMV. When this is low, a slowly rising sawtooth voltage appears at the output of Ai. The rise time of the sawtooth depends on R2-C3. Opamp As compares the voltage across C4 to the instantaneous sawtooth amplitude. The output of A3 drops from 8 V to 0 V when the sawtooth voltage ex- ceeds U[C4]. This change in the output voltage of A3 is delayed by Rs-C6 and passed to A2, so that the MMV is trig- gered somewhat later. The wipers are switched on via Tiand Re when pin 3 on the 555 goes high. Also, C3 is rapidly discharged via Di and RI, while D2 prevents the ...