### Author Topic: Best circuit to turn on circuit when there is enough solar power (no batteries)  (Read 820 times)

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#### gilligan

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##### Best circuit to turn on circuit when there is enough solar power (no batteries)
« on: April 30, 2019, 01:39:25 pm »
I'm looking to build a very small circuit that I want to only run during the day, it's not mission critical, and I want to run it solely off of a small solar panel.

I need to power it up only once I get like 3.5v and I want it to shut down at 3.3v.

I've thought of schmitt triggers but their reference would be moving with the solar energy defeating the purpose.

Then I thought about ZENER diodes and that seems to be plausible, but I do see a good bit of trickle through voltage at the reverse breaking point and worry that will cause problems.

Is there a better way to do this?

#### Freesurfer

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #1 on: April 30, 2019, 06:18:25 pm »
The problem you have there is that the voltage of the solar panel drops when you attach a load to it. Solar panels have a relatively constant open circuit voltage as long as they just get some light, so you cannot base it solely on voltage alone. I'd go for something current based. Like a fixed resistive load connected across the panel, that you measure the voltage across. Then when when you reach a treshold where you know your device will function, let it disconnect the resistive load and switch over to powering your device.

You can probably achieve that with a mix of what you are suggesting, including a zener AND a schmidt trigger. You want to use the zener to establish a stable reference voltage for the schmidt trigger, and the schmitt trigger itself to switch between your device and the resistive dummy load.

#### AndyC_772

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #2 on: April 30, 2019, 06:28:56 pm »
This is a fairly common requirement for which there are specialised ICs available. The key search term is "supervisor", eg. "microprocessor supervisor IC", or "power supply supervisor".

Here's a good starting page to begin your research:

http://www.ti.com/power-management/supervisor-reset-ic/overview.html

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#### mariush

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #3 on: April 30, 2019, 07:08:33 pm »
You could have a very low drop LDO with an ENABLE pin and configure the ENABLE to change on 3v.

Solar panel  -> big capacitor  (so that sudden power on of LDO won't bring the solar battery voltage down and kill ldo right away) -> ldo

Problem is most regulators turn on at some threshold, but turn off only when voltage goes down to some other threshold.

For example NCP114 turns on at 0.9v but shuts down at 0.4v : https://www.onsemi.com/pub/Collateral/NCP114-D.PDF
TCR2EF also turns on at 1v, but shuts down at 0.4v : https://www.digikey.com/product-detail/en/toshiba-semiconductor-and-storage/TCR2EF33LM-CT/TCR2EF33LM-CTCT-ND/4503328

Maybe it would work in combination with a schmitt trigger or two inverter gates in series

#### NiHaoMike

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #4 on: April 30, 2019, 09:41:47 pm »
Look up energy harvesting chips.
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#### daveatol

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #5 on: May 01, 2019, 12:47:53 am »
Have a look at the graph of solar cell voltage/current:

Each trace is for a different solar intensity. You'll notice that the open-circuit voltage doesn't change that much with light intensity, whereas the current changes a lot with changes in light intensity. This means you'd want to measure the voltage while the solar panel is loaded.

One way to do this is to use the supply monitoring IC (or even a TL431) and an inverter (or transistor) to switch between a load resistor when the supply voltage is too low, and the circuit to power when voltage is sufficient. If the load resistor uses more power than the circuit, then you'll get hysteresis as a free bonus.

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#### David Hess

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #6 on: May 01, 2019, 10:55:41 am »
I would use a shunt regulator and connect the load once the current through the shunt regulator is sufficient to power it.

#### gilligan

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #7 on: May 01, 2019, 02:13:51 pm »
Awesome info and will be reading up on each suggestion thoroughly!

One note... this circuit this will be driving will be VERY light load.  Arduino Mini Pro, with a 433mhz transmitter which will only broadcast once a day when a reed switch or something similar is changed.

So, low power the arduino and sample the digital pins every second to couple of seconds or setup an interrupt.  Once the digital pin state changes then transmit that fact and go back to monitoring pins (for sake of easy coding).

So the only thing really drawing any power will be that transmission which will likely happen in the middle of the day when the sun is it's strongest.

Hope that clears up some things.

#### Red Squirrel

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #8 on: May 01, 2019, 08:36:28 pm »
I would recommend at least adding a small battery, as it will help smooth out the highs and lows throughout short periods.  Ex: clouds rolling over.  A small gel cell connected to MPPT controller might even do the trick.  Then you can setup your load to work when the voltage is 13.0 or higher, and set the load to shut off when it's 12.3 or lower. (or something like that)

The Supervisor IC chips are a good idea too, I did not really know about those myself. I would normally do that stuff with Arduino but those chips are probably much lower power usage.   If you use relays latching relays are good to use too as they don't draw power except for when switching.

#### Mr.B

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #9 on: May 01, 2019, 10:07:26 pm »
I have done this type of thing numerous times:
Small LiPo battery – The size will depend on your requirements.
Solar Cell.
Single chip LiPo charger – I use CN3083.
Low quiescent current LDO regulator.
Set up an external interrupt to wake up the Arduino.
Put the Arduino into deep sleep.
When the interrupt is triggered, wake up, do stuff, go back to sleep.
Time is the overseer of all things.

#### NorthGuy

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #10 on: May 04, 2019, 02:33:37 am »
Arduino Mini Pro, with a 433mhz transmitter which will only broadcast once a day when a reed switch or something similar is changed.

Without a battery (and consequently without permanent clock), it'll be very hard to figure out what is "once a day".

#### MarkF

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #11 on: May 04, 2019, 03:07:28 am »
With this circuit from @deba168, the Arduino can monitor the voltage (A0) from the solar panel and turn it on (D3) when it is available.

« Last Edit: May 04, 2019, 03:10:01 am by MarkF »

#### daveatol

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #12 on: May 04, 2019, 03:20:32 am »
Based on the last update from the OP, there's not actually any need to switch on the "load" using a controller, as the load is the controller. It's simple just to have the solar cell connected to a large capacitor, powering the arduino via an LDO regulator, and the arduino using its brownout reset circuit to protect from under voltage. The reed switch is connected to the arduino input, and can use an interrupt from sleep (if desired) to decide to use the transmitter.

The capacitor needs to be sized appropriately to supply the transmitter for its single broadcast. A battery can replace the capacitor, if desired.

#### JimRemington

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #13 on: May 04, 2019, 04:13:30 am »
Nick Gammon posted a fun tutorial on using a small solar panel and supercap to power a bare bones Arduino and an NRF24L01 2.4 GHz radio. The advantage of using the panel to charge a supercap is that the voltage across the cap is a direct indicator of how much energy is stored.

https://www.gammon.com.au/forum/?id=12821

I reproduced it using a much smaller, calculator sized thin film solar panel (2 x BP-242221 Sunceram) and was quite pleased with the performance.

http://solarbotics.net/library/datasheets/sunceram.pdf

« Last Edit: May 04, 2019, 04:34:51 am by JimRemington »

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#### mvas

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##### Re: Best circuit to turn on circuit when there is enough solar power (no batteries)
« Reply #14 on: May 10, 2019, 05:25:21 pm »
Could you connect a Buck / Boost Converter ( \$5.00 Ebay ) between the Solar Panel and the Arduino?

Smf