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Select Fire Rapidstrike SMG

Hello, this is my first post on this forum. Hopefully I haven't gotten too many things wrong.

I have just finished my select fire rapidstrike SMG. I painted it and did the front cut back in September 2015, but it was only rewired to use IMRs in an external battery tray. So I recently re wired it with a PICAXE Microcontroller so now it is Select Fire.

It is using 3 MTB rhino motors and I have a set of Lathed Blasterparts blue flywheels which are much better balanced than the stock rapidstrike flywheels.

The Lipo is a 1000mah 35C constant 45C burst 3s and is housed in the rapidstrike stock. The metal pin inside the stock receiver has been removed so that the entire stock can be taken out without having to take apart the blaster. There are then only 2 screws that need to be undone to remove the battery for charging.

The Selector Switch is a 3 pole 4 way rotary switch with a 3D printed lever on it. It has little click stops so it sounds just like a real gun selector switch.

Full Auto


Semi Auto

The Microcontroller is a PICAXE 18M2 and controls 3 MOSFETs which activate the pusher motor.

One of them turns the pusher on and off and another P channel MOSFET Shorts out the motor to brake it. However, because the PMOS requires 12v to activate it and the picaxe can only output 5v, there is another N channel MOSFET which turns on the braking. Also, for those of you who are worried about turning both of them on at the same time and creating a short, There is a protection diode that ensures the braking can't be turned on at the same time as the pusher.

Here is a link to the circuit diagram:

The paint is almost all from halfords with grey primer, metallic citron red and then masked off satin black. The silver details and light dry brushing were done with some silver enamel and I then used a halfords clear coat over the whole thing.

Here is a video showing it firing as well as some more information:

Neat stock fitted pack, I have wondered if there was enough space for a useable pack in there.
When is it coming out for a game?

OldNoob wrote:

When is it coming out for a game?

I'm not sure when I will actually use it. There aren't really any events where I am so I usually just organise them myself with my friends. If you watched the video you can probably tell that I'm not over 18.

Britnerf events accept under 18's if accompanied by an adult, the adult only has to stay on site although playing is better. Nowhere should now be more than 3-4hrs drive from an event. Blasters like this need to be taken to war!
Shout out if you are thinking of starting a game, age is no barrier to starting stuff, in fact this forum was started by a teenager. Zhom is run by a player under 18 as well.

Design/Forge wrote:

There aren't really any events where I am

Homie where you at? I bet there's at least 10 other people within 2 hours drive from you wherever you are.

Nice one, I'd caught your video before your post (<-- hipster).  Is the select lever mounted okay and easy to use with your thumb?

This maybe a naive question, but if it's easy could you post your pusher circuit diagram? I'm particularly interested in the use of the p- and n-channel Mosfets and how you have protected from a short with the diode. No worries if it's a pain.

Minky wrote:
This maybe a naive question, but if it's easy could you post your pusher circuit diagram?

I have added a link into the original post to a picture of the circuit diagram. The selector switch only gets in the way if you have it in the semi/burst mode and you remove your hand at a weird angle. That doesn't happen in the full auto mode.
The Dark Kitten

Very nice!
It's good to see we have another person in the UK who is openly making me things and building on past ideas.
Yes I am under 18 and still run games
I am sure all the britnerf ran games are accepting of under 18s and under 16s for that matter so definatly get to one!

SSGT had previously told me that to fully 'fet' up a RS build you'd need 4 I now I see how, although I still don't understand enough to know how the circuit works.  I've got to get to grips with Exactly how Mosfets of both flavours work. Up to now I've just seen them as remotely activated switches that can get fried by back EMF but that clearly isn't the case.

EDIT: Moved circuit diagram down the thread to make sense of next post.

Minky wrote:
SSGT had previously told me that to fully 'fet' up a RS build you'd need 4 I know I see how, although I still don't understand enough to know how the circuit works.  I've got to get to grips with Exactly how Mosfets of both flavours work. Up to now I've just seen them as remotely activated switches that can get fried by back EMF but that clearly isn't the case.

You can still think of them in that way you just have to remember that a FET switches "on"/"off" depending on the voltage at the gate pin relative to the source pin and that the source pin is always connected to either the +ve or -ve side of the power source depending on the type of FET being used. The key term there is relative to source - the absolute voltage at the gate, the voltage relative to ground or the voltage relative to any other point in the circuit largely doesn't matter, it's how it compares to the voltage at the source pin that determines whether the FET switches "on" or "off". That's all any "voltage" is really , a difference in electrical potential between two loactions. It's a little like pressure - you often don't care about the absolute pressure relative to a vacuum more often you care about the difference in pressure between two locations - it's the difference in pressure inside/outside something that determines whether or not that something will crush or explode not the absolute pressure inside/outside it similarly it's the voltage across something that determines the current flow and whether or not the component will go pop.

With an N-channel FET the source pin is connected to battery -ve/ground. If the voltage/potential difference between the gate pin and the source pin is 0V (i.e. they are both at the same voltage) the FET stays "off" whereas if the voltage/potential difference between the gate pin and the source pin is greater than the threshold voltage (usually either 2V or 4V depending on the FET) the FET will turn "on". In the case of a P-FET the source is connected to battery +ve but the same rules apply since its only the potential difference between the source and the gate that matters. Just like with the N-FET, if the voltage/potential difference between the gate pin and the source pin is 0V then the FET will be "off" whereas if the voltage/potential difference between the gate pin and the source pin is less than the threshold voltage (usually -4V) the FET turns "on". The subtle difference there is the polarity of the threshold voltage - in the case of a P-FET the source is connected to battery +ve and so the potential difference between gate and source can never be +ve (otherwise the voltage at the gate relative to ground is greater than the voltage of the power supply).

As an example, to help make that easier to understand, assume we're using a 12V supply (i.e. the potential difference between +ve and -ve terminals is 12V) and, for the sake of simplicity, set the -ve terminal to 0V and measure all other voltages relative to that. For an N-FET the source is connected to 0V and so if the gate voltage is also 0V the FET stays "off" whereas if you apply a voltage to the gate that's greater than threshold voltage (i.e. if you apply more than 4V to a standard e.g. "IRF" FET) then the FET will turn "on". In the case of a P-FET the source is connected to 12V and so if you apply a voltage of 12V to the gate the FET stays "off" whereas if you apply a voltage less than 8V (12V plus -4V threshold voltage) then the FET will turn "on". This is why you often need an N-FET to drive a P-FET especially if you are running the FETs from a microcontroller. The microcontroller output pins can only supply between 0V and either 3.3V or 5V (depending on the chip and/or voltage regulator) however your P-FET gate needs 12V if connected to a 12V supply (well, usually -2V relative to source so minimum 10V) to turn "off". That means that if you try to use a P-FET to brake a motor by running it directly off a microcontroller output pin then, unless you're running the motor at the same voltage as the microcontroller, that P-FET will never turn "off" no matter what your code tells the output pin to do meaning that as soon as you turn "on" the N-FET you short the power supply. An N-FET is used to drive the P-FET as it can be driven directly from a microcontroller (although it's advisable to use a logic-level e.g. "IRL" N-FET with microcontrollers and you must use a logic-level FET if you're using a 3.3V microcontroller) allwoing you to connect the P-FET gate to +ve via a pull-up resistor and pull the P-FET gate down to 0V with the N-FET.

In this circuit all the diode does is ensure that both the right-hand FETs cannot be switched "on" at the same time the only issue there is that, without a resistor in series with the diode, if you did set both the "run" and "brake" outputs "high" at the same time the diode would allow a low-resistance current path between the "run" output and ground which could either blow the diode or damage the "run" output pin or both. Also you should really have pull-down resistors on the N-FETs otherwise their gates will be "floating" when the microcontroller is off/for a moment while it switches on (or, even worse, pulled up by the microcontroller's internal pull-ups if they are present/active). Maybe not a massive issue, especially if the FETs themselves are hidden away from potential sources of static charge (you can switch on a FET with a floating gate just by touching it), but it is best practice. A flyback diode in parallel with the motor is also highly recommended to give the current through the motor a path to flow until it dissipates (inductors try to resist changes in current - if you suddenly open a circuit it will either try to arc a gap or create a voltage spike if it can't). Even though the P-FET should do this that assumes it turns "on" the instant that the N-FET turns "off" and whilst almost all FETs have a sort of built-in reverse-biased diode that should cover this IMO it's better to use an external diode that should both be more robust and, more importantly, is easier/cheaper to replace.

Okay, I think I have it.. Though I'm fully prepared to look a fool on this.
Moved the circuit diagram below to help follow.
PMOS -  p- channel Mosfet top right.
NMOS1 - n- channel Mosfet bottom right.
NMOS2 - n- channel Mosfet bottom left.

'Brake' output on, activating NMOS2 making that circuit and bringing power to PMOS gate keeping that Mosfet shut.

'Run' and 'Brake' Microcontroller outputs on.
Run output activates NMOS1 allowing circuit around edge of diagram through motor turning it on.
Still no difference across PMOS keeping that shut.

'Run' deactivates shutting NMOS1 breaking circuit stopping battery power to motor.
Motor continues to power down under inertia, generating power.
'Brake' output off, deactivating NMOS2 cutting circuit across PMOS gate bringing that down to 0V, PMOS activates shorting the motors.

Am I anywhere close?


Almost. Top FET is indeed P-channel, and bottom FETs are indeed N-channel but the way the "brake" output triggers the P-FET "PMOS" is the opposite. Switching the N-FET "NMOS2" "on" doesn't send power to the gate of P-FET "PMOS", or "pull" the gate up to +ve supply voltage, rather it "pulls" the gate voltage of "PMOS" down to 0V/ground (remember the gate pin is compared to the source pin - if gate pin voltage = source pin voltage the FET is "off"). It doesn't help that the P-FET in the diagram is the wrong way around (source should be connected to +ve supply not to the drain of the N-FET/-ve side of the motor - it should really look like this although, again, I'd recommend adding pull-down resistors for the N-FETs, a current limiting resistor before/after the interlock diode and a flyback diode for the motor).

At rest, with no firing or braking, both the "run" the "brake" outputs from the microcontroller (μC) will be "low" (i.e. 0V). This means the gate of N-FET "NMOS2" is "low" so that "NMOS2" is "off" meaning the 10kΩ pull-up resistor "pulls-up" the "PMOS" gate voltage to +ve supply voltage. If the gate of "PMOS" is pulled up to +ve supply voltage then the gate voltage is the same as the source voltage and the P-FET "PMOS" is "off". Obviously if the gate of "NMOS1" is also low then that N-FET is also "off". Although, like I say, if for some reason the pack is connected but the μC is not powered the floating gate pins on both N-FETs mean that the resting state may be undefined - without a pull-down resistor to force the gate pins "low" when disconnected, simply touching one of the FETs (or any other external source of charge) could switch them "on". Alternatively, if the output isn't specifically set to "low" as the μC turns on, any internal pull-ups on the μC outputs may also switch them "on" when you don't want them to.

When firing "run" output is "high" (probably 5V) and "brake" output is "low". This means the P-FET is still "off" but now the N-FET "NMOS1" is "on" and the motor spins.

When braking "run" output is "low" and "brake" output is "high" (but set "brake" output to "high" before you set "run" output to "low", and vice-versa if you switch straight from braking mode to firing mode, otherwise both FETs will be momentarily "on" at the same time and will short the supply) so "NMOS1" is "off" and "NMOS2" is "on" which pulls-down "PMOS" gate turning it "on" and braking the motor.

Since you won't necessarily know when the motor is at rest you'll probably end up with "PMOS" switched "on" and braking the motor constantly until you need to run it (unless you have a timeout on the brake so that it turns off automatically after a pre-determined period of time to prevent wasting power through the pull-up resistor). The beauty of this sort of setup is that, even though P-FETs need to be pulled down to 0V to turn "on", because you use an N-FET to drive the P-FET you turn "on" the "run" output and turn "off" the brake output to run the motor and turn "off" the "run" output and turn "on" the "brake" output to brake the motor.

Thanks SSGT and thanks Design/Forge for posting this and spurring this new edition of 'Minky learns to fet'.

Incidentally,  nice little vid showing just what SSGT was saying in regards to small charges activating the Mosfets without the resistors..

(the lovely Alice Coatduck)

I saw this on YouTube. Totally impressed with the build from the location of the battery pack to the paint job! Great work!

Keep an eye on the War part of the forum as there have been a few hosted in Hendon, which I doubt is too far for you.

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