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Hot Wire Control for Salvaged E-Cig

Prototype circuit for a hot wire cutter

So far in the exploration of salvaging and reusing parts of discarded “disposable” e-cigs I’ve managed to reuse mainly the LiPo cells, in the MP3 player build and the UV torch build I incorporated using the e-cig body tube and indeed the end caps from some of this valuable litter. One area I hadn’t really explored is the small heater element which is essentially a short piece of thin nichrome resistance wire. It seems a waste to dispose of all of this when it’s quite an interesting material. In the e-cig when the user pulls air through the system a sensor detects this pressure change and allows current from the battery cell to flow across the nichrome wire which causes it to heat. The wire is wrapped inside some fire-resistant material which is soaked in the e-cig juice which when the wire is hot, turns into vapour.

length of nichrome wire

At a larger scale, I’ve used longer lengths of nichrome wire to cut and sculpt foam and occasionally have made tiny nichrome wire shapes to cut small slots and holes into foam objects. So I was interested in exploring how I might be able to use these tiny and thin sections of salvaged nichrome wire. In larger systems, the nichrome is tolerant to a reasonable amount of current flowing through it and you can use various items, like a desktop variable constant current DC power supply to drive the wire and control temperature. However, if you simply connect the small salvaged nichrome directly across the recovered e-cig battery you run the risk of drawing too much current from the battery which can cause small unprotected LiPo cells to deteriorate and deform quickly, which can be quite dangerous. I wanted to see if I could use a microcontroller to control the amount of current passing through the nichrome wire and, as it was on my desk, decided to use a Raspberry Pi Pico as the controller.

One way to control current is to use Pulse Width Modulation (PWM) and a MOSFET. In essence, this is a very fast switch between the LiPo and the nichrome which is pulsed on and off so quickly that you can vary the amount of current. In the spares box, we found some AO3400/A09T N Channel power MOSFETs which were in the tiny SOT23 package, but luckily a PCB I’d made for another project had a SOT23 footprint on it and could act as a breakout board.

circuit design

The circuit is pretty simple. A GPIO is connected to the MOSFET Gate pin with an in-line 10K resistor which just acts to limit the amount of current the MOSFET pulls from the GPIO pin. Another 10K resistor is connected between the Gate pin and ground so that the GPIO is pulled low whenever the PWM signal isn’t pulsed. This, as well as pulling the GPIO pin low in the code, means that the nichrome wire shouldn’t stay hot if the GPIO becomes disconnected or the Raspberry Pi Pico is unplugged. The nichrome is attached across from the Drain pin on the MOSFET and the positive of an attached e-cig LiPo cell. The Pico will be is powered separately from its 5V USB source but it’s important to connect the LiPo negative and the Pico ground.

code for Pico

On the Pico, Micropython was used to set up the PWM signal and we used GPIO 15 to drive our circuit. The code is reasonably straightforward and you should be able to see that we turned on the PWM for 3 seconds and off for 3 seconds in a loop. We set the frequency at 500 meaning the PWM will pulse 500 times per second and we then played with the values of the duty cycle. If you plug in a multimeter set to measure current inserted into the positive connection between the battery and the nichrome you can see the changes in the current drawn into the nichrome as you change the value of the duty cycle. At the highest setting of 65535 our small salvaged nichrome drew just over 1 amp from the LiPo cell whilst at values around 30000 the nichrome was still hot enough to cut EPP foam but was only drawing around 350mA.

DMM showing a DC current reading

We’re now armed with enough knowledge that we could potentially make a small hot wire foam cutting tool and if we went for a smaller microcontroller board or chip, it might even fit into one of our E-Cig housings!

General tinkerer! Freelancing writing about making things, rocketry, boats, electronics and a mahoosive pile of unfinished and unstarted! Author of "FreeCAD for Makers" book on Raspberry Pi Press and writes for Hackspace Magazine, Tindie, Kids Code Computer Science, Toms Hardware and more!
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