Page 306 - Embedded Microprocessor Systems Real World Design

Appendix A

Example System Specifications

System Description

The system is a swimming pool timer that cycles the AC pump motor on a swimming pool.
The power input is 9 to 12V DC from a wall-mount transformer.
The pump is a 1/2-hp single-phase AC motor, controlled by mechanical relay. Relay is
remote from control unit, located in weatherproof box near pool pump motor.
Provision is to be made for a switch closure input that prohibits pump operation if the
water level is low.
The user can set the length of time the pump is on and off. An override is available to
permit turning off the pump when it is on for maintenance and turning the pump on when
it is off so that chemicals can be added.
On/off/override time is to be adjustable in 30-minute increments from one-half hour to
23 hours. A display will indicate the on/off condition of the pump, the time remaining, and
whether the pump is in the override mode. The display also will indicate the condition of
the water low monitor.
A minimum number of switches/knobs will be used.

User Interface

Display. Four seven-segment digits: two digits for hours, two for minutes. Also three LEDs:
SET, ON, and OVERRIDE. The LEDs are to be high intensity for daylight readability.
Keypad. There are four keys: SET, ON, OFF, FCN.
Operation. The display will indicate the time remaining before pump switches on or off.
After reset, ON time will be set to 8 hours, 30 minutes. Off time will be set to 8 hours, 0
minutes.
Display will flash to indicate that power has been removed.
After power-up, ON and OFF override will not be allowed until SET has been pressed by
user. Pressing ON will activate ON override. Pump will be turned on for 30 minutes, the
display will show the override time, and the override and ON LEDs will be lit. Each succes-
sive push of the button will increment the override time. Normal time will continue to count

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