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I'm currently working with Raspberry Pis that are going to be powered through an LED night light (it is for being able to observe people working on experiments and giving light to the camera.) The light itself it uses a 7 volt supply - there is no LED in the light but instead using the power that would supply the LED to power the Pi.

The Raspberry Pi uses 5 volts and in the light itself there is only a positive and negative wire. I really don't want to open up the power supply itself though.

I have looked into using a voltage regulator but the problem I find is it needs positive, negative and earth.

Is there anything I can use to bring down the voltage down to 5V without an earth?

Current circuit - just the positive and negative cables going to the pi with from the power supply (no earth available)

Current circuit - just the positive and negative cables going to the pi with from the power supply (no earth available)

This is the lamp that we intend to use. Inside the lamp there is only a positive and negative wire. There is no earth.

Here is the power supply we are planning to use: Power supply

Edit We have decided to go for official Raspberry Pi switches.

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    \$\begingroup\$ A voltage regulator definitely requires only two input pins, generally called + and GND. To reduce 7V to 5V, you will need an LDO, a regulator with a low drop voltage. \$\endgroup\$ Aug 20, 2018 at 19:57
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    \$\begingroup\$ @Osian - You need to edit your question and add clear diagrams of your plan & photos of the various parts, because your statement that the "raspberry pi's [...] are going to be powered through an LED night light" is a problem. Either you mean something else, or if you really do mean that, then (as far as I can tell) the plan itself is misguided. If you show all the various parts that you have, and explain clearly what you are trying to achieve, it will be easier for readers to give accurate replies. \$\endgroup\$
    – SamGibson
    Aug 20, 2018 at 20:20
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    \$\begingroup\$ @Osian - Thanks for updating the question. From the diagram, it seems that you are not powering the Raspberry Pi's through (i.e. in series with) the LED lights, as implied at the start of the question. Instead, the diagram shows the RPi in parallel with the light's power supply. So either the diagram is wrong, or the question text is ambiguous (at best) in the use of the word "through" . It would help if you can fix that contradiction between those two points. You're already getting help and I don't have time right now to write an additional answer, so good luck! \$\endgroup\$
    – SamGibson
    Aug 20, 2018 at 21:24
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    \$\begingroup\$ Ah, the light uses a separate isolated power supply. In that case I retract my previous comment about danger. However, the supply is still unlikely to provide enough power for the LED and the Pi. Look up Raspberry Pi power requirements, and compare them to the power supply. \$\endgroup\$
    – marcelm
    Aug 21, 2018 at 9:44
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    \$\begingroup\$ Hello, Thank you for your guidance - we decided to purchase official power supplies. Thanks \$\endgroup\$
    – Osian
    Aug 21, 2018 at 15:57

2 Answers 2

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You can use a resistor or resistor divider, the problem with this method is the voltage will not be regulated and only works with constant loads. A raspberry pi is a variable load: a resistor or resistor divider won't work.

A voltage regulator is one of the better ways to regulate voltage, so use that. Make sure you read the datasheet and follow the rules on compensation with capacitors. Most regulators need an input or output capacitor (or both). The LED power supply output should be constant (and regulated), if the LED power supply is unregulated it might not work. Also keep in mind that a rasberry pi (for previous generations to v3 500mA will work, a 4 will need more current) needs 500mA of current or more, the LED power supply might not be able to source that much current.

If you don't want to waste power, get a 7805 Drop in compatible DC/DC converter.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Hey @laptop2d, Thanks for your help. I will check tomorrow to see whether the light does give out enough amps as I completely forgot about that. Just to confirm that the ground leg just needs to be placed to the negative returning to the power supply? \$\endgroup\$
    – Osian
    Aug 20, 2018 at 20:49
  • \$\begingroup\$ Yes. A 10 Ohm resistor should be equivalent to the Raspberry Pi if you want to check the supply. \$\endgroup\$
    – Voltage Spike
    Aug 20, 2018 at 20:54
  • \$\begingroup\$ meta.stackexchange.com/questions/126180/… \$\endgroup\$
    – Voltage Spike
    Aug 20, 2018 at 20:54
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    \$\begingroup\$ An 7805 is not a good idea for a 7V input. \$\endgroup\$ Aug 20, 2018 at 21:21
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    \$\begingroup\$ @Wouter I would probably recommend a 7805 compatible DC DC converter. But it sounds like the OP has a 7805 so why not let them tinker with that? It's also not a huge deal since the Rpi has another switching regulator for the 3.3V. You do waste power however \$\endgroup\$
    – Voltage Spike
    Aug 20, 2018 at 21:39
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Using a buck or SEPIC (Buck/Boost) converter would be the best solution if you must use your existing power supply. Check the power requirements on your Pi, the 7805 cannot supply that much power. The preferred solution would be to use a larger power supply that will give you more current. The SEPIC or Buck converter would still be needed. I do not know the lamp design so you will need to stay at 7 Volts. The converter can be gotten for less than the parts you are talking about.

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