Hmax<\/code><\/td>\n300 cm<\/td>\n<\/tr>\n \nMaximum flow, Qmax<\/code><\/td>\n240 litres per hour<\/td>\n<\/tr>\n \nMaximum liquid temperature<\/td>\n 60 degrees C<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\nTwo of those deserve working out rather than reading past.<\/p>\n
4.2 watts at 12 volts is 350 milliamps.<\/strong> That is well beyond anything an Arduino pin can supply and beyond the 5V<\/code> pin as well, which is why this page\u2019s prerequisite is the relay tutorial. The Arduino switches; it never carries the current.<\/p>\nHmax<\/code> and Qmax<\/code> do not happen together.<\/strong> The pump gives 240 L\/h at zero lift, and zero flow at three metres of lift. Everything in between is a curve. Lift water one metre and expect appreciably less than 240 L\/h; lift it three and you get a trickle.<\/p>\nFor a plant watering project, the practical consequence is simple: measure the flow in the arrangement you actually build<\/strong>, with the tube you will use at the height you will use. The method is below and it takes two minutes.<\/p>\nBefore you begin<\/h2>\nRequired parts and tools<\/h3>\n\n- An Arduino Uno or compatible board.<\/li>\n
- One submersible pump. This is Siratek product 2808.<\/li>\n
- A relay module, Siratek 3308 or 3398, or a MOSFET. See below.<\/li>\n
- A 12 V supply capable of at least 500 mA.<\/strong> Not the Arduino.<\/li>\n
- Silicone or PVC tube to fit the outlet, and a container of water.<\/li>\n
- A multimeter.<\/li>\n<\/ul>\n
It must stay wet<\/h3>\n
This is the rule that kills these pumps, so it comes first.<\/p>\n
The pump is cooled and lubricated by the water it is pumping.<\/strong> Run it dry, even for a short time, and it heats quickly and the bearing wears. A few minutes dry can be enough to ruin it.<\/p>\nTwo consequences for any project:<\/p>\n
\n- Never leave it running unattended without knowing the tank has water.<\/strong> A water level sensor or a float switch is not a refinement here; it is what stops an empty tank destroying the pump.<\/li>\n
- Put a time limit on every run.<\/strong> A watering routine that runs for thirty seconds and stops cannot empty a tank and then keep going. A routine with no limit can.<\/li>\n<\/ul>\n
The sketch below has both.<\/p>\n
Submersible means the pump, not the wires<\/h3>\n
The body goes in the water. The joint where the wires leave it is the weak point<\/strong>, and the cable itself is not meant to sit submerged for months.<\/p>\nKeep the cable exit above the water line where you can, and never let the connection to the supply get wet. For a tank that will be left alone, seal the joint properly.<\/p>\n
Relay or MOSFET<\/h3>\n
Either works, and they fail differently.<\/p>\n
A relay<\/strong> is the simpler answer and what the prerequisite tutorial covers. It switches cleanly, isolates the contacts from the control side, and is entirely adequate for something that turns on for thirty seconds twice a day. Its contacts are consumable, which at a few operations a day does not matter.<\/p>\nA MOSFET<\/strong> switches silently, has no contacts to wear, and allows speed control by PWM<\/strong>, which a relay cannot do. It needs a logic-level part, a gate resistor and a flyback diode, and gets you a pump you can run at half flow.<\/p>\nThis page uses a relay, because a watering system does not need variable flow and the relay page already exists. If you want PWM control, the L298N tutorial\u2019s reasoning applies directly.<\/p>\n
The flyback diode<\/h3>\n
A pump motor is an inductor. Switching it off collapses the magnetic field and produces a voltage spike, which arcs across relay contacts and shortens their life.<\/p>\n
Fit a diode across the pump, cathode to the positive side<\/strong>, a 1N4007 or similar. On a relay board it is not strictly required, because the contacts can take it for a while, but it costs almost nothing and visibly extends the life of the contacts.<\/p>\nIf you use a MOSFET instead, the diode is not optional at all.<\/p>\n
Pin reference<\/h2>\n
\n\n\nConnection<\/th>\n To<\/th>\n Notes<\/th>\n<\/tr>\n<\/thead>\n \n\nArduino D7<\/code><\/td>\nRelay IN<\/code><\/td>\nThrough the relay module<\/td>\n<\/tr>\n \nArduino GND<\/code><\/td>\nRelay GND<\/code><\/td>\n<\/td>\n<\/tr>\n \nArduino 5V<\/code><\/td>\nRelay VCC<\/code><\/td>\nCoil current; see the relay page<\/td>\n<\/tr>\n \n12 V supply +<\/code><\/td>\nRelay COM<\/code><\/td>\nLoad side<\/td>\n<\/tr>\n \nRelay NO<\/code><\/td>\nPump red<\/td>\n So it is off when unpowered<\/td>\n<\/tr>\n \nPump black<\/td>\n 12 V supply -<\/code><\/td>\n<\/td>\n<\/tr>\n \nDiode across the pump<\/td>\n Cathode to red<\/td>\n 1N4007<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n![\"A](\"http:\/\/siratek.net\/wp-content\/uploads\/2026\/11\/submersible-pump-wiring.svg\")
350 mA at 12 V: the Arduino switches the relay, and never carries the pump current.<\/figcaption><\/figure>\nWiring steps<\/h2>\n\n- Unplug everything, including the 12 V supply.<\/li>\n
- Wire the Arduino to the relay module\u2019s control side only, as in the relay tutorial, and prove you hear it click before going further.<\/li>\n
- Unplug again. Wire the load side: 12 V positive to
COM<\/code>, NO<\/code> to the pump\u2019s red wire, pump black back to the 12 V negative.<\/li>\n- Fit the diode across the pump\u2019s two wires, the banded end to the red wire<\/strong>.<\/li>\n
- Put the pump in a container of water, with the tube fitted and the outlet somewhere you do not mind getting wet.<\/li>\n
- Plug in the USB cable, then the 12 V supply.<\/li>\n<\/ol>\n
Step 5 is not a joke. The first time this runs it will move more water than you expect, in a direction you did not plan for.<\/p>\n
Software and libraries<\/h2>\n
No library is required.<\/p>\n
Minimal working example<\/h2>\n\/\/ Siratek: run the pump for a fixed time, and never longer.\n\nconst int pumpPin = 7;\n\n\/\/ Relay polarity: see Switching Bigger Loads with a Relay.\nconst int PUMP_ON = LOW;\nconst int PUMP_OFF = HIGH;\n\n\/\/ Nothing may ever run the pump for longer than this.\nconst unsigned long MAX_RUN = 30000UL;\n\nbool running = false;\nunsigned long startedAt = 0;\n\nvoid startPump() {\n if (running) {\n return;\n }\n running = true;\n startedAt = millis();\n digitalWrite(pumpPin, PUMP_ON);\n Serial.println(\"pump on\");\n}\n\nvoid stopPump() {\n if (!running) {\n return;\n }\n running = false;\n digitalWrite(pumpPin, PUMP_OFF);\n Serial.println(\"pump off\");\n}\n\nvoid setup() {\n digitalWrite(pumpPin, PUMP_OFF); \/\/ before pinMode, so it cannot pulse on\n pinMode(pumpPin, OUTPUT);\n\n Serial.begin(9600);\n Serial.println(\"Siratek: pump test. Ten seconds on, ten off.\");\n}\n\nvoid loop() {\n \/\/ The limit is checked every pass, whatever else the sketch decides.\n if (running && millis() - startedAt >= MAX_RUN) {\n Serial.println(\"maximum run time reached\");\n stopPump();\n }\n\n \/\/ A simple ten-on, ten-off test cycle.\n unsigned long phase = (millis() \/ 10000UL) % 2;\n if (phase == 0) {\n startPump();\n } else {\n stopPump();\n }\n}<\/code><\/pre>\nThe maximum run time is checked separately from the thing that decides to pump.<\/strong> That is the whole point of its placement: a bug in the watering logic, a sensor that sticks, a threshold that never clears \u2014 none of them can run the pump for longer than thirty seconds, because the limit does not consult them.<\/p>\nFor anything unattended, this is the difference between a failed sensor meaning a dry plant and a failed sensor meaning a flooded room.<\/p>\n
Measuring what it actually delivers<\/h3>\n
Two minutes, and it replaces guesswork with a number.<\/p>\n
\/\/ Siratek: run for exactly ten seconds, then stop.\n\/\/ Catch the output in a measuring jug and multiply by 360 for litres per hour.\n\nconst int pumpPin = 7;\n\n\/\/ Relay polarity: see Switching Bigger Loads with a Relay.\nconst int PUMP_ON = LOW;\nconst int PUMP_OFF = HIGH;\n\nvoid setup() {\n digitalWrite(pumpPin, PUMP_OFF);\n pinMode(pumpPin, OUTPUT);\n Serial.begin(9600);\n\n Serial.println(\"Siratek: ten seconds, starting now.\");\n digitalWrite(pumpPin, PUMP_ON);\n delay(10000);\n digitalWrite(pumpPin, PUMP_OFF);\n Serial.println(\"done. Measure what came out.\");\n}\n\nvoid loop() {\n}<\/code><\/pre>\nDo it at the height the project will actually lift to, with the tube the project will actually use. A narrow tube and a metre of lift can easily halve the figure on the label.<\/p>\n
From that number, a watering routine becomes arithmetic: if you measure 400 ml in ten seconds and a plant wants 200 ml, the answer is five seconds.<\/p>\n
Expected result<\/h2>\n
Water moving, quietly. A brushless pump is much quieter than a brushed motor, so a pump that is running can be hard to hear: watch the outlet rather than listening.<\/p>\n
Nothing should get warm. A pump that warms up is either running dry or working against a blocked outlet.<\/p>\n
Troubleshooting<\/h2>\n
\n\n\nSymptom<\/th>\n Likely cause<\/th>\n Check or fix<\/th>\n<\/tr>\n<\/thead>\n \n\nNothing happens<\/td>\n Relay not clicking<\/td>\n Prove the control side first, with no load<\/td>\n<\/tr>\n \nRelay clicks, pump does not run<\/td>\n Load side wiring, or the 12 V supply<\/td>\n Measure 12 V at the pump\u2019s wires<\/td>\n<\/tr>\n \nPump runs weakly<\/td>\n Supply sagging under 350 mA<\/td>\n A supply rated for at least 500 mA<\/td>\n<\/tr>\n \nRuns but moves no water<\/td>\n Air lock, or inlet above the water<\/td>\n Submerge it fully; tilt to release air<\/td>\n<\/tr>\n \nMuch less flow than 240 L\/h<\/td>\n Lift and tube resistance<\/td>\n Expected; measure your own<\/td>\n<\/tr>\n \nPump hot<\/td>\n Running dry, or outlet blocked<\/td>\n Stop at once; it damages quickly<\/td>\n<\/tr>\n \nStops after a while, then works again<\/td>\n Thermal protection, if fitted<\/td>\n Find out why it got hot<\/td>\n<\/tr>\n \nRelay contacts pitted early<\/td>\n No flyback diode<\/td>\n Fit one across the pump<\/td>\n<\/tr>\n \nArduino resets when it starts<\/td>\n Shared supply<\/td>\n Separate 12 V, grounds joined<\/td>\n<\/tr>\n \nFlooded the bench<\/td>\n No run-time limit<\/td>\n That limit is not optional<\/td>\n<\/tr>\n \nNoisy running, no flow<\/td>\n Impeller fouled<\/td>\n Rinse it; keep debris out of the tank<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\nNext steps<\/h2>\n\n- Easier first:<\/strong> Switching Bigger Loads with a Relay<\/em>, which this page depends on.<\/li>\n
- Related products:<\/strong> the soil moisture sensor, to decide when to water, and the water level sensor, to know the tank is not empty.<\/li>\n
- Next project:<\/strong> an automatic plant watering system, which is this pump, a moisture reading and a run-time limit.<\/li>\n<\/ul>\n
Sources and attribution<\/h2>\n
All text, wiring guidance and code in this tutorial are original Siratek work.<\/p>\n
\n- The model number
QR30E<\/code>, the 12 V 4.2 W supply, the 300 cm maximum head, the 240 L\/h maximum flow and the 60 degree liquid temperature limit were read from the label in the Siratek product photographs for item 2808. The 350 mA figure is 4.2 W divided by 12 V.<\/li>\n- Relay switching, the startup behaviour and the safety scoping are as set out in Switching Bigger Loads with a Relay<\/em>.<\/li>\n<\/ul>\n
Flow depends on lift and tube diameter, so the figures on the label are maxima and this tutorial gives a procedure for measuring the real value. The warning about running dry is a property of how these pumps are cooled. No third-party tutorial text, diagrams, photographs or code has been reproduced here.<\/p>\n
<\/p>\n<\/body>","protected":false},"excerpt":{"rendered":"
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