Analog line follower
This circuit use 2 LDR's to form a bridge to compare the difference of light received in them, so it would work under most of the lighting conditions. One need not adjust the robot before racing anymore!
The following PCB design is not the same than the circuit plan but very close :-) The PCB is designed for the workshop in the Campus Party 2012 in Berlin/Germany.
The PCB is working and ready for production. here some pictures of the self-etched prototype.
And here is the Video of the working analog Line Follower http://v.youku.com/v_show/id_XNDI4NjgxODk2.html
NOTE: As the sensitivity was not good enough we changed the LDR from 20KΩ to 1MΩ
Based on the first version we changed the circuit to the following:
- 三极管 - Transistor
- 2N3904 NPN , 200 mA, 40 volt, 625 milliwatt transistor with a transition frequency of 300 MHz, with a minimum beta or current gain of 100 at a collector current of 10 mA. 
- 2N3906 PNP , 200-mA, 40 volt, 300-milliwatt transistor with an transition frequency of 250 MHz, with a beta of at least 100 
- 三极管的工作原理 
- NPN 型，当B与E之间电压Vbe>0.5V时，如果三个管脚电压关系是Vc>Vb>Ve，则会处於放大状态；如果是Vb>Vc>Ve 则会处於饱和状态(相当於开关)；如果此时Ve>Vc则仍会处於截止状态
- PNP 型，当B和E之间电压Veb>0.5V时，如果三个管脚电压关系是Ve>Vb>Vc，则会处於放大状态；如果是Ve>Vc>Vb则会处於饱和状态(相当於开关)；如果此时Vc>Ve则仍会处於截止状态
- LDR 
The only change is that we removed the potentiometer in the LDR bridge and we used 1MΩ LDRs to get a bigger potential difference between dark and bright areas. Also the new circuit got a LED as third wheel and to light up the surface between the LDRs. It might help in low light conditions but the benefit might not be that great.
To make the line follower less light dependent LDR's with a very big dark value should be used. It's very important to adjust the LDR'S by height and angle as well the width to each other.Besides this there is not much else to tell but that's a lot of fun building it and let it run (maybe in groups) on the line course or even as small as an A4 paper sheet. See picture in the right pane for printing a line on an A4 paper sheet.