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Author Topic: Nerf Rapidstrike: MOSFET + PWM + Motor braking  (Read 695 times)
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Hello everyone!

I am trying to modify my Nerf rapidstrike blaster with a 3 round burst functionality. I have rewired it as per the attached diagram. For those who don't know, the pusher motor on the rapidstrike has a system where a motor drives a linear actuator gear system to push darts between two flywheels. The "S3_Return_Switch" is depressed when the push moves to the rear, so that the system will drive the motor such that when the trigger is released, the pusher returns home under power then motor brakes when home. Its a pretty simple circuit.

What I would like to do is to use an Arduino to PWM control a MOSFET to allow for precise control of firing while still being able to motor brake the circuit. I am unsure of how to properly achieve the motor braking with a MOSFET due to the issues with the MOSFET being burn out by the generated current. Every tutorial and guide I have read has a diode across the motor which to my mind would totally prevent motor braking. I do not want to rely on know how long the motor take to complete 1 firing cycle.

EDIT: Fixed a mistake in the diagram! Wupps!


* Diagram.png (15.14 KB, 406x466 - viewed 32 times.)
« Last Edit: July 28, 2014, 12:45:26 am by Rhino_Aus » Logged

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Nerf rapidstrike blaster

What's this?

You see if you want the answers to your posts ASAP also which are good  quality answers then you should write detail as much as possible about your hardware and software and the real time environment in which the system is proposed to workout!
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What's this?

I'm sorry; I thought that my description and circuit diagram was enough and the software is trivial and not part of my question; I know how to do the code. The Rapidstrike is a toy foam dart blaster from Hasbro that uses a pair of counter-rotating flywheels to fire darts. It also has a third motor that drives as linear gear system to push darts into the flywheels automatically. It looks something like this:





As I said in my OP; I have modified the wiring as per the given circuit diagram given and want to use an Arduino to PWM a MOSFET to control the pusher motor to fire at a controllable rate of fire in automatic and burst fire. This would mean that S2_Trigger_Switch would instead be connected to the Arduino, and its switching functionality replaced by a MOSFET. The only thing I want to know is how to use a MOSFET in a circuit and still be able to motor brake as it is essential for stopping the pusher motor. Every guide on MOSFETs I have seen has a diode that seems to prevent motor braking to protect the MOSFET.
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I want to know is how to use a MOSFET in a circuit and still be able to motor brake as it is essential for stopping the pusher motor. Every guide on MOSFETs I have seen has a diode that seems to prevent motor braking to protect the MOSFET.

Okay so show us the connections how you are putting the diode and other elements together.
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OKay I think I got down to the nerve of your problem, you need 2 MOSFETS one would be the motor driver doing the half-bridge function as you already have it right now and the other MOSFET actually is controlling the GnD of your motor letting you break the system as per will.

whats more  smiley-eek-blue you may also do PWN on the Gnd side MOSFET to control the amount of breaking you do on the motor!

here's an illustration >> http://hackaday.com/2011/10/28/motor-drivers-half-h-bridge-with-brake-and-more/
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OKay I think I got down to the nerve of your problem, you need 2 MOSFETS one would be the motor driver doing the half-bridge function as you already have it right now and the other MOSFET actually is controlling the GnD of your motor letting you break the system as per will.

whats more  smiley-eek-blue you may also do PWN on the Gnd side MOSFET to control the amount of breaking you do on the motor!

here's an illustration >> http://hackaday.com/2011/10/28/motor-drivers-half-h-bridge-with-brake-and-more/

AHA! This seems to be the answer I was after. I shall investigate how to ensure there is no shorts in my setup tonight. Thanks!
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This is what I am after; but is there a way to do this but with just N channel MOSFETs?
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This is what I am after; but is there a way to do this but with just N channel MOSFETs?

Well you need to workout the circuit reverse so P's are needed and also you can use an H-Bridge.
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So if I am feeling lazy then the best way to do it is to buy a H bridge driver board like this one?
http://www.pololu.com/product/1451
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Well okay it's kind of a beefy one and I don't know your motors depends on the stall current of the motor but if you're that lazy then maybr just go with this.
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