In a whirlwind of bad luck, my condensate pump went up on the furnace about a year after we moved into the house. It's an easy part to replace: some tubing for the pump discharge (reuse what's there), some PVC for the inlet for the furnace (some basic plumbing), a 110V outlet for the pump (provided by code on the furnace itself, so just reuse it), and 2 wires that tell the AC to stop because the pump is full (this is supposed to prevent basement flooding, and is the basis for this hack).The replacement pump I bought turned out to be crap but I was lucky in the sense that the overflow float switch on that unit was wired the same way as on the old one, so it was almost a drop-in replacement (the plumbing part came in as I had to move the pump's placement on the wall, meaning the fill tube from the AC had to be rerouted). However, when I went to replace that dud with a "better" pump, things went awry...
I had to reroute the plumbing a SECOND time, forcing me to start from scratch with all new pipes (damn PVC glue...) and to incorporate some vinyl tubing to make placement this time that much easier. But worst of all, the switch on the "better" pump is Normally Open (the switch closes when the water level is at overflow). On the previous two pumps, the switch was Normally Closed (the connection would go open circuit if the water was at overflow levels). Not having the ability to switch the switch, as this "better" pump housing is completely sealed with only inlet vents for the water, the outlet pipe for the discharge, the 2 wires for the overflow switch, and the power cord, I was stuck. NOTHING is accessible to change that overflow switch. So I thought "well it's a new pump, so until I can fix it, I'll jumper the wires on the furnace and hope for the best."
I went downstairs to get tools for an unrelated project and I find my "waiting to be washed" pile in front of the washer is SOAKING wet, as is the floor, as are all the floor mats including the one out to the garage! Many expletives uttered and towels consumed drying up the mess, and now I have a condensate pump that had somehow "jammed" (it still works but is now untrustworthy) and had caused the AC unit to drip water all around inside the evaporator unit and out the bottom. I'm just damn lucky all that water didn't short out the electrical for the furnace and start a fire (how ironic!). Enter Emergency Engineering, and I can at least run the AC during this muggy Baltimore summer.
My handy bilge pump, with auto-switch, from Hurricane Irene many years before, and one of everyone's favorite orange project bucket, and at least my floors won't get wet, or at least the AC would have to pull 5 gallons worth of mugginess out of the air first (in Baltimore in August, that's like 4-5 hours of run time, I kid you not!).
I knew I needed a relay controlled by the switch on the pump, but at this point, I needed a permanent solution. I ran to Radio Shack to find them closed, so I went to Auto Zone thinking "Hey they have automotive relays! They're 12VDV!". TIP: Don't buy a "relay" from the auto parts store thinking they are the same from 20 years ago. They're very digital on the inside now and will NOT work for this purpose. Maybe I'll do a tear-down of that later...
So, on another weekend, off to Radio Shack AGAIN and this time with a wiring diagram in my head. After 20 minutes of adjusting my shopping list to fit the actual stock on the shelves ("Oh wow, they've added a lot of Arduino stuff and shields and books to their shelves, that's cool!"), I arrive home with $45 in parts (including project box). And I build this.
So this guy was assembled in about an hour, using PVC cement to mount the terminal blocks to the ABS plastic project box (works ok, but you need a clamp, about 10 minutes wait time, and FRESH AIR, that stuff stinks). I just happened to have the cement because of all the stupid plumbing done earlier in this story.
To try and keep things neater, I had the idea of drilling through the back of the terminal blocks so that I didn't have to take up any more real estate with wires looping down from the terminal blocks. The solder points didn't look that great and are potential shock hazards if this thing is connected to high voltage, so I used liquid electrical tape to cover over the soldered portions of the block. Looks messy, and I'm not all that happy with it, but it works and is safer than exposed terminals. I doubt it would pass code inspection, but at this point I just don't want another flood in my basement!
The circuit is admittedly overkill, but I wanted to isolate the low voltage DC control circuit from the furnace circuit as much as possible (likely the voltage is only around 24V but I'm being cautious), and also have the ability to do "other things" with the contacts, such as louder alarms or maybe something that Tweets in the future.
I also need to find a place to mount it where it won't melt in the winter time when the burners kick on.
At rest, the circuit only draws what goes through that green LED (20-30mA). At activation, the draw is about 200-300mA with the two relay coils, the other LED, and the piezo buzzer (nice and loud and annoying, not smoke detector loud, but loud enough). The red LED has its own blinking circuit inside which has the added effect of changing the pitch through the piezo speaker when it lights up, making it more noticeable. If anything is connected to the +12V ALARM OUT contact, then that will draw additional current, but I figure with one of those little 12V 5AH stand-by batteries and some sort of stand-by charger, this circuit should suffice even if I run the AC off of a generator (which is one of my next big projects).
This doesn't reverse the process of changing a NC float switch pump to work with a NO furnace, but that can wait for another day...