The circuit is a 555 monostable, The push-switch is to
trigger and generate a reset pulse for uC. The diode is for
OR' ing later. The High will go thru the diode but the Low of
555 cannot drink any current as diode blocks.
The bottom part of the Circuit is the CD4093 Schmitt nand
based flipflop. Power on Soft Start with Reset.
See the trigger pin 2 being used with a Switch to ground, a
Green Button to start a timer. The timer output pin 3 goes
high and turns on a Relay. The relay contacts could be used to
drive a motor or lamp. In this configuration Pin 5 control
voltage sets the time. When pin 3 goes high the pin 7
discharge goes into high impedance. Now C starts charging thru
Ra. When 2/3 Vcc or 15V is reached (10V) across C; Pin 6
comparator senses it and changes the state of the internal
Flip-Flop. The output goes low, Relay tuns off, pin 7
discharge turns on and discharges the cap C. Motor is off.
Now the timer is ready for the next cycle when you press
start. Remember that when the Timer was on and the Motor was
running; pressing reset would stop the motor prematurely. This
would be the Red Button.
20th Apr 2020
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