pythagorean cup circuit? - Page 1
Forum section: electronics / beginners

author: lennyp
author_rank: Newbie
timestamp: September 08, 2019, 03:36:26 pm
content: Hi everyone!i want do do a little project for myself but pretty quicky hit a dead endi wanted to give new purpose to some parts i had lying around so my idea was to re-purpose an oldservo with a broken driver IC to wind up an old mechanical watch.to power this thing i wanted to use two solar panels from another unfinished project.however these solar panels are super weak an cannot run the motor on their own. to accomodate for thati wanted to use two supercapacitors in series (to match the voltage rating to the maximum output voltage of the solar panels).i was thinking to build a circuit that functions similar to a pythagorean cup (https://en.wikipedia.org/wiki/Pythagorean_cup)but i have absolutely no idea how(see attached schematic).let me elaborate:starting from 0V the switch is closed until the capacitors hit around 3V (the solar panels will generate around 3-3.5V at the place where i want to put the whole contraption).if the voltage rises above that level the switch must go into a closed state until either the capacitors hit 0V or preferably ~1V since the motor starts to go super slow around that mark.rinse, repeatto me, itfeelslike there should be some simple way accomplish this with diodes/resistors/transistors/capacitors only, but my knowledge of analog circuits is limitedany help/ideas would be greatly appreciated. however i would like to keep the complexity as low as possible and avoid buying exotic components

author: ledtester
author_rank: Super Contributor
timestamp: September 08, 2019, 04:51:17 pm
content: WHat you are trying to do reminds me of the "Symet" solar powered robot featured in the book "Junkbots, Bugbots & Bots on Wheels". It introduced the acronym "BEAM technology" which stands for Biology, Electronics, Aethetics, Mechanics.Here's the schematic of the Symet. The idea is to hold the charge in the capacitors until they reach a certain voltage and then dump the energy through the motor. As such it kinda stutters along one step at a time. In strong sunlight, however, you can achieve continuous motion.The book recommends a red flashing/blinking LED.And here's a video:https://youtu.be/Ib3iUIr__0UOther solar powered BEAM robots employed a 1381 voltage trigger, and this use was called the "Millersolar engine":http://solarbotics.net/library/circuits/se_t1_mse.htmlThe company Solarbotics still sells BEAM components and kits.

author: lennyp
author_rank: Newbie
timestamp: September 08, 2019, 06:09:17 pm
content: wow! this looks like it could work.i built this circuit right now replacing the transistors with bc547/bc556 and a 2.2k resistor and used a white LED.i am charging the capacitors using the solar cell while monitoring the voltage (very slow).charging the capacitors with a psu let the motor spin until ~0.9Vdo you know how to calculate the threshold voltage for turning on and provide some insight on how this circuit works in general?

author: ledtester
author_rank: Super Contributor
timestamp: September 08, 2019, 06:27:25 pm
content: Found this explanation:https://electronics.stackexchange.com/questions/158408/how-does-the-circuitry-work-in-the-symet-robotBasically after enough voltage is built up current runs through the flashing LED which turns on the transistors allowing the caps to discharge through the motor. To increase the trigger voltage you might just try adding another flashing led in series.Apparently the circuit using the 1381 voltage trigger is more efficient:http://solarbotics.net/library/circuits/se_t1_mse.html

author: lennyp
author_rank: Newbie
timestamp: September 08, 2019, 06:37:59 pm
content: update:when charging the capacitors to 4,5V and the plugging them in the LED is constantly on and very bright.at ~3V the LED turns off and then blinks dimmly a couple of times between 0.98V and 0.9V. the motor isspinning the whole time.i find this unexpected.i will read up on the links you've posted. thanks for the help!the solarbotics stuff in general looks really interesting!i will experiment a bit with this circuit and will keep you updated in this thread.

author: ledtester
author_rank: Super Contributor
timestamp: September 08, 2019, 07:12:21 pm
content: The book recommends covering the leds as their operation is affected by light.

author: lennyp
author_rank: Newbie
timestamp: September 08, 2019, 07:50:22 pm
content: by replacing my supercaps with a smaller one the circuit is working as expected (to cut the waiting time).i do not have a FLED so the LED is constantly on (duh).the circuit operates between 0.85V and 3V.thanks for your help @ledtesterthese BEAM robots are intriguing. i think i will try some of these circuits and will probably writean update if i ever settle on a design for my watch-winder