The circuit is shown above. C1, R1 are power-on reset circuits, and K is a start/stop switch. The IC uses AT89C2051. Its P1.O and P1.1 ports are also the input terminals of the internal comparator. In order to ensure the normal operation of the program, an external pull-up resistor is required. R4-R11 is an LED current limiting resistor. If the brightness is not enough, it can be used. The LED dynamic display uses DN74000190A. The following figure is its schematic diagram. To simplify the circuit, only the last two digits of the LED dynamic display are used. This circuit is very simple, its main work is software design. The key to software design lies in the preparation and arrangement of digital display forms. Since the pins of the dynamic display are time-multiplexed, it is more complicated to compile the digital display table than to use the seven-segment LED. You can draw a table on the draft paper first. According to the number of O-9, fill the O in the bright segment and fill in the segment in the extinction. The resulting binary code is converted to a hexadecimal code to obtain the desired code table. When the program is running, first call 1.29 pin as the encoding of the timing digit, temporarily store it in R7, and then adjust the encoding of the ten digits and R7 to perform the logical AND operation. As a result, the P1 port is displayed with a half digit of two digits. Then adjust the 2.26 pin to the timing ten-digit code temporary storage R6, and then adjust the code of the one-digit number to perform logical AND operation with R6. As a result, the P1 port is displayed to display the remaining half of the two digits, and the complete digital display is synthesized.
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The program is designed to clock from 0 to 99 seconds to make it easier to check the program and the coding table. Appropriate changes to the software can constitute a 0 to 99 minutes, O-99 hours of the timer. Used in life work. Such as car washing machine used in car beauty shop. According to the manufacturer's instructions, the oil must be changed every 100 hours of operation, but the boot time varies from day to day. It is difficult to change the oil on time, and this problem is solved after installing this timer on the machine. This article is simplified by such a circuit.
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