Small ATtiny 45 circuit supplied by solar cell.

Hi all,

i would like to power a small circuit involving an ATtiny45, a temperature probe and an OLED Display with a small solar cell.
The circuit only needs to be switched on momentarily by pressing a button to read the temperature.

The Idea was to have a solar cell keeping a supercap topped off constantly, by pressing the button the circuit is powered and by letting go power is cut off.

The target voltage is 5V and current draw while powered will be < 50mA

Do you guys know of any similar circuits or examples?
My problem lies in figuring out the complete chain (Solar → Boost converter? → Cap → ??? → circuit).

basically I need a 5V solar powered supercap battery.

Thanks for any help :slight_smile:

Cheers,
ZR

I'd probably go with a rechargeable battery.

You'd have to either measure or calculate/estimate the current and the amount running-time needed, or you might just have to experiment to see how long the capacitor "holds". [u]This capacitor[/u] should just fine, but I'm not so sure about [u]this one[/u]. (The small one has a recommended discharge rate of 1mA or less.)

My problem lies in figuring out the complete chain (Solar -> Boost converter? -> Cap -> ??? -> circuit).

You shouldn't need a boost converter as long as your solar panel puts-out 5V or more.

Capacitors discharge exponentially, which means you loose a LOT of voltage very-quickly. For the best use of the capacitor, Solar -> Capacitor -> Regulator -> Circuit. But with this arrangement with the capacitor (or battery) before the regulator, the capacitor (or battery) has to handle the full solar voltage. The greater the voltage, the more the capacitor (or battery) can discharge before falling to the point where your circuit won't run.

If you can arrange the solar cell voltage greater than the supercap, and the supercap greater
than the circuit its powering, add (linear) voltage regulators at each step (or even just
resistor/zener divider as the current is quite low). You need to protect the supercap from
over-voltage, hence the first zener or regulator, and the second gives controlled voltage
to the load. The second can be a DC-DC converter of course as mentioned, so boost is
possible (most supercaps are 2.7V or so).

Use a schottky diode on the output of the solar cell to prevent back-powering it when the
sun goes behind a cloud.

A zener shunt regulator circuit isn't the best (its cheap) since it will have to dissipate
power constantly and has to be rated appropriately.

Thanks guys for the replies :slight_smile:

For now i've ordered a small 3V 35mA Solar Cell and a boost converter 1-5V -> 5V. I will try to set up a circuit with a Supercap (probably not the one for $ 100) and try it out. The fallback solution will be to use a standard non rechargable AAA Battery.

The boost converter will be in between the cap(s) and my circuit.

The circuit will be ON only for about 1-5 seconds at a time, with hours of inbetween time in a well lit room, so the solar thing might just work out.

Generally , Solar Cells and boost converters dont work well, as the Solar cell wont be able to deliver enough current to start the boost converter.
Boost converters draw far more current at startup than when running.
You are much better off using the Solar Cell to charge a rechargeable battery, which then powers the Micro.

Another way to go is get a 5v solar cell, about 200 mah, and a USB lipo charger module off eBay, and let this charge a 500 mah lipo battery, with a Schottky diode in line with the positive of the solar cell to prevent feedback, and maybe just use a LM335 - TO 92 package temp sensor that uses very little current, as your temp sensor...

Simple but effective, just buy a decent solar cell, as the smaller ones are made from the ends of the sheets and have at best a 80% efficiency.. this all depends on how much sun you have, as the charging only starts around 4,2v even though the module specs 4,5v...

I am using this system to power a remote Adafruit trinket board, and works well... so far..

Let me know if you need more info or details, as i am currently working on the ATTiny 85 ATM and trying to rationalize a good system for remote powered units comprising of these chips..