Sync signal on warning lights - Page 1
Forum section: electronics / beginners

author: SEKCobra
author_rank: Newbie
timestamp: June 01, 2024, 07:19:59 am
content: I am trying to figure out how a common industry practice in warning lights works. "All" of them output and receive a sync signal on a single wire. I've put an oscilloscope to it, and while the devices are 12V fed, the sync wire outputs a roughly 5V square. I don't know if they all implement it the same, but for now I'm trying to figure out how it works at all.The signal is just a 5V pulse/square wave, both devices put this out when probed individually. When you connect the sync wires, they start flashing in sync and there is a single signal. Now my question is how they achieve this. Are they just putting a microcontoller pin to the wire and doing carrier sensing to see if someone else is pulsing and if so switch to input?Like my best guess is:microcontroller A:Put out signal for 700msListen for signal for 700msrepeatmicrocontroller B:same as A, but sees a signal during the sensing period. Syncs to A, stops output until signal disappears.Do you think that's how it is done? And if so, is that safe/what kinda protection circuit would be used for that? The same pin is also connected to 12V to change settings of the devices.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 03, 2024, 01:38:01 pm
content: There is a "one-wire" CANBUS spec. SAE J2411

author: SEKCobra
author_rank: Newbie
timestamp: June 03, 2024, 03:39:37 pm
content: I don't think there is any signal being transmitted, it's really just regular pulses to sync the timing between these devices.

author: jwet
author_rank: Frequent Contributor
timestamp: June 04, 2024, 05:20:40 pm
content: Guessing here-If they are somehow programmable with over the 12v line, then there is likely a microcontroller involved. Its likely pretty simple if its just flashing lights. The sync pin could be a resistively coupled (10K) port pin with some protection diodes to the high and low rails. It might have a high value pull down (100k) so it doesn't float. A node looks at it sync pin as an input and if not wiggling, it blinks at its own rate, if it sees activity on its sync pin, it follows it. If it sees none, it makes the sync an output and drives it at its blink rate. Eventually others see this and get quiet- there are some inherent race and deadlock conditions that you'll have to work out. Your method of pulse and quiet could work- think it through. The trick is to have the sync bus be somewhat high impedance so devices don't kill neighbors. Kind of a fun basic network problem.

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 04, 2024, 05:55:07 pm
content: I would have the light follow the 5V signal directly, for example using a BJT and a MOSFET.I assume the 5V signal is open collector output. Then, the microcontroller needed would be extremely simple, and even implementable using a '555 (or a dual 555, me hobbyist not sure about optimal implementation).The idea is that falling edge causes the logic to switch to mode 0, and a rising edge to switch to mode 1. Both switches also restart the timer.In mode 0, there is a pulldown on the 5V signal, which is released when 700ms is elapsed.In mode 1, the pulldown is enabled when 700ms has elapsed.This way, the fastest of all the low and high durations (which is not necessarily the same light) will win, causing all lights in the string to be in sync.