The sump pump is a device that works hard to keep your home dry but is prone to failure, and if you have any valuables in your basement or crawlspace, even a light flood can cause a lot of damage and potential health problems (mold!). It tends to fail in a big storm when it gets really busy pumping water away from around your home (worse if you lose power too). Heard a few horror stories and not having some kind of backup or monitoring it is the perfect recipe for a disaster waiting to happen.
My trusty SonarMote is of course an independent sensor which alerts me when the water reaches an unsafe level in the sump well. It’s mounted on the bottom of the sump well lid and it measures the distance to the water surface. My gateway app graphs that level and also provides a means to alert me via SMS or email.
This year the pump’s mechanical float decided to get stuck again and then fail, so I had to either buy a whole new pump, or find an alternative to the float part. Turns out there’s a device called The Basement Watchdog. It’s very simple in its operation – has its own float, which triggers a circuit that activates a 120v outlet for a fixed amount of time, cheap and perfect.
As soon as I installed this, I noticed my SonarMote measurement graph was messed up. That’s because the device would tend to empty out the well very sporadically and for a short delay, and the level would only vary by very little, not very visually helpful.I wondered how hard can it be to hack the Watchdog to make it stay on for longer and pump more water. Unsurprisingly the circuit is based on the TI ne555 timer chip which after a little investigation is obviously running in monostable configuration. Here’s a nice video walkthrough that explains this mode in more detail if you’re not familiar with the 555 timer. In a nutshell there is a trigger (the float) which activates the 555 timer output to go HIGH (activates relay and turns pump ON) for a fixed amount of time (configured by a resistor and capacitor RC discharge circuit), after which it goes LOW (and switches off the pump).In my Watchdog, the timer is determined by a 100K resistor and 47uF capacitor. The 555 monostable timer formula is Tw=1.1*R*C which results in a ~5 second activation of the pump, which is way too low and explains the unappealing graphs I’ve been getting:
To change this output time there are 2 options – desolder the resistor and solder a larger value (a bit messy and risking to damage the circuit), or solder a parallel capacitor to the existing one. I chose to solder another capacitor since capacitors in parallel combine their capacitance and required no desoldering. So adding 2 wires to the existing cap and using another cap at the other end makes it easy to change the cap overall value or even use a variable capacitor. If you wanted to change the resistor instead, you could add a potentiometer and case-mount to to allow changing the timing any time later, but I thought that’s overkill, I just needed a bit longer delay. So here’s the 220uF capacitor I added and taped to the relay to prevent any movement/shorts: