Help me understand this simple circuit - Page 1
Forum section: electronics / beginners

author: Moron
author_rank: Newbie
timestamp: May 04, 2022, 11:33:21 pm
content: Hello, I bought a simple blinking LED circuit so I could learn more about electronics, but I can't figure how it works.First of all, it came without schematics so I had to make one myself. I don't know if it's 100% right, but it ended up like this:" alt="" class="bbc_img" />I removed the 5v and GND connections that go to each opamp to simplify it a bit.I get that the LEDs will light up when the base of the transistor receives enough current, but I can't figure out what's happening with those opamps.My assumptions so far (correct me if I'm wrong):1 - Opamp A will receive 2.5v on its noninverting input, while opamp B will receive on its inverting input.2 - Opamp A will try to make its inverting input the same voltage, so it will output enough voltage for that while at the same time charging that 47u capacitor. As the capacitor charges, more voltage will be needed.3 - As opamp A outputs more voltage, opamp B will receive more voltage on its + input. The lower the resistance on that variable resistor is, the faster it will happen.4 - When B has enough voltage on +, it will stop sinking current and will start to output it.I can't understand what happens from step 4. Will not that capacitor be reversely polarized? On Falstad simulations, A is also going to be inverted and no current will go to the base of that transistor, shutting it off completely. Why does that happen?Can somebody help me? I'm very new to electronics and I'm also not very bright. I would be grateful if you could do an ELI5 on what I'm doing wrong or what sources I need to learn.English is also not my mother tongue, sorry for any mistake.I attached some images of the pcb.

author: ledtester
author_rank: Super Contributor
timestamp: May 04, 2022, 11:54:36 pm
content: fwiw, it's a breathing led circuit -- you can see it in action at 22:00 here:LED heart kit build and test (#94)https://youtu.be/vTo5wj_7V3Y?t=22mThere's also a schematic on this aliexpress page (also attached below):https://www.aliexpress.com/item/2251832654823830.htmlSecond schematic found athttps://www.electronicspoint.com/forums/threads/lm358-breathe-light.271835/Here's an explanation:https://www.electronicspoint.com/forums/threads/lm358-breathe-light.271835/#post-1635319

author: ledtester
author_rank: Super Contributor
timestamp: May 05, 2022, 01:06:01 am
content: Here's an an ltspice simulation which you can play around with:The period is approx. 1 second. Two things that will help in analyzing the circuit:- the output of U1B is either Vcc or GND- the inverting input of U1A is always Vcc/2Update: It's just the standard triangle wave generator applied to the base of the transistor. Here's a Falstad simulation:https://www.falstad.com/circuit/e-triangle.html

author: Moron
author_rank: Newbie
timestamp: May 05, 2022, 08:11:37 pm
content: Thank you!I spent this afternoon playing with your simulation and I finally understood it. It's indeed a triangle wave generator going to the base of that transistor. I never saw one before, so watched several videos to learn how it works.I'm going to finally solder everything and give it a try.