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I'm planning to build an ebb-and-flow hydroponic system (nope, not for weed) and as my electronics skills are close to zero, I'm trying to put things together before soldering anything. I'm a decent software developer, so coding is not the issue here. I know it's ambitious, but well... I want to learn :)

I'm facing power problems and I'm not sure about my solution.

My set-up includes :
- a barebone arduino (5v, 70mA)
- a submersible pump (12v, 3W, 250mA), controlled by an arduino pin and a mosfet
- a RTC DS1307 (2mA)

I made these working together, using USB for the arduino, and a 12v battery for the pump. To achieve a definitive set-up, I plan to use a laptop charger (15v, around 4A) and to split its output to 2 home-made lm2576 circuits (12v and 5v, both rated for an overkill 3A).

Next step would be power LEDs... and I'm getting even less confident about it. These LEDs look like what I need : 3W, warm white (forward voltage : 3.5-4.5v max, 750mA max).

I know LEDs need constant current, and among many others, this circuit seems simple and efficient enough.

If I understand everything, 1 LED would require 4V, 2 LEDs in series : 8V, and so on... Current must stay at 750mA (max). I plan to use only one LED as I don't think my basil will require more, so I'll stick to 4V.

Here are all my search results, but I still don't know if these circuits will work together :

15v laptop charger
    \ lm2576 (12v) --> pump
    \ lm2576 (5v) --> ICs
                              \-> constant-current supply (0.5V dropout) --> 1 LED (4.5V max)

Arduino will turn the LED on and off through (I guess) another mosfet, just as the pump.

So there I am, humbly asking for a educated eye on these questions...

Thanks in advance,
D

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  • \$\begingroup\$ Good job on the question. All looks OK to me except I am not clear on how you plan to drive the LED. The circuit you linked to is a linear circuit, so it will draw full LED current from the power supply, and dissipate any extra Voltage in a wasteful, heat producing way. So you can use it if you connect it to your 5V LM2576 regulator, but don't connect it to your laptop charger. Also, there is a chance that the pump will need more than 3A, briefly, during startup. Try it and see what happens. Make sure it is reliable before you leave it unattended. \$\endgroup\$ – mkeith Sep 8 '15 at 22:16
  • \$\begingroup\$ Why not get a 12v supply from the start and skip one regulator? \$\endgroup\$ – Passerby Sep 8 '15 at 22:51
  • \$\begingroup\$ Thanks @mkeith for your fast answer ! My schema isn't very clear (space-padding is disturbing), but yes, I planned to connect the LED driver to the 5v regulator :) It's good to know my guesses were not hat wrong... And thanks again for the warning about my pump ! \$\endgroup\$ – djezzz Sep 8 '15 at 23:03
  • \$\begingroup\$ @Passerby : it's the only one I have :) \$\endgroup\$ – djezzz Sep 8 '15 at 23:04
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Continuing on from mkeith here, if you want to switch the linear LED regulating ciruit on and off, I'd use a second N-channel mosfet. The 'ground' end of the constant-current circuit should attach to the drain of the mosfet, while the source of the mosfet should attach to the actual ground. Run the gate to the arduino and hey presto.

The whole LED regulator circuit with the switch will probably need about 1.2V or greater to work nicely. Add the LED in and you might have anything up to 5.7V, which is too much for the 5v LM2576. I'd make a crude 1.4V reference using a resistor feeding current into a couple of series diodes, with a cap in parallel with the diodes. Attach the ground pin of the LM2576 to the 1.4V reference and it should regulate around 6.4V, which gives you a bit of margin to work with. See how much heat the whole thing produces and heatsink accordingly- you might get away with only a little.

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  • \$\begingroup\$ Hi @Jefferson Allan, and thanks for the update. The linear circuit designer has an option to add an uC to the circuit : instructables.com/id/Circuits-for-using-High-Power-LED-s/step8/…. By the way, I don't know how you (or him) can tell the circuit voltage dropout : you say 1.2, he says 0.6... is there a component specification I should focus on to understand ? \$\endgroup\$ – djezzz Sep 24 '15 at 9:11

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