HALF ADDER

HALF ADDER

Arithmetical operations are often an essential function of logic circuits. This truth table describes the simplest possible operation, adding two 1-bit binary numbers A and B:

The half adder circuit generally needs two outputs, these inputs are sum (S) and the carry-out (Co). The logic of each of these inputs is normally determined separately. Comparing the output values in the S column with those in the truth tables tells us that the operation for summing is equivalent to exclusive-OR. The operation for producing the carry-out digit is AND. The complete half adder needs only two gates. If required, the half adder can be built entirely from NAND gates by using an exclusive-OR circuit to produce S. The exclusive-OR circuit

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RIPPLE COUNTERS

 The clock shown in diagram 22.5 is running at 1 MHz in the simulation of a 3-bit up-counter. Traces show the clock from top to bottom and A,B & C shows the output. At one mega hertz clock rates or less, we may ignore the propagation delays of  few nanoseconds.

 Figure 22.6 shows the result when we increase the clock rate by ten times and outputs are also plotted on 10 time scale. The Propagation delays are of more relative importance and we can observe them in the plots. 11 ns is a typical delay interval of the 74LS73 J-K flip-flop that is used in simulation . 

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Synchoroneous Counters

If we Clock all the flip-flops at the same time it will eliminate the ripple effect. Instead of having each stage to be clocked by the output from the previous stage, they are all clocked at the same instant by the clock input signal. The figure 22.7  below demonstrates such a counter composed of from 3 J-K flip-flops. It is thus known as a synchronous counter.

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ROBOT ACTUATORS

Motors

In order to perform any action robots make use of actuators. The drive motors are the most important actuators of mobile robots. In a three-wheeled robot there are two drive wheels that are located at the front. The third wheel of the robot acts as a caster. To make the robot run forward or backward the drive wheels normally run at the same level of speed. If the left wheel is made to turn faster than the right wheel, the robot can pursue a course for turning it to the right. If the right wheel is made to run at faster speed, the robot turns its direction to the left. If left wheel runs in forward direction and the right wheel runs in backward direction, the robot perform a spin to the right. If the right wheel moves in forward direction and the left wheel moves in backward direction the robot spins in the opposite direction. The motors used in the robots are controlled by a fine arrangement of transistors that is known as an H-Bridge. This usually contains four MOSFET transistors that are controlled by two MOSFET inverter gates. Motors are also installed in robots to enabling them to move their arms.

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Desoldering Procedure

The Desoldering process, obviously, is the opposite proccess of soldering. We need desoldering in order to change defective components, other use is to correct construction errors, we can also redo wiring jobs, and to salvage parts. One widely used method of de-soldering makes use pre-fluxed copper braid to absorb molten solder by means of capillary action. The de-soldering braid, or wicking, goes by a variety of brand names and is easy to use.

What we do is that braid is simply placed in the molten solder, and then we see that capillary action draws the excess solder up into the braid. The used piece of braid is then cut off and disposed off. While working with smaller solder joints, the action is process is often improved by way of sandwiching the braid between the soldering iron tip and the joint that is to be desoldered.

There are many people who prefer to make use a vacuum desoldering tool. Desoldering is done by “cocking” the tool: by holding the tip close to the joint needed to be de-soldered, melting with a soldering iron, and then pressing the trigger on the tool. The continuous suction action sucks the excess solder up into a holding chamber, from where it can be easily removed later. This tool being used is normally called a solder sucker.

If you have a plan of performing a lot of de-soldering (e.g. for salvage purposes), you might tend to make investment in a dedicated de-soldering station. This station contains a dedicated vacuum pump and an especially designed soldering iron that have a hollow tip. The molten solder is sucked up through the hollow tip of the iron and held in a filtered container for removal. When you desire suction action, it can be triggered by a foot or knee pedal. But a drawback is that, these dedicated de-soldering stations are very expensive to purchase. Make sure that you can justify the cost before investing in one, that is, the benefits you are going to derive must exceed from the costs that you pay for it.

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