How does this automotive circuit work/ how to wire it up? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: June 01, 2021, 10:57:49 pm
content: so I found this.. what I guess is a 3rd gen honda accord blinker module, I'm not sure it's that but I'm pretty sure.there is a sticker on it but it's unreadable, all the text has rubbed off...anyway, I don't understand how it's supposed to work... I drew up a schematic of it here:[ Attachment Invalid Or Does Not Exist ]the zener diode (ZD1) seems really confusing for what I think the circuit does... almost seems like something related with ignition for there to be a zener diode for one of the connections (for protection)but yeah the more I look at it, the more I get confused.maybe someone can help me understand this circuit...(also, connection 1 is ground in the circuit)EDIT: TR1 is 2SC1214 not 2SC536 as in the circuit!

author: ledtester
author_rank: Super Contributor
timestamp: June 02, 2021, 02:27:10 am
content: Here's my crack at it...Clearly the relay gets turned on if there is a high-enough voltage/current at D3 or D4.So a high enough signal at 6 will turn on the relay via D4. I think the zener is there just to provide some input protection.The 2SC536 looks like a general purpose NPN transistor:https://alltransistors.com/adv/pdfdatasheet_sanyo/2sa608_2sc536n.pdfThe 2SA844 looks like a general purpose PNP:https://alltransistors.com/adv/pdfdatasheet_hitachi/2sa844.pdfAs for how a signal at (2) affects the input at D3... I resorted to simulating it. Here's a graph of the OUT port (green) as (2) (blue) varies ramps up from 0 to 5V and back down again. The two are equal when (2) is < 2.5V. When (2) is > 2.5V the output drops to 0V.The 100u capacitor is interesting... might have something to do with an RC timer.

author: MarkF
author_rank: Super Contributor
timestamp: June 04, 2021, 01:50:38 am
content: Except that R2 (the 22k resistor) doesn't go directly to ground.So, I'm thinking that #2 must set it to flash between dim and bright.And, #6 flash bright and off.I just can't figure out how #4 and #5 are wired to the bulb.

author: ledtester
author_rank: Super Contributor
timestamp: June 04, 2021, 02:10:53 am
content: Quotegood point!

author: MarkF
author_rank: Super Contributor
timestamp: June 04, 2021, 03:25:58 am
content: I see three different configurations with either one or two filaments:None of which really work?Is the schematic correct???

author: james_s
author_rank: Super Contributor
timestamp: June 04, 2021, 06:16:44 am
content: Don't forget that turn signal flashers are required to detect a burned out bulb and flash quickly in that situation. Some of the circuitry may be to monitor lamp current and trigger the rapid flashing if it is low.

author: MarkF
author_rank: Super Contributor
timestamp: June 04, 2021, 07:40:41 am
content: Quote from: james_s on June 04, 2021, 06:16:44 amI think my turn signals just stay ON and don't flash if one of the bulbs are burnt out.Good point however.I just can't figure out how the bulbs are wired.It seems like there is something wrong with the schematic around the relay.And I haven't even considered how both the front and back signals are connected.Or left and right sides.There is much missing.

author: mikerj
author_rank: Super Contributor
timestamp: June 04, 2021, 10:27:56 pm
content: Could be an intermittent wiper circuit. If pin 2 is fed by a variable resistance (or number of fixed resistors switched in by one of the control stalks) it would form a timing circuit with the 100uF cap and the threshold detector. Pin 6 is likely connected to the windscreen washer, to make the wipers operate for a while after the washer has been operated.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: June 05, 2021, 03:34:59 am
content: Quote from: james_s on June 04, 2021, 06:16:44 amI don't think it's "required". It happens because there's the burned out bulb changes the timing of the oscillator circuit.https://www.sunautoservice.com/flickering-car-lights/#:~:text=Why%20Does%20My%20Turn%20Signal,indicates%20a%20bulb%20is%20out.&text=Turn%20signals%20operate%20at%20a,by%20flashing%20and%20clicking%20faster.

author: richard.cs
author_rank: Super Contributor
timestamp: June 05, 2021, 09:26:37 am
content: Quote from: JustMeHere on June 05, 2021, 03:34:59 amHistorically it was incidental, thermal flashers (common until maybe the late 1970s?) slow down with less load or stop flashing completely. Modern ones definitely do a detection and a deliberate behaviour change, but I am not sure what the actual requirements are. They often also disable that detection in hazards mode (so it works normally however many lamps you've smashed). One I took apart had a very fancy relay with a low impedance coil that did the detection, but I'm sure recent ones do it electronically.