Sunday 15 November 2015

Car Smoke Machine

A prank that's been doing the rounds in my office is that if you leave your car keys unattended on your desk, you can expect that your car probably won't be where you left it when you leave for the evening.

I've been kicking around ideas on how to improve on this gag, and at the same time defend my car from it happening to me.

My idea was to orchestrate a scenario of deliberately leaving my key for someone to try and move my car, and setting up a surprise for them when they get in. If I could make the car appear to be breaking down, would they own up thinking they broke the car, or just put the key back and deny all

So, what do we need to make it seem a car's breaking down? Noise and smoke.

The noise is quite simple, take a vibration motor from an Xbox/Playstation controller, wire it up and put it in a small container of screws & random bits of metal, so when it vibrates, we get a metallic rattle.

The smoke, not so easy. All the consumer smoke machines I could find required mains power, and would be far too bulky to conceal in the car, so I have to roll my own.

Thankfully vaping and e-cigarettes have made battery powered, small, smoke producing devices quite cheap and readily available.

I purchased a few e-cigarette "Atomisers" from amazon and did some testing. See this post for a write up of how I wired up the eCigarette.

Unless there's a person 'drawing' on the device, the smoke cloud just sits there, so I'm going to need to create a pump or fan to force air through the atomiser.

A quick test with some old computer fans that I had lying around showed that they weren't powerful enough, so I found a battery operated air-mattress pump that would do the trick.

The pump appears quite bulky, but it runs on 4 D Cell batteries. As we're not going to need those, a large proportion can be cut away, streamlining the whole thing.

To drive the motor of the pump, I'm going to use the same SN754410 that I'm using to power the e-Cigarette (this is why I used the SN754410 in the previous post, as it's capable of driving two motors, or in this case, one motor and one e-Cigarette.)

This is how the original circuit was planned. The input voltage is 12v as this is the most readily available source in the car. (The EN pins and motor logic will all be connected to the microcontroller - left off in this diagram).
Testing this circuit turned out that 1 amp per 'motor' wasn't powerful enough to drive the pump, so I ended up putting the eCigarette coil and the pump motor in series, and joined the two motor channels of the SN754410 in parallel to double the available current. This isn't an ideal solution, as the resulting heat of the eCig coil is a bit excessive, but I only need it to work in short bursts, and I can control that from the micro.

The revised circuit

Here is the first working test of the smoke machine:

As the pump makes enough noise, I decided that the vibration motor was a bit unnecessary, so left that off the final design. Essentially the device is a simple micro-controlled smoke-machine.

Thursday 5 November 2015

Wiring an interface to an e-Cigarette

I'm currently working on a project that requires the use of an e-cigarette.
I'm not a smoker, so as far as that aspect is concerned, I might be a little off on the terminology, but eCigarettes, vapourisers, whatever you want to call them, essentially follow the same principle. There's a battery, a small reservoir of vaping fluid, a wick, and a wire coil.

Current is passed through a coil which causes it to heat up, in turn heating a wick soaked in fluid which turns it into a vapour.
We can ignore the battery component, it's only really the wire coil I need for the project. I do however need to control it, so I needed to find a way to interface it.

The coil connects to the battery by a screw-type connector which appears to be specifically for these devices. In addition, the connector also contains a small gap which acts as an air inlet for the user to take a drag on it.

This causes problems when trying to create a connection, as connectors run the risk of blocking the inlet.

Coincidentally I found that a male TV aerial connector is an ideal size to create a push-fit connector. The next task is to decide how to control the current flow. Simply switching it on and leaving it will cause it to overheat.

Coils like this can be controlled in similar way to motors. In fact, the IC I'm going to use to control this is really designed for driving motors, but works fine for the purpose of this.

The chip is the SN754410 H-Bridge motor driver - which is completely overkill for this, but as this is forming a part of a larger project, the reasoning will become clearer later.

The pinout for the chip is available on the third page of this datasheet. Treat the vapouriser as if it was a motor - polarity/direction of travel doesn't matter.

For the motor drivers voltage I'm using 12v, which, again, is overkill for this part in particular, but the coils are very tolerant of it, and use the speed control function of the chip to control how hot the coil gets.

That's all for now, stay tuned for the 2nd part of this project...