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BASIC 
ELECTRONICS COURSE 
Page 20 INDEX

The next circuit we will cover is a LED FLASHER. It flashes a single LED and produces a higher illumination from a LED than the Flip Flop project. You will see how it does this in a moment. 
The LED Flasher and FLIP FLOP have been presented as "beginners circuits" because they are simple, with a great effect and demonstrate the principle of transistor operation. 
They are ideal for animating and you can "see" how they work because this is the way electronics designers see a circuit in operation. By seeing a circuit working in their mind, they can "see" if it will work.   
These two "flasher" circuits are also very reliable in operation and will operate every time they are switched on. The value of the components are not critical and the supply voltage can drop considerably without affecting the frequency or reliability. You may think it's easy designing new circuits but lots of flasher circuits have been designed but many were very fiddly to get going - only a few survived the "test of time."   

THE LED FLASHER CIRCUIT
The circuit looks completely different to the Flip Flop circuit described in the previous section and if I said it was basically a very high gain amplifier, you would be surprised. But that's what it is and in the "5-PROJECTS" section, you will be able to make up this circuit without the 10u electrolytic and create a very high gain amplifier.  In place of the 470k resistor a TOUCH PLATE has been added and by placing your finger on the plate you will be able to regulate the resistance and control the operation of the circuit. 
To understand this circuit fully you need to go to the  HIGH GAIN AMPLIFIER project and carry out the interactive experiments, then return to here.  
The LED FLASHER circuit consists of an NPN and PNP transistor. We have shown how an NPN transistor "turns on" when the voltage on the base rises above 0.7v and current is allowed to flow into the base. This causes the resistance between the collector and emitter leads to reduce and effectively reduce the voltage between these two leads. 
The PNP transistor is a "mirror" of the NPN transistor. It works in exactly the same way except the voltages on the leads are to opposite polarity. When the base voltage is lower than the emitter (for the BC 557), the transistor turns on and current will flow in the collector-emitter circuit. But collector-emitter current does not flow yet. We already know that the "characteristic voltage" across a red LED is about 1.7v and the base voltage has to drop 1.7v plus 0.7v for the transistor "characteristic base voltage."
"When the base is 2.4v lower than the positive rail, current will begin to flow through the LED. In fact the voltage on the base drops much more than 2.4v because the first transistor "turns on" (as you will see) and this causes the PNP transistor to turn on. 
Now we can go to the start. When the power is switched on, the 470k base-bias resistor charges the 10u electrolytic (in the reverse direction - but they can be charged slightly in reverse) and when the base rises to 0.65v to 0.7v, the transistor turns on and causes the voltage on the base of the PNP transistor to drop to only a few volts. 
This allows the PNP transistor to turn on and the LED illuminates. At the same time, current flows through the 22R resistor and when current flows through a resistor, a voltage drop is produced across it. This voltages drop raises the voltage on the positive side of the 10u electrolytic and this makes the negative side rise ALSO. 
The effect of this is to turn on the NPN transistor VERY HARD and the whole circuit becomes FULLY TURNED ON. The illumination from the LED is very HIGH because the current limiting resistor is very low. 
We have already mentioned that the current limiting resistor for a LED and 9v supply should be about 470 ohms and if a lower value is fitted, the LED will be damaged. So, why put a 22R in the circuit for the dropper resistor?
The reason is to produce a very bright flash from the LED. One point we failed to mention in the previous pages is the LED will accept a very high current for a very short period of time without being damaged. 
This circuit does this and nothing is damaged. 
The circuit is now fully turned on and the 10u electrolytic continues to be charged due to the voltage drop across the 22R. While the electrolytic is charging up, the charging current is high but as the voltage across the electro increases, the charging current reduces and the NPN begins to turn off. This makes the PNP transistor begin to turn off and the current through the LED and 22R reduces. The voltage-drop across the 22R reduces and this brings the positive side of the 10u down towards the 0v rail. 
The voltage on the base of the NPN is reduced by this action and the NPN turns OFF. The whole circuit is now fully turned off. The energy stored in the electrolytic creates a voltage across it and this is gradually removed by the 470k discharging the electro. When the electrolytic is fully discharged, it can begin to charge again (in the reverse direction) by the 470k.  

These actions are best presented in an animation (below), where you can see the effect of the electrolytic charging and discharging.

The action of the capacitor (electrolytic) is quite complex but you can see it charges slightly in the reverse direction at the beginning of the cycle to turn the circuit ON then it is charged in the forward direction due to the voltage developed across the 22R resistor. This charging current keeps the first transistor ON and after a short period of time the charging current drops to a level that cannot keep the transistor on. The first transistor turns OFF slightly and the voltage across the 22R drops. This makes the right-hand side of the capacitor fall and the left-side falls too. The voltage across the capacitor is gradually removed by the discharging effect of the 470k resistor and during this time the circuit is OFF.  When the voltage across the capacitor rises to 0.65 to 0.7v, the circuit turns on again. 

MAKING A PROJECT

Now we come to the stage where you can build some of the circuits we have discussed in the course. Talking Electronics has produced a PC board containing projects to give you "hands on" experience with the circuits we have covered. It is called "5 PROJECTS" and contains five separate projects on a small PC board. The first section of the PC board is a HIGH GAIN AMPLIFIER and shows how a transistor "turns-on" by placing a finger on a TOUCH PAD. 
The second section of the board is a LED FLASHER  as shown in the page above.
The third is a FLIP FLOP as shown in the previous page of this course. 
The fourth is a FIBRE OPTIC display. It uses the LED Flasher project or Flip Flop project to produce a miniature display suitable for model train layouts.
The fifth project is a SIMPLE SIREN. It changes tone according to the pressure of your finger on a touch plate.  
The total cost of the kit, PC board, batteries, and including pack and post:  us$15.00
It is called "5 PROJECTS" and will allow you to build 4 separate circuits on one PC board and an extra PC board is included for the Fibre Optics project, making 5 separate experiments. 
The "5 PROJECTS" kit can be ordered by clicking HERE.

When you have your kit, go to the chapter: "5 PROJECTS" and start on the projects. 


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