Important considerations

Some important factors/ features we need to consider, as pointed out by sir while we design the mouse are as following:

• the voltage drop across the collector and the emitter in a BC 547 is 0.6 V, hence we can consider AC197 - germanium tranistor which has a voltage drop of 0.2 V.
• Light sensor can be given 3.7 V directly from the Li-ion battery as compared to 5V it is currently getting though 7805 IC.
• As LCD is only required for debugging and not every time we can put a manual switch to control its On/Off state.
  

and we need to search around for suitable battery pack and its charger, or we can make one with Nokia battery - BL-5F or BL-5C. but then we would need a custom way to charge the battery pack.