Miniature smoke machine for laser experimenters

Here's a simple design for a device to create small amounts of smoke on demand to make laser beams visible. It uses a dispensing syringe to pass standard smoke machine fluid through a tube, heated by a soldering iron element. I knocked this up from materials I had to hand, and a more elegant solution shouldn't be hard to devise. If you can't be bothered with this, you can buy cans of artificial smoke from Maplin.

Overall view. Everything is mounted on a metal plate, although a more elegant assembly could probably be made using a complete soldering iron. I used a 25 watt Antex element, and to get a suitable temperature, it is run at half power by connecting a 1N4005 diode in series with the mains supply. You will definitely need to reduce the power, and if half power isn't suitable, use a light dimmer to adjust the temperature. Of course you could also use a temperature controlled iron! Temperature needs to be around 150-200 deg.C.

The large coil is a safety guard (made from coat-hanger wire) to protect the hot element from accidental contact with anything. A coil was used, rather than a tube, to reduce the heating of the guard - with a coil the airflow will be good, so it should stay fairly cool.

For the heating tube, I used some 1mm I.D. stainless capillary tube, held against the stainless element shaft with a couple of coils cut from a spring. This was purely because I didn't have any copper tube, or any bits to fit the soldering iron element. A much easier method would be to use copper or brass tube (e.g. the capillary tube from an old thermostat), and braze (hard solder) it to the soldering iron bit.

The coupling from the tube to the syringe is a short length of heat-resistant 1mm I.D. silicone rubber sleeving, and is actually a lot more involved than it appears in the picture. After some experimentation, it was found that a one-way valve was necessary to prevent smoke generation when not required, and dripping of fluid when not in use. A flow limiter was also necessary to avoid drops of liquid being ejected from the nozzle.

Both the valve and flow-limiter functions are achieved as shown on the left. The end of the 1mm syringe dispensing needle is blocked (I forced some wire into the end and slightly crimped it - a tiny blob of epoxy resin would probably also do the job). A hole is then made in the side of the needle with a small round file. When the silicone sleeving is slid over the needle and the end bound with wire to seal it, this acts as a one-way valve, the liquid pushing the silicone sleeve away from the hole.This is just like older style cycle tyre valves.

The flow limiter is simply a piece of copper wire inserted into the bore of the needle to reduce its effective cross-section, flow being set by the diameter of the wire (you can stretch copper wire to fine-adjust its thickness). To avoid the wire falling out, bend it slightly before insertion. Leave enough extra length so that it can be removed with pliers if necessary.

I found that the smoke density was much more even if the end of the tube (bottom in picture) was crimped almost closed to restrict the output flow, increasing the velocity.

It still has a bit of a problem with spitting droplets of liquid out the end of the tube (so keep it well away from high-quality optics!)

I tried this longer tube version, using brass tube which flattened as it was wound round, reducing the bore. This seemed to suffer less from the droplet spitting problems but still isn't perfect.