DIY solar powered low power soap dispenser

I’m looking to make a DIY solar powered soap dispenser.

The logical steps would be as follows:

  1. Solar panel charges supercapacitor via ADP5090 chip
  2. Proximity sensor wakes up from sleep mode and is activated
  3. MCU turns on and sends a command to the motor to turn on
  4. Motor or motor pump is turned on and dispenses soap
  5. Motor turns off
  6. MCU goes back into deep sleep
  7. ADP5090 recharges supercapacitor if need be

I found these awesome solar to dc regulator reference designs on digikey’s site:

This is what I’m particularly interested in:
ADP 5090 solar-to-dc-regulator
page 6-8 in the attachment below contains schematic and BOM for parts.

The solar panel and capacitor they were referring to in the BOM datasheet:

ADP5090Data SheetRev. C | Page 18 of 21 TYPICAL APPLICATION CIRCUITSFigure 33. ADP5090 Based Energy Harvester Wireless Sensor Application with PV Cell as the Harvesting Energy Source (Trony 0.7 V, 60 μA, Alta Devices 0.72 V, 42 μA, Gcell 1.1 V, 100 μA), Shoei Electronics Polyacene Coin Type Capacitor PAS409HR as the Harvested Energy Storage, and Panasonic Primary Li-Ion Coin Cell CR2032 as the Backup Battery

I didn’t find the brand on Digikey but did find another they mentioned.
This brand claims 25% efficiency!!

This is the proximity sensor I was considering:
semtech-corporation SX9310ICSTRT proximity sensor
SX9310 datasheet
Ultra Low Power Consumption
Active Mode: 70 uA
Doze Mode: 8 uA
Sleep Mode: 2.5 uA
2.7 – 5.5V Core Supply (VDD)
1.65 – 2V Host Interface Supply (SVDD)
1.65 – 5.5V Compliant Host Interface (VPULL)

These are the relatively low cost motors I’m considering to use:
Rated voltage: DC 3V
No-load current: 20mA

DC Voltage: 3-6V (Title claims it can work at 2.5V)
Working current: 130-220mA

I want to use the lowest power MCU possible.
Right now I’m consider the same MCU from the Nano 33 IoT
It seems to have a low micro current usage per Mhz and low current usage in sleep mode.

Given that the motors have a near 3 volt operating range.
Would it be better to use the following components for the ADP5090 chip that are rated at or near 3.3 volts to minimize power losses and efficiency losses?:

Solar panel, powered by indoor bathroom lights, or if it’s daytime then light coming in thru windows
Current @ Pmpp 55.1mA
Voltage @ Pmpp 3.35V

Current - Quiescent (Iq) 50nA

or (depending on current usage of motor to get it to start may need larger current output)

Linear Voltage Regulator IC 1 Output 500mA 6-TDFN (1.2x1.2)
Current - Quiescent (Iq) 55µA

These capacitors look like a lot better power options than the ones mentioned for ADP5090:
3.3F, 2.7V

More expensive but less farads capacity but rated for 3.3V
330 mF, 3.3V

I’m not too familiar with capacitance leakage but I think I read about it somewhere.

Also found this site via the forum for potential knowledge about the subject:

I’ve attached the sensor and ADP5090 chip datasheets for reference in addition to the links provided above.

ADP5090-2-EVALZ_UG-782.pdf (452 KB)

SX9310_datasheet_Rev2_STD_200115.pdf (1.21 MB)

Try this capacitance calculator: Capacitor charge and discharge calculator | MustCalculate Try this calculator it will help with the super cap life etc. Battery pack calculator : Capacity, C-rating, ampere, charge and discharge run-time calculator of a battery or pack of batteries (energy storage) You will probably finish up using micro or nano amps for your calculations. This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!

Wouldn't it be simpler to use a rechargeable battery?

Or at least get the project working with a battery and then develop a variant that uses the super-capacitor.


It probably would be but I'm trying something new and getting rid of needing a battery.

Found this great site below.

The ADP5090 specs say
Voltage Out 3.5 V
Voltage In 80 mV ~ 3.3 V (from solar panel)

I basically made a spread sheet to do calculations offline from the url above.
These are the numbers I got:

V (volts) 2.7 applied voltage to capacitor
C (farads) 3.3 capacitance
R (ohms) 1.33 resistance
τ = (seconds) time constant

E = CV^2/2 (joules) 12.0285 energy stored in capacitor
τ=RC (seconds) 4.389 time constant

I wish the site link you gave had some formulas to explain how they got the results.
I was confused about what exactly is the from and to voltage.
I thought supply voltage would be from the solar panels but I ended up using the voltage the capactor is rated for that I mentioned in the first post the 3.3F, 2.7V capacitor.

Also for the from to voltage I assumed the circuit will discharge from 3.3V to 3V to supply power to motor and rest of circuit like the proximity sensor and MCU. I was generous with respect to motor pump time, but it probably could be less.

I think the output current based on the site seems sufficient for either of the motors I was planning on using. However, the original dev kit by Digikey seems to supply 3.5V and 800mA output.
The supercapacitor they are using is:

I was curious is an oscilloscope or true RMS multimeter a good way to measure the starting amperage of a dc motor? I thought as a rule of thumb the starting current is typically 3/4 times the nominal current of the motor. Correct?

In other news....the company Alta devices that made the single junction solar cells of 29.1% efficiency seems to be going out of business I think .... bummer :sob: :cry: :fearful:

The ATmel processors have very low power use, in the uA range when in deep sleep. Look for an ATtiny running on the internal clock.

For your project, I'd simply start building. Forget about the power part for now, first figure out how much you really need!

The hard part seems to be the proximity sensor. How is that going to wake up the processor? It's normally the other way around: processor wakes up (every 0.1-1 seconds or so, however fast you need it to react), does a measurement, goes back to sleep. That's going to take a relative large amount of power over time, though during the day you're likely to be able to keep your capacitor (or battery) charged fully. During the night, that's a different story of course.

When you have your sensor/controller combo working, measure the power it takes. Then you're in a position to actually think of how big a capacitor or battery you need.

So I found out that I will cause the 2.7V capacitor to break/leak it if is supplied with a greater voltage i.e. 3.3V.

At DC a capacitor has infinite impedance (open -circuit), at very high frequencies a capacitor has zero impedance (short-circuit). All capacitors have a maximum working voltage rating, its WV DC so select a capacitor with a rating at least 50% more than the supply voltage.

I attempted to start the dc geared motor rated at 3V at a lower voltage and it moved once and after that it doesn't move any more. I hope I haven't fried it.
I powered it via a variable votlage dc power supply. I may have gone to maybe to 3.1V but only briefly. I don't think this should have fried the motor though.

I also am able to power the dc pump with a minimum of 2.35v.
I slowly raised the amperage up to .16A and it started up. The power supply then read .12A as the nominal voltage running the motor.

2.35V* .16A = .376W = .376J/s

If I round up voltage to 2.5V then working voltage of capacitor based on quote above should be ~5V.

I found a 5V, 2.5F capacitor.
If I had a similar solar panel that the ADP5090 was using (it has a .08 V to 3.3V range and it's designed for indoor light 200 - 1000 lux)
Based on the calculator in the posts below.
It would charge up in 12 seconds based on a lets say low light situation of 0 -.25V.

These are the links to the calculations.

I think running the pump to dispense soap 2-3 seconds would be good enough. But I wanted to hold enough charge in the supercapacitor for 3-4 dispense rounds. I just always thought that once a capacitor is initiated to release its energy it has to release it all. I'm still unsure if it can be delivered in fractions of its rated capacity and different time intervals.

Here is a filtered list of 5V different farad capacity capacitors:

Before you start trying to calculate storage and loads.
You need to workout how much current it will take to depress the soap dispenser.
Not from the motor specs but from some good get your hands dirty experimentation.

Have you got a soap dispenser to try and work out the energy required to activate it?

Tom... :slight_smile:

Hi, I will utilize gravity to help with dispensing of soap, the pump will be located at the bottom of a large glass bottle and the pump will actually do the work to suck it out and dispense it. I guess the viscosity of the soap will play a role in the current used to dispense it out. The other thing will be prevention of excessive dripping soap like a runny nose.

Hi, I will utilize gravity to help with dispensing of soap, the pump will be located at the bottom of a large glass bottle and the pump will actually do the work to suck it out and dispense it. I guess the viscosity of the soap will play a role in the current used to dispense it out. The other thing will be prevention of excessive dripping soap like a runny nose.

Have you looked at "perstatic pumps"?

peristaltic pump - Buy peristaltic pump with free shipping |

Most are 12V however.

Tom... :slight_smile:

Thanks for the option.
I've heard of them but not sure what the advantage is.

This pump below provides the best cost option:
DC Voltage: 3-6V (Title claims it can work at 2.5V)
Working current: 130-220mA

That little pump, like most water pumps, will not pump soap. This are centrifugal pumps, based on spinning an impeller very fast, pushing the liquid out using the centrifugal force. That doesn't work in a viscous liquid like soap. The pump probably doesn't have the suction to pull in the soap, and the moment the chamber is filling with soap it won't have the torque to spin any more.

A peristaltic pump uses two or three rollers to push against a tube, rotating them to push the liquid through. You can dose the liquid very accurately by running the pump for a specific time, and the rollers prevent liquid from passing through the tube when the pump is not operating.

A quick search on Google learns me that automatic soap dispenses indeed use that kind of pump.

Great idea. I want to see your project in action. I found an interesting review about How Hot Do Solar Panels Get - Blog About Solar Energy | Websolarguide