Recommendation for 12v Power source for a Project


I am working on a project that requires the use of 12V source to operate a closet door latch:

Now it needs to be on battery power since I won’t be able to have access to a wall outlet.

My current investigation goes to the tone of using a voltage regulator to down convert the 12V to 5V for the logic. I still need to check how to not draw current while the photon is asleep, but step by step.

Before the voltage regulator I will use some filtering and then a relay to latch open the the door lock.

My question is… What kind of power source would you recommend? Since it’s 12V and the current battery shields don’t offer such recharging abilities nor fuel gauge monitoring, I am leaning towards AA or D batteries in series, but using 8 seems a bit too much! And I don’t thing they will alast long considering the mechanism draws 600ma when activated and the Photon has its own power consumption. I only plan to use wiFi for reporting when a door opens. So my plan is to use BLE to open the latch while WiFi Sleeps. And when the door is open connect, send an event and then shutdown again until another “open” event exists.

Any ideas welcomed! I tried searching for 5V latches but was unsuccessful!

If you’re in the range of Bluetooth LE, you might want to consider Bluz as well. It uses so little power it can run of a coin cell battery for months, and is compatible with the Particle ecosystem. You can Particle.publish right from it, through the Bluz gateway to the Particle cloud. And you can program it with the same tools you use for Particle devices.

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@frlobo, do you need the door to stay locked if you lose power or does it need to unlatch?

It should stay locked in absense of power.

Only need high power consumption when door opens… so I think 3 seconds…

@rickkas7 that’s a good option. However I can’t run on a coin cell cause I need the power to move the latch. I am in store for either a larger 12v battery that I can monitor or charge easily or external AA batteries that I can do so from the outside and replace easily. It would be nice to have monitoring capabilities but that’s priority 2

@frlobo This place carries a lot of good power converters. I’m sure there are some that would fit your application that also have an Enable/Disable pin which allows you to power down the regulator when it’s not being used.

May take a little modification of your setup, since it doesn’t have an angle on the plunger, but would this work?

@frlobo, here are some things to consider:

  • A 2S or 3S LiPo battery will produce 8.4v and 12.6v respectively (fully charged). I suggest using a 5000mA/h 2S rechargeable LiPo battery and charger which are easily available as combos on Amazon. These will provide plenty of power for both the latch and the processors.
  • To use the 2S battery (8.4V), you will need a 6V latch
  • The latch can be powered with (as @RWB kindly linked) a 6v buck regulator. You can control the latch power via the enable pin on the regulator.
  • Another buck regulator can be used for the Bluz/other BLE/RedBear Duo/Photon power.

For the door control, you could use a Bluz for the BLE and fire off an interrupt to wake a sleeping Photon. Or you could use a RedBear Duo to do both WiFi and BLE. Or you can forget the reporting and use just a Bluz or other BLE device. Or you can use a MFRC522 RFID scanner to wake a Photon which controls the power to the latch and sends a publish. Or, you could… :wink:


I saw this. But it is not ideal for my setup… and I still need to have a high current source available. But thanks for the info.

Thanks all for the treasure trove of info.


So what you propose is that I control de latch using the buck regulator.

if I decide to go with the 12v setup, I will need 2 buck regulators, one for the MCU and the other for controlling the latch thru the enable pin.

If i decide to use the 8v setup, I will need one buck regulator for both the MCU and the latch. But in this setup I will use a relay of some sort to control the latch? Or am I missing something?

I like the idea of the Bluz as an access control and the Photon as the Master MCU. Where the Bluz, wakeup the photon from deep sleep and the photon then opens the latch and reports. Or only reports and the bluz opens the latch. I will investigate further
your ideas!


This is a simplified quick drawing to see what you think @peekay123

@frlobo, powering both the photon and the solenoid from the same regulator is not my favourite approach, primarily due to the large inductive load when switching. If you stick with this design, make sure to add a large (100uf) and small (0.1uf) caps near the photon’s Vin pin to decouple noise and switching. You could also use a FET instead of an opto-coupler and transistor but that is entirely up to you.

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You could use a INA219 to measure the battery voltage levels.

I think the INA219 would be more energy efficient than a resistor divider setup but I’m not sure about that.

I used a FET in the past to power a 40 Amp 12v relay that consumed about .250 amps @ 12 v and it worked just fine. Here is a good link showing how to do it:

INA219 datasheet says it typically takes 6uA in powerdown mode (p4).

@RWB @chrisb2 Great Resources Thanks!

@peekay123 What do you think about this setup?

PD: Ignore the 1k value. i haven’t checked what value of R i need.

Question 1:
Doesn’t the step down transformer have the 100uf and 0.1uf filters already?

Question 2:
How much current will the Buck regulator consume from the battery? Will this be something to consider? I am still debating between lipos and AA batteries (x 8) for simplicity. But I need to consider charging intervals.

To keep the battery simple you could just use a single cell LiPo battery and then use this boost converter to take the 3.7v battery voltage to 12v @ 600mA for a short period while the latch opens. That is approx 2.4 amps at 3v and that would be no problem for some of these larger LiPo cells, plus they have hi discharge rate cell that are better for high discharge applications.

The less battery cells the better in my opinion. You can then just use one of the single cell USB charging circuits for a simple drop in charger.