Fog Machine Timer

A couple of years ago I got this fogger on eBay pretty cheap. It needed some help. The pump was stuck so I disassembled it and got it cleaned out and working again. I wanted to use it for Halloween this year but I don't want to have to press the button every few minutes. A long time ago I made a long interval timer that would be perfect for this. I know they make a timer for $20 but that's almost what I paid for the fogger......

First thing was to see whats inside the remote pendant. Wow, a switch and a light. that's easy. It uses a standard computer IEC connector so I don't even have to hack up the original cable. I had a couple of IEC jumpers laying around.

There's so much room in that pendant I was tempted to put a relay in there with a headphone jack connected to the coil. Then I could hook that into the doorbell and be done.....

If you're looking for a quick way out that would be it.

I never take the easy way out when it means there's a gadget that wont be built. From the way it's wired the white and black wire are connected directly to the power line. That's great since it means I can use them to power a small transformer to run the micro-controller. That opens up a whole bunch of possibilities.

So now I have a source of 110V power and if I add a CDS cell and a neon light I can even get a signal to the micro to tell me when the machine is ready. In this machine the light is ON until the machine is ready. When it goes out then you can hit the button. That pumps a little of the fog juice into the hot chamber vaporizing it.

This was the only non standard part of the build. I mounted them so the CDS cell was facing the center of the tube and covered it with heatshring tubing. I pinched the top of the tube shut with a pair of needle nose pliers while it was still hot.

There is a warning in the manual to not hit the button before the light goes out. Oddly the pump is wired across the thermostat so it wont get any power until the machine is hot. That kind of eliminates the need for the ready light since the controller can pulse the thing all day long and it wont hurt anything.

I added it anyway so the LCD can show a status of "Heating" or "Ready" along with the timer settings.

The UI is simple 4 pushbutton switches Dur+ Dur- Int+ Int- to set the duration and interval and a toggle switch to turn it off. The duration is in millisecond increments. and allows for 100 to 1500 milliseconds (.1 to 1.5 Seconds). The interval is in second increments and allows for 30 to 600 seconds between pulses. These limits can easily be changed.

Maybe another feature in V2 will be a float sensor to stop the timer if the tank goes empty. That brings with it a third status of "Fill Tank". I added the code but it's too close to Halloween to get a float switch installed. For nowI'll leave a jumper on that connector.

I wrote this in PicBasicPro. I also uploaded the hex file if you want ot use it as is. There is a free student version of picbasic available online you can use if you want to make changes.

I tried redrawing the schematic but its still a little sketchy....

You can omit the pullup resistors on the front panel switches since the pic has internal weak pullups. I like to add 1K resistors anyway.

I used a 10Mhz ceramic resonator in place of the crystal and 2 caps on pins 9 & 10. Either way is fine. When I tried to test it with a stop watch I got a measured time of 1 minute & 59.96 seconds when set to 120 seconds. Close enough for me....

I used a small wall wart and attached it to the side by using longer screws through the existing screw holes. This keeps it neat and simple.

Its really handy to have some small screwdrivers to get in there and tighten those screws.

Its looks pretty neat and the original cable exit is perfect for the power cord.

The board was left floating in the case. I should mount it but I'll have to wait till after Halloween to do that.

I used a 16F73 micro-controller that doesn't have eeprom so there is no permanent setting storage. The next version will get and upgrade to a 16F876 which has non volatile memory. Then when you change the interval and and duration it will be stored for use at the next power up. Right now it always defaults at power up to 750 mS pulses every 150 Seconds.

The two chips are pin compatible so its just a firmware update needed.

I did add a status led that blinks when it operating so I have a heartbeat. Next version that changes to a bicolor led so its either a green heartbeat, a yellow standby indicator, or a red error indicator...

Its not pretty and nothing is labelled yet but its fully operational in time for Halloween!

Just a little warning. No matter how tempting it is don't use a computer cable to plug this into a power outlet. The first time the relay fires it will connect the ground to the hot line!

My first thought was to use a hallowed out power brick to house the
controller since I had one on the shelf. That only increases the risk that some unsuspecting person would plug it in the wall.

I would suggest adding 750mA fuses to each lead from the cable.

Even worse if you plugged the unmodified factory pendant into the wall it would burn your finger when you hit the button. its easy to see that the button is connected between the black and green wires. Its such a cheap switch it would pop violently when you pushed it. I'd add fuses to that pendant too.

Its just a poor design from the factory and one that many of these machines share. Even in its unmodified form right out of the box its dangerous to a beginner who thinks "Hey I know what goes in there".