Count Down Timer with PIC16F84A

The choice of material to read or drive is the definition of three digits (Bit), RA0, 1,2 PORTA of the PIC at the entrance of 74HC138A. To select a material for reading or driving an output of 78HC138 is '0 '(Low), forcing the transistor (2SA1015) to conduct and thus is the material with the feet of PORTB. This function ensures that there will never be found simultaneously connected to PORTB two materials, since the remaining transistors are off. Switches BCD (inputs), the start button (input) and LED Display (outputs) are controlled solely by the pins RB0 - RB7 of PORTB. Depending on what material is selected and modified the state of PORTB (If it is input or output).
Count Down Timer with PIC16F84A - schematic

Prevention of short circuit through the breakers PORTB BCD Because through the contacts of BCD switches may be shorted pins of PORTB when it acts as output (driving), have been placed in preventive circuit pathways to prevent flow of current. Pull up Operation Since the BCD switches on this circuit give reasonable '0 'in closed contacts, to identify the microprocessor has the entrance to each leg to be used consistently logical '1'. Internal Pull-up operation is applied only when the situation in the PORTB pins made from output, input, and if you specify zero '0 'in place of the registrar RPBI OPTION. If the PORTB output serves as the pull-up function does not. At the stop button of the timer is connected to PORTA added external resistor Pull-up, because this function is only supported on the feet of PORTB. Relay Output At the foot of RA3 PORTA is connected to a relay coil, a switched contact, where you can drive any secondary low power device. Each leg of the PIC can provide power in the range of 25-mA. If the relay used in the circuit for the power consumed by the coil is 23-mA, which is very close to the limit. Therefore to avoid possible destruction of the PIC to drive the relay is not directly but through a transistor can provide current in the range of 150-mA. Timing Circuit For the timing of the processor used a ceramic filter (Resonator) or 10MHz crystal and two 22pF...

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