Team MedBots

[Shakey]

What do you intend on using it for?

[Max]

Melty brain spinner! I only need to drive each motor in 1 direction so no real point using a motor driver. So plan is to have a mosfet driving each motor then a fly back diode across the MOSFET so that when the MOSFET is off you get braking of sorts.

As in writing this I'm realising that the diode will try to 'brake' the motor between each of the PWM pulses which probably isn't good.

I'll have an arduino mini running the thing and an accelerometer to measure the radial acceleration and hence the rotational speed.

I doubt it will work very well but it should be a fun project.

[Shakey]

Yeah you may aswell just go for a regular driver chip. Or a half bridge driver chip if you really only need single direction. It will probably make the coding behind it a bit simpler and it worries about all the brake/coast/power etc itself.

[Max]

So something like this:
http://www.ti.com/lit/ds/symlink/sn754410.pdf

I guess that doesn't have built in braking but it's not the end of the world if the motors don't need brake. Would I still need a fly back diode?

[Shakey]

From the data sheet I don't think that one is built for PWM switching. They normally say about stuff like that.
https://www.pololu.com/product/713
https://www.pololu.com/product/2135

Those two chips are pretty decent dual motor driver chips. They are bi-directional but that doesn't matter, just only tell them to turn one way. And they come on a breakout board (I had some of the drv8835's not on boards. I think 2 of them are in my carpet now :S ).

[Max]

Thanks for all your help so far.

If I were to stick with mosfets and diodes and used a configuration like this:


Would I get a problem with the motor braking between pwm pulses and would I get motor breaking when the MOSFET was just turned off?

[Rapidrory]

You want a setup like this (just a random pic of google image):



the Fet needs to be on the low side of the circuit.

Also, you may want to drive it with more than 5v at the gate as I've found that they can get warm even just driving ant motors..

[Max]

Ok, thanks Rory.
Does anyone know of the lemon rx provided direct access to the ppm stream?

[Max]

I'll take that as a 'I don't think so but not entirely sure"!
I see from the pictures that there are 2 pins/connection points on the lemon rx board that it is not very clear what they are used for. Does anyone know what these extra pins do?

[Rapidrory]

Receivers give out the following type of PWM servo signal:



You'll need a micro controller to convert the servo signal into a PWM signal suitable for driving a MOSFET or motor driver chip.

The other two pins are likely programming pins; I've never personally done anything with them.