Picture of line follow robot, no programming required
This is the line following robot which i designed and built for an NCEA electronic design assessment. the idea came to me when playing around with light dependent resistors (LDR's), realising that the motor speed can be varied when wired in series with the LDR and a transistor. I decided to name the robot HEMI, simply because of its hemispherical shape.

Step 1: Overview

Picture of overview
You will notice that the pictured robot is built to micro dimensions(it measures 45mm in diameter), but this isnt neccesary, and this instructable will explain how to build it according to the parts that you can get hold of. the line follow robot is a great first robotics project, giving you good idea of how to achieve autonomous behaviour using simple electronic components and some mild soldering skills.

parts needed include:

 an NPN transistor
a resistor (the value of which later be determined later)
batteries (between 6 to 12 volts depending on size of robot and motor requirements, I used two 3v button cells).
two motor and gearbox assemblies
a switch
a light dependent resistor (LDR)
and a suitable shell. (mine was the clear case from a toy, it measures 45mm in diameter).
a multimeter or ammeter

note: the picture does not include the transistor because at the time of taking the picture i was still in the process of designing and didnt think that the transistor was neccesary. also, only one resistor was needed.

it is cool!!! but i cant clear with the schematic plz sent proper diagram

shoshu14 days ago

bro, this is a cool one ..but can you plzz sent me the circute diagram ..so that it is easy to find.... plzz broo plzz. ;) :D

RaulL3 months ago

Does it work with only one LDR?

which transistor did you use . what was the no. / or will any npn transistor will work ??

I live in Vietnam but i very like this thing

hi this is adi10 months ago

can i use 2 ldr's

hi this is adi10 months ago

can i use 2 ldr's

sagar thakur11 months ago


your project is awesome but can you please specify the parts more clearly like which ldr you are using because i am stucked in between maybe because of the ldr.....it will be really helpful if you reply fast...

jrastogi1 year ago

i have made the one by the circuit diagram shown above but it doesnt work only the motor2 runs can you please help me

jrastogi1 year ago

hello i am using 6v aa batteries can you please show me the circuit diagram

leebates1 year ago

Where did you get the motor and gears from? Are these out of a toy? I can't seem to find anything that small available to buy.

Maybe you could post the link?

can you email me how does this work...this is really the simplest awsome LFR..
so cool!!!!!!!!!!!!!!!
rexxxdavid2 years ago
awsome work dude... as you said it wont work under certain light conditions. i tried to add a led under the robot as the light from the led reflects to the ldr. it will also give a cool look to the robot. but i cant find the right place to fix the led. i hope it will work fine under any light enen in the dark if you can adda light source...
how the robot turns left&right according to the line?
which controls the robot?
profpat2 years ago
yup, i got it, just the arrow for the emitter is on the collector, but the connection is correct for a npn transistor, thanks!
profpat2 years ago
hi, according to your drawn schematic, the transistor is a pnp!
Nitro-methane (author)  profpat2 years ago
Sorry, I must have made a mistake there as well, if you look in the parts list of the first step I wrote NPN.
profpat2 years ago
nice one! can we use a npn transistor instead of a pnp transistor?
Nitro-methane (author)  profpat2 years ago
yes, i actually made a typing error on one of the previous replies, i was meant to say NPN. Sorry.
WHAT transistor should i use please help ME FAST
Nitro-methane (author)  Mahmoud Alaa2 years ago
I used a BC548B transistor. Most standard PNP transistors will work.
Hope this helps
thanks very very very much , and can i have your permission to add this circuit to my new 'able ??? i will appreciate that very much :D
Nitro-methane (author)  Mahmoud Alaa2 years ago
sure, thats fine. And it would be great if you could acknowledge it to me in your instructable.
ok thanks very much and i will be posting my able as soon as i can
benmurton2 years ago
Hey Oliver

Nice job, you may remember me, Mr Taylor introduced us when you made that cool as jet boat.

Have you thought about putting a little LED facing down next to the LDR with a shield between the two? This would allow it to work more reliable. Another idea is using an infra red LED and a infra red photodiode, this eliminates interference from normal visible light yet again making a more reliable.

Really awesome instructables! Two thumbs up and my vote!

Message me or come and see me at school if you need any help or have any cool ideas that you feel like sharing. :)

Benjamin Murton
Sundar0052 years ago
That's really a good one. But I have a doubt. What happens if the line to be traced is other than black; say red or yellow?
Mayank3 2 years ago
My circuit is not working can you upload a video plzz
Nitro-methane (author)  Mayank3 2 years ago
Sorry for the late response, I will try and upload the video again when I get the chance but there are no guarantees. As for your circuit, I can only suggest that you tell me what parts you used and I may be able to help.
AndyGadget2 years ago
Very nice.  Those motor / gearbox / wheel assemblies are neat; Where did  you get them? 
Nitro-methane (author)  AndyGadget2 years ago
Sorry, also the wheels are from a lego set.
Of course! I didn't recognise it as a servo with the top cover off.
(And Lego sets always come in useful for small (and not so small in the Technics sets) wheels.)  Neat set-up.
Mayank3 2 years ago
My circuit is not working can you upload a video plzz
vishalapr2 years ago
First time I have seen a line follow robot without any programming, I would be making this soon!
Did you buy those tiny motors online or at a local store? When I use such small motors with gearboxes they are not strong enough to move anything.
This project is the functional equivalent of EMILY, the "Electromechanical Inebriated Ladybug" that was a cover story in "Popular Electronics" about half a century ago. That robot was bigger, and had a relay between the transistor and the motor, because it used motors and gearboxes from toy cars, but it worked the same way. This version, with a little paint, would look even more like a ladybug.
Nitro-methane (author) 2 years ago
They came from two Brocken 9g micro servo motors, thanks, I'm glad you like it
makesomeso2 years ago
can you make a vid of it in action?
My thoughts are exactly like Kiteman. This is just awesome:). Please post a video if you can.
Kiteman2 years ago
This is really cool - I have a soft spot for robotics thst do not need microprocessors to do their thing.

Any chance of seeing a video of it in action?