Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: reSpawn on November 16, 2008, 08:46:36 AM

Title: Relays, and much more. Help needed.
Post by: reSpawn on November 16, 2008, 08:46:36 AM

Hello,
Today I bought a second-hand RC Car without remote(he lost it) and I'm trying to control it with an Atmega8. The thing is that I know that if I remove the car's PCB I need to get some HBridges to control those motors, and I'm looking to find a way to only connect some pins(maybe where the receiver is connected) from the atmega to the pcb.
So ... I disassembled the car, and now I have physical access to it's PCB.
Here is a photo of it:
(http://img201.imageshack.us/img201/3580/picture021hh2.th.jpg) (http://img201.imageshack.us/my.php?image=picture021hh2.jpg)(http://img201.imageshack.us/images/thpix.gif) (http://g.imageshack.us/thpix.php)
Currently I'm investigating the PCB, but as I don't know much about electronics.
How does the "MCU" in there control the motors? The same way I want to? with an HBridge? If so, I could just extract the HBridge from there, or could you guys tell me another method?

Title: Relays, and much more. Help needed.
Post by: reSpawn on November 16, 2008, 11:54:16 AM

Edit:
Alright, after a bit of research I figured out that it will be too hard to adapt the current PCB and I have to get 2(or 3) H-Bridges so I can control the DC Motors using digital signals from my atmega.
Can someone tell me some HBridges that can be controlled with a digital signal(0V and 5V) and can drive a motor that draws at 7.2V 1.8A when starting up, and 0.8A in idle ?

Title: Re: Relays, and much more. Help needed.
Post by: octagonx on November 20, 2008, 04:37:47 AM

why don't you create the remote controller? I believe the RC you have has the standard 27MHz and 45MHz frequency.

I'm not sure about the 0.8A in idle (and where did you get that value?), but L298 motor driver has 2 H-bridge and can give a maximum of 2A each.

Title: Re: Relays, and much more. Help needed.
Post by: Admin on November 25, 2008, 03:23:05 AM

Yea, I agree. Better to rip all that 80's tech out of that thing and just add in some motor drivers of your choice.

Lots of motor drivers here that'll fit your spec:
http://www.societyofrobots.com/robot_parts_list_electronics.shtml

Title: Re: Relays, and much more. Help needed.
Post by: szhang on November 25, 2008, 10:55:49 AM

Hm... Haven't seen a RC car with relays in a LONG time.

.8A idle is a rather big value, you might actually exceed the current limit for the LM298 during a stall.  Just for reference, the motors on my robot (built on a RC car base) draws about .5A idle and stalls at around 7A.  Make sure you measure the stall current (hold the wheels stationary and apply 7.2V), the startup current is most likely an average, you want to know the peak current draw of the motor.

I am using a MC33887 driver (http://www.pololu.com/catalog/product/711/pictures), which has a current limit of 5A, but has overcurrent protection.