This big digital display count-down timer goes to 0 from set time of 1 minute to maximum 99 minutes. It uses the subtraction method for the timer.
I am using 74HC192(Presettable Synchronous Up/Down BCD Counter) as the counter.
There is 74HC190 and so on in the counter which can be downed in the count. I used this counter because I used the clear terminal as the second counter. The clear signal may be unnecessary.
The pulse signal (being the once per minute) from the second counter is inputted to DW(Count Down). The counter is count-downed by the low to high transition of the signal.
The output of BR(Borrow) becomes the L (the low level : 0 V) when the counter becomes 0. Next, it changes into H (the high level : +5 V) when the counter becomes 9. The counter of the upper palces is count-downed by the change of such BR.
When the LD(Load) terminal is in the L condition, the original value of the counter can be set with BCD-SW. At the time of this condition, because the value which is the same as the set value of BCD-SW spreads through the display part, too, you can confirm the set value in the display part. Because the thumbwheel switch is used this time, you find the set value even if you don't confirm by the display.
When the LD is not in the H condition, the counter doesn't work. The condition of the LD is controlled with the control circuit which is controlling the operation of the timer.
The BCD(Binary Coded Decimal) counter is the counter which handles from 0 to 9. The counter of this kind has the binary counter as well as the BCD counter. The binary counter is the counter which handles from 0 to 15 and it says the hexadecimal (Hex) counter, too.
http://www.hobby-elec.org/e_counter.htm
