How it works

The burn wire circuit works by "short circuiting" the battery across a small piece of nichrome wire. Because the wire has low resistance, lots of current flows from the battery through the wire which heats the wire up very quickly.

To implement the scheme described above, we need an electrical switch that lets us briefly "short" the battery across this piece of nichrome wire. That circuit is shown below. Practically, our switch (and battery) can't handle a complete short, so we throttle the amount of current allowed through the switch by pulsing the signal on "BURN1."

The code to do this is quite simple:

import board, time, busio
import pulseio
import neopixel

pixel = neopixel.NeoPixel(board.NEOPIXEL, 1, brightness=0.5, auto_write=True)

def burn(FREQUENCY=500,DUTY_CYCLE=0,DURATION=1):
    print('----- BURN WIRE CONFIGURATION -----')
    print('\tFrequency of: {}Hz\n\tDuty cycle of: {}%\n\tDuration of {}sec'.format(FREQUENCY,(DUTY_CYCLE/0xFFFF)*100,DURATION))

    # configure burnwire PWM pin, wont turn on b/c duty_cycle=0
    burn1 = pulseio.PWMOut(board.BURN1, frequency=FREQUENCY, duty_cycle=0)
    time.sleep(0.5)
    pixel[0]=(255,0,0)
    # start PWM on burnwire pin
    burn1.duty_cycle=DUTY_CYCLE
    # wait for a set amount of time
    time.sleep(DURATION)
    # turn off burnwire pin
    burn1.duty_cycle=0
    pixel[0]=(0,0,0)
    # make sure it can't turn on again (special)
    burn1.deinit()
    print('----- BURN FINISHED -----')

We perform the pulsing (known as pulse-width modulation: PWM) on the board.BURN1 pin and then deinitalize the pin to prevent inadvertently using the pin again in the future.

Using the code

To use the above code, save it on the pycubedmini board in a file title burn.py. Plug the mainboard into a Z panel board USING THE CABLE LABELED +Y (since our flat flex was designed when we had burn wires on the +/- Y panels). Next, make sure you have a battery connected to the mainboard. The simplest way to do this is using the VBATT and GND connector on the mainboard. Plug the mainboard into your computer and open Mu. Inside Mu, open the serial terminal (REPL) and type import burn and press enter. Now you're ready to test! Now you can enter something like: burn.burn(FREQUENCY=1000,DUTY_CYCLE=22000,DURATION=3) and as soon as you press enter, the burn wire will be active. The terminal will print something like this:

>>> burn.burn(FREQUENCY=500,DUTY_CYCLE=25000,DURATION=2)
----- BURN WIRE CONFIGURATION -----
        Frequency of: 500Hz
        Duty cycle of: 38.1475%
        Duration of 2sec
----- BURN FINISHED -----

Note the code will turn the neopixel red while the wire is active.

In the REPL, you can re-enter burn.burn() and proceed to tune the performance of the burn wire. I recommend you keep FREQUENCY at 1 kHz, and try and keep DURATION as short as possible (<5 sec).

IMPORTANT

You need to carefully increase the value for DUTY_CYCLE to avoid damaging the circuit, batteries, or the board. This value is a 16bit integer with 0xFFFF (2^16) representing 100% on and 0x7FFF (2^15) representing 50% on (isn't hex cool?). Note in the code you can see how duty cycle is calculated: (DUTY_CYCLE/0xFFFF)*100

Keep in mind, the wire will get 🔥 HOT 🔥. Over time, as the wire gets heated over and over again, it is possible for it to break. Unsolder the wire and attach a new one. Try and make it look as close as the original: short, solder all around the wire. You can't actually solder to the Nichrome wire, so you're basically just holding it there with solder and trying to make the best electrical contact you can.

spacecraft-design-lab-2019 / documentation

burn wire testing #196

How it works

Using the code

IMPORTANT