29V Christmas lights: would like to mod with ESP32 - Page 1
Forum section: electronics / general

author: lezginka_kabardinka
author_rank: Regular Contributor
timestamp: December 23, 2023, 07:59:01 am
content: I just bought some tree lights (240 in a string) which run off the following adapter (photo attached), which has a built in effects selector button. I’m sure this is an “H-bridge” type, I scoped the outputs and I see PWM waves. It’s the usual 29V type.Any idea what kind of chips drive the H-bridge in these kinda things, and whether I could tap off the required voltage to power the esp32 without any fuss with regulators etc?My aim is to run micro python on the esp32 and have a basic http server so I can send effects commands; at the moment it’s just using a momentary button to toggle through each effect in sequence and select one.Big Clive tore down one of these “Jutai” type adapters, but it didn’t seem to be one of these. I’m asking because I don’t wanna destroy the sealed casing without there being a good chance of it being easily modifiable.Thanks chaps.

author: lezginka_kabardinka
author_rank: Regular Contributor
timestamp: December 23, 2023, 08:01:18 am
content: Final photo…

author: lezginka_kabardinka
author_rank: Regular Contributor
timestamp: December 23, 2023, 08:05:59 am
content: Truly the final photo actually

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2023, 12:04:08 pm
content: Yes, they just us an H-bridge. The LEDs are connected back to back, giving two channels. The standard colour combination is red & yellow, or green & blue, but warm and cold white, are also available,which look more like gold and silver.I wouldn't bother trying to modify it. It's probably easier to buy a 30V PSU and build your own controller. It shouldn't be difficult: an LM317L for the ESP32 and an H-bridge for the LEDs. The only thing I'm not sure of is whether the LEDs have an external resistor, or if it's build-in to the PSU.

author: wraper
author_rank: Unknown
timestamp: December 23, 2023, 12:10:18 pm
content: Quote from: Zero999 on December 23, 2023, 12:04:08 pmESP32 consumes too much current to do that. You need a buck converter as minimum. LEDs do have a resistor but AFAIK PSU is current limited too and drops voltage under high load rather than going into protection.

author: lezginka_kabardinka
author_rank: Regular Contributor
timestamp: December 23, 2023, 07:43:43 pm
content: Found the H bridge; seems to be some "591" and "491" BJTs, but the microcontroller is unmarked...

author: lezginka_kabardinka
author_rank: Regular Contributor
timestamp: December 23, 2023, 07:49:08 pm
content: Mains header

author: lezginka_kabardinka
author_rank: Regular Contributor
timestamp: December 23, 2023, 07:50:28 pm
content: 29V HEADER

author: lezginka_kabardinka
author_rank: Regular Contributor
timestamp: December 23, 2023, 07:51:02 pm
content: WHOLE PCB - SMD SIDE

author: lezginka_kabardinka
author_rank: Regular Contributor
timestamp: December 23, 2023, 07:52:00 pm
content: WHOLE PCB - THROUGH HOLE SIDE

author: lezginka_kabardinka
author_rank: Regular Contributor
timestamp: December 24, 2023, 07:46:23 am
content: Done some reversing; what on earth is this biasing arrangement on the PNPs?Edit:: oh, oh yeah of course, it’s setup so the bridge can be controlled with only two inputs from the MCU gpio:When an input is high it turns on the NPN it’s attached to, and said NPN pulls the end of the 7k5 resistor connected to its collector to ground. therefore switching on the PNP on the opposite side of the bridge, thus both halves of that phase of the bridge (PNP & NPN) are fully on, and current flows from positive supply through the PNP, through the load, then through the NPN to ground.Doh.

author: Zero999
author_rank: Super Contributor
timestamp: December 24, 2023, 10:38:20 am
content: The NPN transistors turn on the PNP transistors.Here's a simulation.Note Q2 and Q4 mustn't be turned on simultaneously.

author: lezginka_kabardinka
author_rank: Regular Contributor
timestamp: December 24, 2023, 06:28:17 pm
content: Quote from: Zero999 on December 24, 2023, 10:38:20 am