Day 6 (8th June 2011)

Completed the Report and gave for printing.

Day 5 (7th June 2011)

Added a Reset Function.Aref Voltage was found to be 4.94V (expected 5) hence the mapping had to be changed.Also corrected a few bugs that caused indefinite correction when out of bound inputs were given.
Started working on the report.

Day 4 (6th June 2011)

Built a constant current source using a PNP transistor  with constant base current. Since the collector current is a constant multiple of the base current, the current is independent of the load(provided the transistor is in active region).

In this case a Zener diode of breakdown voltage 6.2 V was used to provide a constant base voltage of 23.8V. The drop across the base emitter is a constant 0.7V. Therefore the emitter current is given by

Ie=Vee-Vz+0.7/(Rpot)

Thus by choosing an appropriate resistance using the potentiometer the required current can be obtained. Since Ic is almost equal to Ie , a constant collector current is maintained.

For this working the Zener diode has to be maintained in breakdown. To ensure this a minimum amount of current should flow through the diode. This was done by choosing R1 such that the required current flows through the Zener diode. 

Day 3 (3rd June 2011)

Added 2 more relays for diodes  in groups of 2 and 1 to get a better resolution.Added a feedback network using a voltage divider circuit and an op-amp buffer.The voltage across the LED's is divided by 6 and fed back to the microcontroller.Based on the keyboard input a slightly lesser number of LED's than required is switched on and the voltage across them is fed back. Then diodes are turned on 1 by 1.Once 3 diodes are turned on one more LED is turned on and the diodes are turned off.This process is repeated till the load voltage crosses the set voltage.The present error and the previous error(when load voltage was below set voltage) are compared and the better case is chosen.

Day 2 (2nd June 2011)

Mounted 4 relays on the bread board and laid wires on the board. Tested switching with the ULN2003A using a 12 V and a 15V supply. Connected 15 LED's in groups of 8,4,2,1. The relays were connected in a way so as to be able to choose any number of LED's between 0 and 15 . Interfaced 4X4 tactile keypad switch matrix and LCD. Wrote code for open loop system by approximatin the drop accross LED to be 2.1V. 

Day 1 (1st June 2011)

Went to S.P. to buy components. Bought 6 relays, 20 LED's, 2 ULN2003A, 1 7805 and single strand wires.
Searched for relay board. One with 8 relays costed Rs. 1100. Hence individual relays were bought.