68HC11 as a Digital Timer

The timer is proposed an effective model for controlling an external load 8A/380V. Equipped with a 68HC11 mu.P associated with an 8 MHz crystal, it can get very precise programmable durations. Two lines are planned: from 0 to 99 seconds and from 0 to 99 minutes. Both displays allow programming in conjunction with a 4-button keypad. They also display the count of elapsed time. The relay output double contact work controls the external load.
68HC11 as a Digital Timer - schematic

The choice is enabled by 68HC11A1FN lequartz 8 MHz and operates in BOOTSTRAP with Moda MODB = = 0. It runs on an internal program residing in the EEPROM. The displays are multiplexed, the attack of the segments being shared and managed by the port B via resistance of 390 W . PA3 PA3 lines and provide the multiplexing transistors T1 and T2 controlling the common cathodes of the displays. Line PA5 blink a LED diode in the range "min" to show that the counting is done well. The port C lines PC0 to PC4 as inputs, 4 keys on the keyboard connected to the inverter and "min / sec. PC5 to 7 lines are outputs. FP5 control relay via T4 and FP6 the buzzer using T5. Port E is not used. PD0 and PD1 lines are drawn at +5 V for the R20 correct starting the program. The reset mu.P the power is obtained by R12 and C3. Power is supplied by the 12V DC externally obtained either by battery or by a block sector. The relay and the buzzer used that tension. Cons by feeding mu.P and viewers is by bringing the 78L05 5V to 12V classics. 1 68HC11A1FN programmed ( to download the binary program ) Q1 BC549B .. 5 or eq. 1 red LED 5mm 1 1N400x 1 Reg 78L05 R1 .. 7390 W R8 / 9 3.9 k W R10 470 W R11/14 3.9 k W R12 27 k W R13 18 k W R15 1 M W Network 4 .. 19 R16 R 22 k W + 1 common C1 / 3 33 pF in 2.5 REB LCC...

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