Digital timer I

74HC00, 74HC191

We're now going to build a digital timer using a counter IC. Since the digital timer works by counting the number of clock pulses, it is much more accurate than analog timers so long as the clock pulse repetition period (time) is maintained constant and free from any change.

Since the counter IC works as a presettable binary down counter, you can preset the count in 4-bit binary numbers, as Figure 1 shows.

The clock pulses are generated by a NAND gate, fed to the CK terminal of the counter IC, and displayed by LED 1. Now, set S1 - S4 to the count you've selected from Figure 1 (if the count you've chosen is 11, it is equivalent to a binary number of 1011, so turn S1, S2 and S4 ON, and turn S3 OFF).

Then turn power ON. LED 1 starts flashing on and off during counting the preset number. LED 2 lights up after the preset number has been counted. The setting of this digital timer can be changed by varying the clock pulse repetition period (time).

You can make this period longer by using a larger value for C2 or R6. So try changing the timer setting.








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