AEMLOVE revision 5

A project log for Designing a solar harvesting blinking gadget

Based on earlier solar harvesting projects I wanted to design a solar harvesting gadget

Jasper SikkenJasper Sikken 06/04/2019 at 09:480 Comments

In previous AEMLOVE designs I used the STATUS2 output from the AEM10941 to flash a LED which is high for ~80ms every 5 seconds. I think 80ms is actually pretty long and it can be shorter while keeping brightness perception. I learned about 1.5V powered LED flash circuits that have very low average current (~10uA) and wanted to make a similar circuit. 

Normally the 1.5V is not enough to light the LED because red led forward voltage is at least 1.7V. So how does it work? First the capacitor is charged to 1.5V through the two resistors. On the left it is 1.5V and on the right 0V.  When the mosfet is closed the left side suddenly changes from 1.5V to 0V the right side remains 1.5V lower than than left, and so it is -1.5V Then shortly there is 3V over the LED and it flashes until the capacitor has discharged. 

And since the LED can be powered from a very low voltage that is basically doubled it was possible to try out LEDs with higher forward voltage, like a white LED. Since the AEM10941 has two regulated output voltages 1.8V and 1.2V I could design the circuit twice and experiment with it.

I read here that the eye perceives 10ms flashes at full brightness and after that it decays to 50% in 20ms. This means for a 10ms flash you actually perceive 30ms. This could really reduce average current

In revision 5 I made following design changes

Below is the circuit diagram

What I've learned from Revision 5

I was disappointed by the LED brightness and life time. So I made a simulation in LTSpice. Not perfect but it gives me an idea.

And I found that peak current is actually really high and the duration very low, something like 5ms. 

That basically explains why it was not perceived as very bright. In addition it was not low power. So I decided this is not the circuit for me.