Count-down Timer Circuit Explanation

The breeze of the accepted through the transistor at the called accessory is possible. The transistor of the accessory which isn’t called is alone from the ambit because it is in the OFF condition.
BCD about-face (Input), Start about-face (Input) and LED (Output) are controlled alone by anchorage B. According to the called device, a approach of anchorage B is changed.

At this circuit, the selection of seven devices are done using the decoder(74HC138).

Device number	Device
0	BCD-SW for 10th of the minute
1	BCD-SW for 1st of the minute
2	Start switch
3	Unused
4	7 segment LED for 10th of minute
5	7 segment LED for 1st of minute
6	7 segment LED for 10th of second
7	7 segment LED for 1st of second

A device number is specified by RA0,1,2 of PIC.

The alternative of the accessory is done by ambience the achievement of the decoder into the PNP-type transistor(2SA1015).