555 Timer Ic

One commonly used circuit is the 555 IC which is a highly stable controller capable of producing timing pulses. With a monostable operation, the T(time) delay is controlled by one external resistor and one capacitor. With an astable operation, the frequency and duty cycle are accurately controlled by two external resistors and one capacitor.The application of this integrated circuit is in the areas of PRECISION TIMING, PULSE GENERATION, TIMING DELAY GENERATION and SEQUENTIAL TIMING.
                                     
 

Clock Controller

                    The Clock Controller V1.1was designed to be an exemplary of using 'C' language to control timer0interrupt, 7-segment LED and keypad scanning. It provides 1-bit sink currentdriving output, for driving a relay, opto-triac, say. Many projects requiring7-segment display and keypad interfacing may get the idea from the Clockcircuit and software.
 

                       

                        Circuit diagram of the Clock Controller  P10-P1.7 drives 7-segment commonanode LED with sink current. P3.0-P3.3 also drives a base pin of 4-PNPtransistor, 2n2907 with sink current. As shown in the figure, the 2nd 2-digitLED that connected to P3.2 and P3.3 is rotated 180 degrees to the 1st 2-digitallowing the pt. segment to be used for 1 second blinking. P3.0-P3.3 alsoconnects four momentary switches while the other legs are tied to inputport P3.4. During display and key switch scanning, a logic '0' is shiftedfrom P3.0 to P3.3,  if there was a key pressed, P3.4 then became low.P3.7 is a 1-bit sink current driving, an example in the circuit uses a2n2907 to drive a small electromechanical relay 5V.

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Multiplexing

Multiplexing is a very efficient technique for controlling many components wired together in a matrix/array.  In this example, I'll be talking exclusively about multiplexing an array of LEDs, but the same basic principles apply to other multiplexed components (sensors, buttons, etc).

   
In a multiplexed array of LEDs, only one row of LEDs is on at any given time.  It seems like this would limit the types of shapes we can display on the LED matrix, but it actually doesn't.  This is because the arduino (or whatever is sending data to the array) is switching through each row so quickly (hundreds or thousands of times a second) that we do not perceive the flashing on and off of each consecutive row.  You can read more about this phenomenon, called persistence of vision, on wikipedia.            
                                  

So how do we send data to one row at a time?  If we connect five volts (red) to one row and connect ground (blue) to the other three rows and cycle through each row one by one, it will look something like figure 1.  Now image that while one of the rows is at +5, we connect one of the columns to ground.  As shown in figure 2, this will cause the LED at the junction of the +5 row and GND column to light up.  This way, we can address each of the 16 LEDs in the matrix individually using only eight leads (four to the rows and four to the columns).



Now look at the image below.  Imagine if we very quickly turn on the LED in the upper left corner (position 1,1), then the LED at (2,2), then (3,3) and (4,4), and we cycle between these four LEDs very quickly (hundreds of times a second).  It will appear that all four of these LEDs are on a the same time (as shown in right image in the image below).  Study the diagram below and convince yourself that this is true.
                    

Soldering Techniques

1.     Study the diagram and PCB layout.

2.     Check all the components and study their pins and orientation.

3.   Clean the leads of components if needed to remove rust or grease with a blade.

4.     Place resistors first then IC base, capacitors etc.

5.     Use little flux to the joints to increase the melting point of solder lead.

Perf board Circuit Construction

Printed circuit board, or PCB, is used to mechanically support the electronic components and to connect them electrically using conductive pathways called tracks etched from copper sheets. Besides etched PCBs, there are Common or General purpose PCBs to make temporary or permanent circuits boards. Here explains the types of General purpose PCBs.

Perf Board

Most experimenters are familiar with "Perf board" which is a pre-drilled circuit for creating prototypes of simple circuits. It's not too expensive and for more easier to get started with than etching PCB's.

                               

 The components are mounted by inserting the leads through the most appropriate holes then are wired on the back side, usually by bending the leads over to the desired connection point.

Transistors

Transistors may be in plastic or metal can packages. Value of the transistor is printed on its body. Pin numbers are generally assigned as 1,2 and 3 from the facing side.
  

General purpose NPN transistors have pins 1-Collector, 2-Base, 3- Emitter. That is CBE. In PNP types the pins are reversed. 1-Emitter, 2-Base and 3- Collector. That is EBC. Metal can transistors have a small projection in the rim of the body.

The pin close to it is the emitter. Pin opposite to the emitter is Collector and the middle pin is base. Pin assignment of some common transistors is given below.

 

    

Transistor	Type	Pins 1 2 3	Transistor	Type	Pins 1 2 3
BC546,547,548,549 550, BC 337,AC 187	NPN	CBE	2N 2222A,2N 3904	PNP	EBC
TIP 120,121,122	NPN	BCE	TIP 125,126,127	PNP	EBC
BD139	NPN	ECB	BD140	PNP	ECB
BF494,495	NPN	CEB	MPSA 92,42,44	PNP	EBC
C2570	NPN	BEC	BC636	PNP	BCE
C1730	NPN	ECB	SK/CK/BEL100P	PNP	EBC
BD677	NPN	BCE	AC188	PNP	EBC
D882 / 2SD882	NPN	ECB	BC557	PNP	EBC
D313/MJE 13005	NPN	BCE	BC558	PNP	EBC

5 Band Resistor Color Codes

This calculator converts resistor values to or from 5-band or 4-band colour codes.

To convert from value to colours:

Select the resistor type (4 or 5 bands) using the radio buttons. Type the resistance in the box below and select the units and (optionally) the tolerance from the drop-down boxes. The colour bands will be displayed below on the image of the resistor.

To convert from colours to value:

Select the resistor type (4 or 5 bands) using the radio buttons. Hover the mouse over the coloured bands on the image of the resistor to display the colour selector box, then click on the required colour. Repeat for all bands (tolerance is optional). The resistance will be shown in the box above the image, along with the units and tolerance.

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