See if you have connected the electronic components in the right direction , counted IC pin correctly, and all that sort of thing. Bear in mind that a single missed connection, wrong connection, or reversed component can mess up the whole works.
What are you suppose to do if ,you have doublechecked the circuit (compared to the circuit diagram) and still doesn't work .Its time to reach for some test equipment.The basic test equipment you really need is a simple multimeter.Some more elaborate kinds of equipment can be helpful at times, but that multimeter is all you should ever need.
Check the power source level voltage, making sure that it falls within the specified range.Olso double check the polarity of the power source of your electronic circuit to see if you might have connected it with + and - leads reversed Then use multimeter to check power source voltage to all the ICs.The circuit diagrams show which pins are to go to the positive side of the power source and which pins are to be connected to the negative.This sort of test can uncover wrong wiring that can escape closest kind of visual inspection.
For example,you are supposed to have +5 Vdc at pin 14 of a IC of a circuit, and you find there are no voltage there,that shows there is a connection missing .It is entirely possible to have poor breadboard connections and broken wires under the insulation,both invisible from a visual check.
