This modification will allow you to control the motor speed of a Nerf Stryfe and thereby the impact force of the Nerf darts.
The Stryfe is my favourite Nerf gun, but like all electronic blasters in its stock form it is kind of weak. There are multiple instruction on the web on how to perform a voltage mod, but to change the speed you would always have to use different batteries. You might ask why would you want it to run on a lower speed? Well if you have ever heard how loud it is after the voltage mod the question answers itself. Also, to see your friends faces, when you crank up the speed is priceless due to the noise the gun is making.
As an added feature, the speed of the motors is indicated by nine LEDs on the side of the blaster.

Btw. if the only thing you would like to do is to run the Stryfe on 14.8 volts, just take a look at step 2.

Step 1: Stuff You Need

Materials

Nerf Stryfe
5 mm thick ply wood (take a look at step 5)
Transparent red plastic
Optional: paint, plastic primer, modelling clay, filler
Hot glue, or sugru
Perf board
Silicone grease

Electronics

Atmega 328
7805 voltage regulator
2 x 22pF, 1 x 47 and 1 x 100 uF capacitor
16 MHz quartz
TIP120 npn darlington transistor
NPN transistor BC547
Relay
Si-Diode
Potentiometer 0-80 kOhm
9 x 220 Ohm resistors, 2 x 10 kOhm resistors, 1 x 1 MOhm reistor
9 straw hat LEDs, 3 red, 3 orange, 3 yellow
4 x Li-ion cells 14500 (e.g. from UltraFire, on amazon: Batteries and charger)

Tools

Phillips screw driver (PH 0)
Drill and 5mm drill bit
Soldering iron, solder
Hot glue gun

Step 2: Taking the Stryfe Apart

Start by taking the Stryfe apart. The outer shell is held together by 13 long (marked with green) and two short (marked with red) screws.
The second picture shows the internals. I've marked all the screws you will have to keep with red. Take all the screws out and remember where the ones that are marked go.

Btw. if the only thing you would like to do is to run the Stryfe on 14.8 volts, simply remove the thermistor I have marked in the second picture and use four 3.7 volt batteries.

Step 3: Altering and Painting

Altering and painting the gun is of course optional, but since it is my favourite part, I will walk you through my process.

As you can see in the pictures, I've altered the looks of the gun. I use modelling clay for bigger alterations and car body filler for smaller ones. In my experiments most modeling clays don't stick well to the plastic (even after sanding it). So I let the modeling clay set, break it off, use two component glue to reattach it and then the car body filler to remove unevenness. With this method I never had any problems.

Before you paint the gun you should sand it. This will help the paint to stay on longer. After you are done sanding all the pieces (especially the trigger and switches) clean them and prime them with a plastic primer.

Painting will work best, if you either hang the pieces from a clothes line or stand them upright in a cardboard box. The first way will allow you to paint it from every angle, but the spray paint will go everywhere, so don't do it close to your house. Like always when you are painting stuff, multiple thin layers of paint are better than one thick one.

Step 4: Drilling the Holes

It is not an easy task to drill the holes, as shown in the pictures. Mainly because you will have to use a very low drill speed, else you will just melt the plastic.
I used a 6 mm drill bit to drill the hole for the potentiometer, but when I tried to mill the holes for the LEDs, I just couldn't get them right. So I ended up making them smaller and using a diamond file to get the perfect fit. I used 4 LEDs for one hole and 5 for the other, but feel free to use whatever combination you would like.

After drilling the holes I used two different tones of grey to match the mass effect colour scheme and dry brushed them to give it a weathered look.

If you are interested in dry brushing, here is how I do it:
I like to use a flat, soft brush with a size between 14 and 20. Make sure, that it is absolutely clean and dry. Carefully dip it into the paint and use a paper towel, to wipe off most of the paint. Then carefully use long strokes back and forth, to get the tone you like. If you would like to add accents to the edges, brush in just one direction away from them.

I like to use Revell Email paints, since there quality is amazong. I brought the Nerf gun to the Mini Maker Faire in Groningen and even though more than 70 kids used it and some of them were scraping it over the floor it still looked good in the end.

Step 5: LEDs

Drill holes into the plywood to fit the LEDs (to prevent them from shining through the wrong hole). Afterwards glue your transparent red plastic to it and glue everything to the inside of the gun.

Cut two small pieces of perf board and place the LEDs into it, as shown in the second picture. Solder the cathodes (shorter leg) of the LEDs together.
Now solder one 220 ohm resistor to each anode (longer leg) and add a wire to each led connecting it to a pin as shown in the last picture of this step.

Insulate everything with either sugru or hot glue. Make sure, that the mechanism still works and fixate the wires once again with either one of the materials. This way the wires won't get tangled.

You can add the potentiometer to the same side of the gun as the LEDs, but I prefer to have most of the wires on one side and therefore didn't do so.

Step 6: Electronics

The controller reads the voltage drop over the potentiometer R11 (violet part) using analog input A5 and translates this into the motor setpoint. This setpoint is then translated into a current signal at PWM pin 3 which feeds the base of the TIP120 Darlington transistor and controls the actual motor speed.
The setpoint signal also controls the power of the LED Pins D5 – D13 either directly via the PWM function or indirectly by turning the ports on and off in a regular interval.
The board marked with the green area contains the current supply for the Atmega controller as well as a self-latching relay (U3). The relay is turned on with the software by powering D4 and keeps the circuit closed when the original push-button of the Nerf is released. When the potentiometer is turned to the leftmost position for 3 seconds, the controller stops powering D4 and powers itself off.

I used an Arduino UNO to upload the following code to the Atmega:

int motorPin = 3;  //  motor control 
int inputPot = A5;  // input potentiometer
int potMin = 943;  // minimal value at PotRes = 0
int potMax = 1023;  // maximal value 
int holdPin = 4;  //voltage stability

int LEDPins[] = {13, 12, 11, 10, 9, 8, 7, 6, 5};
int LEDCount = sizeof(LEDPins)/sizeof(int);    //number of LEDs


//Motorsteuerung
unsigned long motorTimer = 0;  //motor control in ms
int motorLoopTime = 100;  //response time of the motor in ms
int outTime = 2500;  //how long has the potentiometer turned to 0 until the motor turns off? [ms]
unsigned long outLoop = 0;
int powerSet = 0;
int powerOut = 0;
int maxPower = 120;  //maximum speed of the motor
int minPower = 30;   //minimum speed of the motor
int LEDFak[] = {0, 0, 0, 0, 0, 0, 0, 0, 0};


void setup(){
  Serial.begin(9600);
  pinMode(inputPot, INPUT);
  pinMode(motorPin, OUTPUT);
  pinMode(holdPin, OUTPUT);
  delay(20);
  //turn the relais on, as soon as the potentiometer is set to 0 during the start
  if (analogRead(inputPot) <= (potMin + 5)) { digitalWrite(holdPin, HIGH); }
  else { digitalWrite(holdPin, LOW); }  
  for (int ii=0; ii<LEDCount; ii++) 
  { 
    pinMode(LEDPins[ii], OUTPUT); 
  }  
}

void loop(){
  if (millis() > (motorTimer + motorLoopTime)){  
    motorTimer = millis();
    int motorSP = analogRead(inputPot);
    //digitalWrite(holdPin, HIGH);
    
    //turns the relais off, as long as the potentiometer is turned for a certain time to 0
    if (motorSP <= (potMin + 5)) { outLoop = outLoop + motorLoopTime; }
    else { outLoop = 0; }
    Serial.println(outLoop);
    if (outLoop > outTime) { digitalWrite(holdPin, LOW); }
    
    Serial.println(motorSP);
    int motorPower = map(motorSP, potMin, potMax, minPower, maxPower);
    //Serial.println(motorPower);
    analogWrite(motorPin, motorPower);
    
    //calculate the LED settings
    int LEDVal = map(motorPower, minPower, maxPower, 30, LEDCount * 100-30); 
    
    for (int ii=0; ii < LEDCount; ii++){
      if (LEDVal > 100) { LEDFak[ii] = 100; }
      else if (LEDVal > 0) { LEDFak[ii] = LEDVal; }
      else { LEDFak[ii] = 0; }
      LEDVal = LEDVal - 100;
    }
  }
  dimLEDs();
}

void dimLEDs(){
  float delayTime = 500;
  
  for (int ii=0; ii<LEDCount; ii++)
  {
    if (LEDFak[ii] == 100) { digitalWrite(LEDPins[ii], HIGH); }
    else if (LEDFak[ii] > 0)
    {
      int onTime = (int)(delayTime * ((float)LEDFak[ii]) / 100);
      digitalWrite(LEDPins[ii], HIGH);
      delayMicroseconds(onTime);
      digitalWrite(LEDPins[ii], LOW);
      delayMicroseconds(delayTime - onTime);
    }
    else { digitalWrite(LEDPins[ii], LOW); }
  }
}

Step 7: Reassembly

Now it is time for the reassembly. Cut off the rail of the trigger switch, as shown in the first picture. Else the trigger will activate the switch every time it is pulled (honestly because of the relay it doesn't really matter, but it is just cleaner).

The pictures show you how to reassemble the switches, make sure to put some silicone grease on them, to prevent the paint from scraping off and to make the blaster smoother.

All that is left to do is to load it up with four 3.7 volt batteries and you are good to go.