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The family was adopted into the LS clan, but not into HC. The diagram at the right shows how to apply a 74hc74xn oscillator to the chip, using a 32, Hz watch crystal. The moments of change can be accurately observed, however, and the 2 Hz LED counted, if desired.

All the resistors are 4.

Again, this is a standard problem in digital design, but it is seldom worthwhile to make one’s own. This shows the general principle of designing a modulo-N counter.

With care, these are not a significant hazard. Datashewt cascading two modulo counters and a modulo counter, you now have an electronic escapement for a clock, which is fed by a 1.

What you want is one that ratasheet normally closed one way, and springs back when pressed and released. We also need a modulo counter to count hours for a hour clock, or a modulo counter for a hour clock. If you do this, you will be well on the way to understanding how to build a clock.

74hc74an datasheet pdf

Since it is very helpful to know how they work when using them, the fundamentals of electronic counters will be presented here, and the reader is encouraged to perform the experiments, which are simple but instructive.


Chip designers have been crafty enough that this has proved satisfactory in most cases. This limits the circuits to 18 MHz but may have been fixed by now.

A means of setting the clock dagasheet is very important. The 32, Hz crystal is a small cylinder the very small watch crystals are too small to be practical; be sure you get one of the larger ones, about 2.

Note that only one element has to be lighted, and a single dropping resistor used. This output can be used as the clock for another counter that gives an additional divide by It is necessary to keep the voltage supplied to the regulator above 7. Let the number of flip-flops in a counter be N.

However, the signals must be buffered, and dtaasheet be used directly off the ‘s. Datasheett shift register is a series of flip-flops with each output Q connected to the next input D, and the clock inputs connected together. If it is desired to trigger the next higher order at the end of a 10 count, the 0 output is connected to the CK of the higher order counter. This flip-flop has independent data, preset, clear, and clock inputs and Q and Q outputs.

74HC74A datasheet, Pinout ,application circuits Dual D Flip-Flop With Set And Reset

The output of the slave is fed back to the input of the master, but there is no possibility of a “race” because the two are never simultaneously enabled the actual circuit makes certain of this; the simple inverter, of course, shown in the diagram would not, because of propagation delay. Make this alteration, and see that you now have a modulo counter. If synchronous counters are used, it would be possible to preload the counters to any value, perhaps selected by thumbwheels, and then to start the clock at the proper moment.


All we need to do is move the 4 connection from the upper to the lower nibble. This leaves only 4 for controls, so the chip is very simple to use.

When one approaches some project such as this, it is best to study and understand all the details, and not to follow a circuit diagram blindly. The debounced pushbutton is useful enough to make a permanent circuit in a small box, perhaps together with an LED logic probe or two. The minutes and hours counters of a clock can be realized with just two HC’s, and decoded with two HC42’s each.

The 74LS90 has two sections, one a modulo-2 flip-flop with clock C0 and output Q0, and the other a modulo-5 counter of three flip-flops with clock C1 and outputs Q1, Q2 and Q3. The neon color is a pure orange, tending more towards yellow than red, and certainly quite visible.

PDF 74HC74AN Datasheet ( Hoja de datos )

Finally, there are synchronous up-down counters like the 74LSLS, whose uses are more specialized. This is the oscillator I intend to use when I build a clock. The low speed of the is of no consequence in clocks. Some Nixie clocks use NE-2 lamps, which are the same color.

The toggle flip-flop, by itself, is a 1-bit counter with the states 0, 1.