Robot: problem with load current

Hi, I have been using Arduino for a year and I am in love with it. Im currently building a robot and Im in the final phase of hardware assembling. My robot is a PIR security bot with wireless camera mounted on it. A Sharp IR detector is used for turing the camera up or down and stop the bot before it hits something. The bot can also be controlled via a hacked RC board.

Every singal circuit I've soldered are tested to work fine. However, yesterday night when I was trying to put all the things together I found out its not workig well. The motors move only when I unplug the power source of the Servo and Ardunio (which was also powering the IR detector and a LCD). I was using 2 9V battery to power these things. I guess it may be my power (current supply) is not enough. I have ordered a 4600mAh 7.2V battery on ebay an hour ago and I hope it will work out fine.

In my current design, I didnt connect the 2 9V batteries in parallel. Each 9V battery powers different stuff.

9V battery 1: Arduino, LCD, IR detector, 4 PIR sensors, AND gates (74LS32)

9V battery 2: Servo, Remote control board, 2 Motors (wheels), OR gates (74LS08), L293D motor controller IC.

I figured out that the Remote control doesnt work only whe I m sending signal to the Servo. (At that moment, nither the servo or the motor spins). If the MCU is powerd off and Servo is unpluged, I can remote control the bot but it is pretty slow comparing to my test on just the remote control and the motors I've done before.

Is there anyone think that the supply current is not enough. I hope I get this right. :-[

Basically 9V batteries sucks :-)

You can't expect to drive 2 motors a servo + other stuff from a single 9V battry for more than a very short time.

Thanks, as you know I have ordered a 4600mAh 7.2V NiHM or Li-Po (Ive foregot which type) Do you think it will work out?

It's hard tio say without knowing details about each of your cmponents / motors / devces current cunsumption, but it should be better than a 9 V battry, if your stuff can live with the lower voltage.

but it should be better than a 9 V battry

I don't understand this - a PP9 has a capacity of around 4 or 5Ah.

I don't understand this - a PP9 has a capacity of around 4 or 5Ah.

You have got to be kidding, are you? A PP9 is an old carbon zink non-rechargable battery that costs like $15+. I'm sure it can supply some amps but is it practical for any application these days? Most people when talking about a 9 volt battery mean the small 2" inch long ones, that supply vary little current and are quite expensive for the MAH rating provided.

http://www.batteriesinaflash.com/special-application/eveready-276-carbon-zinc-9v-battery-neda-1603-pp9-6f100

http://www.batteriesinaflash.com/alkalines/9v-batteries/duracell-9v-1pk-retail-card-alkaline-mn1604-6lr61-battery

Lefty

No, I wasn't kidding - a PP9 is what I think of when someone says "a 9V battery". Maybe it's an age thing. :(

I think it might be a british thing!

:wink:

I’ve never seen such a battery - looks to be as big as a some SLAs!

For the OP:

Make sure you get the right charger for your battery; LiPo batteries can be dangerous fire-emitting devices otherwise!

Thanks guys. I have done a little calculation about the total curret consumption and I guess in the worst case it should draw not more than 800 to 900mAh.

One thing I am not sure is that the small DC motors I am using are from a Tamiya Twin gearbox. Under no load @3V it should draw about 150mA.

One motor is faster than another one and it prevents the robot to move straight. I did one very stupid thing is that I added a 100O ohm resistor parallel to the power pins of the faster motor.
It does slow down the motor under 9V ( 650mAh) battery. But do u think it is a good idea when I switch to the new battery?

One motor is faster than another one and it prevents the robot to move straight. I did one very stupid thing is that I added a 100O ohm resistor parallel to the power pins of the faster motor.

That is not really the proper way to make your robot move straight.

The proper way involves optical encoders on the wheels (quadrature encoders if you care about direction), along with PWM, so you can monitor the speed of the wheel, and speed up or slow down each motor to make them match speed.

:)