# Low current in a motor used in a line following robot

I'm trying to make a line following robot for my students, the problem is that I have not done any calculations (I don't know how).

The motor can't move the robot unless I connect it directly to the 9 V power supply, but when i connect the other compenents(arduino) in parralel to the same 9v powersuply the motor doesn't get enough current when put on the floor so it just freeze's.

Where do you think should i start ?

The transistor I used is: NPN C1815 the motor is a brushed dc motor

• 9V power supply as in PP3 battery?
– Majenko
Apr 30 '16 at 23:06
• yes a 6F22 general purpose 9v battery
– joe
Apr 30 '16 at 23:11
• Does the motor have any form of gearbox?
– Majenko
Apr 30 '16 at 23:21
• You should not use a 9v battery for a motor project - it lacks the power to deliver voltage under load. Additional, were it actually to deliver 9v it would destroy a typical hobby servo, which is intended for 4.5v - maybe 6v. You are also missing the base resistor on your drive transistor. May 1 '16 at 4:49
• @Paul. 9V > 6V! Though 6V is probably on the low side for the regulator to work properly. You could use rechargeable AAs, which would give around 4.8V. PS nowhere in the question it's said that the motor is actually a 9V motor! May 1 '16 at 16:20

Due to only a tiny amount of gearing (what is it - 2:1? 3:1? something like that) your motor is operating near the stall point unless it is able to pick up enough speed (at which point it would smash into the nearest wall and pulverise itself).

Consequently it is drawing far more current from the battery than you'd like. And that is, without the Arduino connected, about all the puny little PP3 battery can provide. Add the Arduino etc into the mix and you are over-stepping your current budget by enough to mean the motor cannot start turning (the 1A stall current).

So what to do? Well, two options:

• Use a bigger battery.
• Increase the gearing of the motor.
• (my personal recommendation) Do both.

Increasing the gearing of the motor will decrease maximum output speed, increase the torque, and most importantly, decrease the start-up current. The whole load on the motor will be reduced making it much easier to start.

By increasing the power of the battery (use a 2S or 3S Li-Ion depending on what your motor's voltage limits are) you have a much bigger current budget to play with.

• ok, thank you sir, i will give your recommandations a try
– joe
Apr 30 '16 at 23:49

Noticed that there is no base resistor between transistor and Arduino output pin, you may have blown the transistor and/or output pin, if not try a 1K.

• i used a 1k resistance i forget just in the schematic
– joe
May 1 '16 at 9:32

ok i solved this using gears, and two separate power sources (9v transistor radio battery) i also used a 1.5 amp transistor for driving the motor thanks everyone

• You seem to have ignored most of what was said. Do not use 9v "transistor" batteries for motors - not one of them, not two of them. Do not power a hobby servo from 9v. May 2 '16 at 18:13
• What is a 1.5 amp transistor? May 3 '16 at 7:14
• @ChrisStratton i did use a voltage divider for the servo, and i used one 9v low current battery for the motor and a 9v for the arduino, thats what i have in hand, i also changed the transistor to another one with a max Ic current of 1.5 ampere! i didn't ignore anything of what was said i hope, (i'm in a remote area, if i buy the proposed battery it will take about a month to arrive, by then students will be at holydays..
– joe
May 3 '16 at 10:59
• Voltage dividers are not appropriate for power unless feedback controlled. Feedback controlled voltage dividers are called linear regulators. You should not use lithium batteries for such a project, nor 9v, just get ordinary AA's. May 3 '16 at 14:59
• ok @ChrisStratton thank you very much for the information about voltage divider, i've never heard of Feedback controlled voltage dividers, i'll do a search about it and i will buy AA's because i can find them easily. thanks again
– joe
May 3 '16 at 21:37