My knowledge of electrical engineering isn't great but faced with a challenge I do have a habit of learning enough to see it through. There is a probably a very simple solution to this.

I've been pondering different routes to achieve a 'chuffing' smoke effect on a model train. I'm looking to use a Hall effect sensor to pick up 4 magnets spinning on the axle of the train. Each detection triggering a puff of smoke up the chimney.

To generate the smoke I'm going to be using a vape heating coil of 1.5ohms. There will also be a small 3V fan in an enclosure with the coil to ensure there is airflow and push the vapour out the chimney. Total load would be a smidge over 2amperes at 3V.

However, I would like the fan and coil to be continuously powered at a base load so there is always a little activity. I would then like the Hall effect sensor to come into play to produce 'chuffs' of more intense blasts when the locomotive moves.

There is 12VDC on the locomotive to utilise, but I've got some miniature variable voltage regulators (MP1584en) left over from another project. I was thinking I could use two of these to have a supply of 3V for the 'chuff/HIGH' and ~1.5V for the idle/LOW.

What I need is a way of switching between them with the Hall effect sensor.

I've got a few A3144 Hall effect sensors and some IRF9540N P channel MOSFETss that I was originally planning to use, but a second opinion wouldn't go amiss.

Thankyou to anyone who can spare me a few moments!

(ps. The more I think about this the more issues I for-see. If the locomotive were to stop in a position where the Hall effect sensor was activated, it would be good to have a way to default back to the lower voltage state after a few seconds.)

  • \$\begingroup\$ I think it will be better to drive the coil with low side switching. Have a capacitor charged up with a pullup resistor, then dump the capacitor through the coil. That way you won't reset everything when you energize the coil (which is going to be a problem otherwise unless you have the ability to deliver V/1.5 Amps). \$\endgroup\$
    – user57037
    Commented Jan 21, 2021 at 5:25
  • 2
    \$\begingroup\$ To be honest, I would get rid of the hall sensors and just program an MCU like the PICAXE to do puff on a fixed or randomized timer and do something else the rest of the time. Then instead of switching the output voltage of a linear regulator (which would require switches/transistors in the feedback loop of the linear regulator which is dicey), I would instead PWM the voltage to the vape unit using a single transistor driven by the MCU. If your raw voltage supply is stable enough you can PWM the supply directly without a a linear regulator. This is much more efficient without a regulator \$\endgroup\$
    – DKNguyen
    Commented Jan 21, 2021 at 5:26
  • 1
    \$\begingroup\$ In case you do not understand mkeith's low side switching comment, he is saying to use NMOS instead of PMOS. NMOS is most convenient on the low-side, PMOS is most convenient on the high side but the two are not equal and NMOS is more convenient in general since it can be driven more simply with a ground referenced signal and most devices that will drive it are ground referenced. \$\endgroup\$
    – DKNguyen
    Commented Jan 21, 2021 at 5:32
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    \$\begingroup\$ You could keep the hall sensors around so the MCU knows when the train is in motion or how fast it is going which could be useful. Without them, the train is liable to puff while just sitting there if just on a simple timer. \$\endgroup\$
    – DKNguyen
    Commented Jan 21, 2021 at 5:37

1 Answer 1


I'd suggest using an MCU as well, you can still use 1 magnet on the axel to know the train speed (you can average and divide by 4 on the MCU itself) and control the coil from the MCU.

With a PWM (generated from the MCU) you can adjust the power of the coil in the rest and active state. The same goes for the fan.

However, the coil response of a vape is a few seconds, so it might be difficult to do short puffs. You would probably need the more sophisticated implementation to continuously produce smoke.

Perhaps continuously producing smoke in a cavity and modulate the fan speed, with a fast, small, fast response fan, to produce the puffs.

A small arduino is probably a good way to start.


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