New circuit for 3W led emergency light (my first project) - Page 1
Forum section: electronics / projects

author: mijuc
author_rank: Newbie
timestamp: March 19, 2023, 03:43:54 pm
content: Hi guys.I am new to the world of electronics and pcb design. I'm curious what you guys think about this circuit and if it would work. I want to adapt the old LED emergency lights, which worked on lead acid batteries, to lithium ion batteries (18650). It would use a single 18650 cell with a bms.I would not change the cases, and on them I have the option of selecting the operating mode and the charging indicator, as well as the test button, which I would not use. As for the modes of operation, I chose to have 3 modes.1. Low luminous (lower current)2. Normal mode for emergency lights3. High luminous (higher current)The case is of the closed type, so I worked with a regular capacitor dropper, because it is the cheapest. Which will convert my AC input (220V) to 7.5V. The circuit with LEDs has a voltage drop of 5V (I want to use them at 100mA, i.e. when their voltage is 6V and at 300mA, i.e. at a voltage of 7V). For the led driver, I used a PT4115 chip that will control my current (it's easier for me to use 2 resistors for current control than another 555 circuit to dictate PWM on the dim pin). Due to insufficient voltage from one 18650 cell, I had to make a boost converter with MT3608 chip. I know that I should test the circuit on a breadboard first, but I don't have all the components, and I don't have anywhere to buy them because I live in Serbia.. and it's not worth it for me to wait for them from China, because the delivery will take a month or 2. So I'm interested in yours opinion, where I went wrong and better options for this circuit. I've seen people rebuild circuits from old lamps but I didn't like the concepts they were working on.

author: Benta
author_rank: Super Contributor
timestamp: March 19, 2023, 08:51:34 pm
content: Sorry, don't understand your circuit. You have connectors that have no relation to each other, two "S" inputs not connected to any output etc.Draw a proper schematic.

author: jonpaul
author_rank: Super Contributor
timestamp: March 19, 2023, 09:07:50 pm
content: capacité dropper,dangerous, unreliable.Off,the shelf, modules,easier, need,a,transformer.But the Duracell emergency plug-ins are perhaps $15..20. for a,pack of,3..4,pcsJon

author: mariush
author_rank: Super Contributor
timestamp: March 19, 2023, 09:32:40 pm
content: Decide how you want to arrange the leds first. Being an emergency light, you may want to stay away from long series of leds, because if one led in the series fails, then whole led series could be turned off.You know the forward voltage of white/blue leds is around 3v..3.2v so if you make series of 3 you'd need at least 10v (and that's also enough for a 2S battery charger chip), for groups of 4 leds you'd need at least 12v.If you tweak the design to have 28 leds, you could have 7 channels x 4 leds each.There's 8 channel led drivers which allow to set the maximum current through a resistor, and you could talk to the led driver through i2c or spi to set the brightness of each channel to some level (and the ic will pwm the leds to reduce the brightness)Capacitive dropper would work, but it wouldn't give you safety, though I don't think it matters.There are small transformers available that can be soldered to a pcb ... for example this 12v 4.5VA transformer is around 3 eur in volume :https://www.tme.eu/ro/details/bvei3821191/transformatoare-pcb/hahn/bv-ei-382-1191/If you use a bridge rectifier/4 diodes and a capacitor to smooth, you're looking at around 15v after rectification and up to around 2.75w so you could have groups of 4 leds , 8 channels x 4 leds = 32 leds in total... or 7 channels x 4 leds = 28 leds.I think you'd want higher voltage and a 2S or 3S battery pack anyway because at 3w, a single 18650 would last for under one hour.example of shift register like driver :https://www.digikey.com/en/products/detail/stmicroelectronics/STP08DP05MTR/1653915no brightness control, just shift bits in (1 for each channel) to set the state of each channel. But you could dim the display by turning on every other group of 4 leds, or turn on and off to pwm the leds.

author: Zero999
author_rank: Super Contributor
timestamp: March 20, 2023, 09:53:25 pm
content: Quote from: Benta on March 19, 2023, 08:51:34 pmLook at the PDF attached to the original post.Why boost the voltage then drop it back down again?

author: Benta
author_rank: Super Contributor
timestamp: March 20, 2023, 10:27:03 pm
content: Quote from: Zero999 on March 20, 2023, 09:53:25 pmI did. There's J1, J2 and J4 plus two unconnected "S" inputs.Looking again, there's an output from a boost converter that's also an input.

author: BBBbbb
author_rank: Unknown
timestamp: March 22, 2023, 10:35:42 am
content: Quote from: mijuc on March 19, 2023, 03:43:54 pmwww.comet.rs- best stockthere are other that might not have good stock but do orders from Mouser, Farnell... usually a week of wait (Mikroprinc, Kelco,...)Farnell ships, but the customs is a bit of a coin flip (by customs I mean mostly clearance fees from certain couriers, like DHL)