The $35 Pump Watchdog
A small alarm that catches a water pump about to die quietly. Buildable with basic tools, about $35 in parts.
Water pumps fail quietly. A float sticks, a line freezes, a dugout runs low and the pump keeps running dry until the motor cooks. The first sign is usually an empty tank, a flooded field, or a repair bill — days after the moment that caused it.
This design watches two things that together tell the truth: how much water is left and whether the pump is running. When those two go wrong together, it yells. It is the starter version, and it is yours for free.
The one rule that matters: never fail silent. A watcher that goes quiet when it loses a sensor reading is worse than no watcher, because you'll trust it. Every failure mode in this design ends in an alarm or a flagged fault, never in a lie.
How it thinks
- Every second: read the water level, read the pump's current.
- Pump is “running” when the current clamp reads above a threshold (calibrate once: pump on vs. pump off).
- If the pump is running and the level is below your low mark for 10 straight seconds: alarm. The alarm holds as long as current keeps flowing.
- If the level sensor is lost or fouled: flag sensor fault, keep everything else running, and never clear a live alarm on a lost reading.
- Same logic catches the mirror case: pump running far longer than normal with the level not moving — a stuck float or a dead pump.
The picture
Parts (about $35 + cable)
| Part | Notes | ~USD |
|---|---|---|
| ESP32 dev board | Any flavor. The C3 supermini is cheap and proven. | $5 |
| JSN-SR04T-2.0 waterproof ultrasonic | 20–600 cm range. Buy the 2.0. Clones (AJ-SR04M) work but check the mode pins. | $8–12 |
| PVC guide pipe + cap | ~30 cm, open at the bottom. | $3 |
| SCT-013 current clamp | Get the 1V-output version for a 3.3V ADC (or add a burden resistor to the 50 mA version). | $8–10 |
| Buzzer + LED + 2 resistors | Active buzzer so you don't need a tone library. | $2 |
| 5V USB brick + cable | Often a spare. | $0–6 |
| Enclosure | Any dry box. Mine is a poplar offcut box, because it hangs on a cabin wall. | $0–10 |
| Cable (probe run + clamp run) | Measure before you order. Standard lengths are short; sites are not. | $5 |
Mounting rules (the stuff that bites)
- Straight down, perpendicular. A tilted ultrasonic reads through the water at an angle and reports nonsense. This is the #1 reported failure.
- Guide pipe, open at the bottom. Kills ripple false-reads and rain splatter.
- Keep 20 cm of headroom. The sensor's floor is around 20 cm; closer reads go invalid.
- Clamp one conductor only — the live lead, not the whole jacket. Clamping both wires reads ~zero, forever.
- Calibrate once: read the current with the pump running, set the threshold near half of that.
Test it before you trust it
- Slide a card under the sensor at the low mark — alarm should fire and hold while current continues.
- Clamp the clamp around a desk lamp's lead and switch it on — the “pump running” state should light.
- Unplug the sensor mid-test — the box must flag a fault, not go quiet. (This exact case caught a real bug in my firmware: a lost reading used to clear the alarm. The digital twin caught it before any solder did.)
What it is not
Not a certified safety device. Version 1 watches and yells; it does not switch mains power, and you should not wire it into your pump's power feed. Keep it out of anything life-critical.
Want it sized to your site?
I do the design side: parts, wiring, enclosure, firmware, and a test plan made for your exact setup — your pump, your power, your distances. $29 per setup. Email kit-180@ilands.app and tell me what you're pumping and from where.