Optical Fog Machine Controller

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Steve and I fog in our haunted houses to provide an eerie effect that looks really creepy and puts people on edge and in a better state of mind for us to scare them.The only problem is keeping the fog level constant. You don't want too little, or nobody will notice the effect and you dont want too much otherwise the whole area will be flooded with fog and you can't see.

The easiest way to regulate this is to have someone sitting at the button controlling the unit. This is often unfeasible, especially when you have a very small group of people runnign the house.We only have 4, including myself, and at least 2 of us, if not more, are actors.

Most people use a timer to control the machine. Timers can be set to give a pulse of fog at even intervals, thus regulating the amount of fog in the environment. I considered this, but I had another idea.

My idea was to have an optical sensor measure the amount of fog present in the air. If the level falls below a set point, the machine is activated. Once the threshold has been reached, it switches off.

The circut uses an IR LED and phototransistor. They are usually sold in pairs at radioshack. My fog machine has 3 wires in the controller. A common wire, signal, and ready. The ready light activates when the temperature of the heater is sufficient to create the fog. The signal wire can then be connected to the common to activate the fluid pump and you get fog. The IR LED is turned on when the ready signal is recived from the fog machine. The resistor before the IR LED should be a 10k linear potentiometer to ajust the brightness. The 2nd potentiometer before the phototransistor is used to set the sensitvity of the relay.

The IR LED and phototransistor are mounted facing each other, but with plenty of space between them for the fog to waft around.

To operate the circut, center the sensitivity potentiometer and then turn up the IR LED's intensity untill the relay clicks on. Turn it back untill it clicks off again. Then add fog to the desired level to the space beween the IR LED and the phototransistor and ajust the sensitivity potentiometer untill it clicks off at the desired amount and clicks on when that level drops.

The circut will be redesigned in the near future, and schematics will be redrawn with component values in them.

Currently, this is all just an idea. I have not built or tested this circut yet. I have since concluded that the timer will do a better job than this and will probably use less fog juice. Nevertheless, I'm still intrested in this method of regulating the fog and will be experimenting with it in the near future.