SparkFun Forums

[teekay_tk]

hi
well i have an application where a very tiny pic based circuit is being used.
it is in sleep mode all day using less than 10uA. but once in a day it wakes up for a minute, uses 20mA(max) for the whole minute and then goes to sleep for another 24 hours.

as these are disposible, i am thinking of using a small solar cell and charging a capacitor. within the 24 hours. then using the capacitors charge to run the circuit for that minute. thus using no battery at all and getting much longer life than i would get from a button cell. overall size is very restricted. is that possible. can i have a circuit example for the power side of it. and also, how can i calculate the charge capacity for the capacitor for this applicaiton.

thanks

[silic0re]

http://library.solarbotics.net/circuits/se.html

[npk]

Maybe, you gave no criteria for success, so i will just babble. Anyhow, Assuming that the pic runs @ 3.3V

The sparkfun supercap (there are bigger caps out there, but they're even more expensive) can hold the energy:

E_capacitor [Joule] = 1/2 * C [Farad] * Volt^2
= 1/2 * 10F * 2.5V^2
~ 30 J
Your pic's usage is Power * Time. Power is Voltage * Current.

E_used [Joule ] = 3.3V * 20mA * 1 minute
~ 4 J
Meaning you have enough juice in the capacitor for a few minutes.

Question: Do you need a step up switching circuit? Or, how will the voltage change in 1 Minute:

V(t) = V_initial * exp(-t/RC)

The pic `resistance' is 3.3V/20mA = 165 Ohm. And RC is the RC time (Ohm * Farad is second.) The RC time tells you how long it takes for the cap to fall to 1/e of
V(t) = 3.3V exp(-t/27.5 minute)
V(1minute) = 3.18V

So you're driving the PIC just fine. I think PICs stop working @ 2.5V, so you can run for about 4 minutes with just the capacitor.

-------

A battery is better. Assume you can get a lithium 3V @ .25 A-hour. That means you can have 3*.25/(3.3*0.02)=11.3 hours of your high power mode or 3.*.25/(3.3*10e-6) which dominates the 23,000 hour (958 day) you would get in low power mode. So you could run for 678 days on one watch battery.

[teekay_tk]

ahann
thanks for the maths, i am always afriad to get into it. i was looking for a node that would "run for ever". but you are right 650 days is pretty much. i found these tiny hearing-aid battries that boost 650mAh standing 5.5mm high and 11mm diameter. with two of these to get 3V, and pic tunning down to 2.5 V, I may be looking for another years run.

thanks for you help..again. also the solar engine circuits were very interesting. i may build one just for fun.

[MilanMerhar]

You need to remember that Solar Cells are diodes; in particular, when charging a battery or SuperCap, they're diodes wired in the direction that DISCHARGES the battery in the dark. It's only the energy from the light that drives current in the charging direction.

So, to eliminate this "dark current discharge", you need to put a diode in series with the solar cell to pass charging current and block discharge current. To minimize voltage drop, use a low-forward-voltage Shottky diode or even a Germanium diode like an 1N34A or 1N270.

Getting back to your hypothetical design, you'd probably need an eight-cell panel outputting around 3.6 volts (open circuit, bright light) to overcome the diode drop and get a SuperCap charged to around 3.3 volts. That will allow you 0.5 or 0.6 volts drop during discharge before your circuit gets into trouble.

[teekay_tk]

thanks

i'll keep this in mind for my first experiments with solar cells tomorow


I have finally bought the flexible from robot store