Disposable e-cigarette Teardown - Part 3

DANGER: 

There is a really good chance that the battery is still holding quite a lot of energy. The fluid runs out first. So if, during disassembly, the unit triggers (quite likely), then a big current will flow and the heating element will get hot. Similarly any accidental short between the leads to the battery could easily cause a fire.

Cut the leads to the element (one at a time) and to the battery (again one at a time) as soon as you can access them. 

Also have a contingency plan such as fire blanket (not a bucket of water) or open window through which the flaming device can safely be thrown in the event of it catching fire.

If in any doubt that you can do this safely, then don't do it.

In the third part of this series of posts, we'll take a look at exactly what's going on in one of the minimal disposable e-cogs -- the type that appears to jus have 3 components: a battery, a heating element and something that looks like an electret microphone.

The 'mic' turns out to be a lot more than just a microphone. For a start it has a blue LED on it that lights for a few seconds when it detects a strong pressure difference. I got this to work by attaching a short length of tubing to it and then blowing into it. 

I attached a load resistor to the output in order to measure the output voltage when the e-cig is activated. No real surprise here, but it matched the battery voltage closely.

Here, you can also see wha happens a the output when the e-cig is activated. Here is the wiring diagram for the e-cig.

The mystery 'mic' is acting as a high-side switch. That is. one side of the heater element is connected to the battery negative (GND) and the 'mic' (when activated) connects the other end of the heater to the positive terminal of the battery.

I think that the 'mic' might actually be a pressure detector plus microcontroller MOSFET and LED all in an electret-insert-style case. The back of the case being a thin PCB with the LED on the outside. The only reason that I think the 'mic' might have a microcontroller in it, is the way that the output is timed and that the LED blinks just before the e-cig deactivates.

Here's my best guess as to what's in the 'mic' can.

The pressure sensor may actually be a microphone, and the microcontroller does some thresholding and to decide if the air rushing over it constitutes someone drawing on the e-cig and then sets the output low to turn the P-channel MOSFET on. The LED does not seem to be in parallel with the output load, as applying a voltage back to the output when the e-cig is not activated does not light the LED. So, it looks like the LED is either connected to a different GPIO pin to the MOSFET, or its on the gate side.

Anyway - my collection of handy LiPos, of various sizes, is gradually increasing. There are just so many of these out there discarded on the street.