DIY Digital Turn Signal Controller

[Step'67]

This post is to simply document my programmable turn signal controller design in the event that other members may wish to use it/for future reference. This project all began when I went through my third turn signal indicator relay, all bought locally from parts stores. The problem was that the indicator relay would function properly for several days, then would begin to flash quicker and quicker as time went on. These relays are thermally activated. As the "element" internally heats up, it triggers the switching action of the relay. I also had another issue with these relays, they're not instant-on. Some of them take a few seconds to provide power to the indicators after the turn signal switch is activated, presenting a major problem at high speeds in tight traffic. So began the creation of my own design...

I know most of you aren't a fan of electronics, the more simple the better, and less things to fail. However, in this design there is just one relay that may fail some long time down the road, and if it ever does, the circuit is designed to fail closed so that the indicator light will still illuminate, allowing the driver to manually flash the light until a new relay is installed. My design inspiration for this came from my love of electronics/microcontrollers and the want for a programmable flash delay and instant-on illumination. Is it overkill? Yes, but it was a fun quick project.

This isn't a tutorial, so a background in some electronics and Arduino understanding will be needed to replicate this device, but it is quite simple to construct and all materials are easily available. A run down on how it works: A relay is placed in line with the signal relay contacts on the fuse block under the dash, just like the original thermal relay (Indic1 & Indic2). This is then connected to an Attiny85v microcontroller (a basic Arduino), through a transistor, for control of the flashing. The problem is, the relay contacts always have a 12v signal whether or not the turn signal switch has been activated. I needed a way for the circuit to see when a left or right turn indication has been requested so that the relay would begin pulsing, and only pulse when a signal was wanted (so that the relay wasn't unnecessarily turning on and off constantly). This was accomplished using what is known as a voltage divider (a series of resistors that divides the 12v running through the chassis wiring down to within the useable range of the microcontroller). The "input" of one of these dividers is tapped into each of the indicator light wires heading into the firewall (the dark and light blue wires, consult a wiring diagram). The outputs of the two dividers are then connected to the microcontroller. This does the following: When the signal switch is turned on (either for a left or right indication), power is sent through either the dark or light power wires (Turn1 & Turn2). The microcontroller senses this voltage and the program instructs it to begin pulsing the relay on and off at the set interval (650 milliseconds), flashing the lights as the thermal relay should. Once the turn signal switch goes to the off position, the microcontroller senses the lack of voltage running through the wires, and turns off the pulsing. This is a very simple design and does its job without fail. The entire circuit is powered by a voltage regulator, stepping down the power from 12v to 5v for the microcontroller. Below is the circuit schematic:



Parts:
2_ 100k resistor
2_ 10k resistor
2_ 22k resistor
1_ 1k resistor
1_ 7805 5v regulator
1_ 12v SPDT relay & protection diode
1_ 2N3904 transistor
1_ ATTiny85v microcontroller
1_ 8 pin IC socket
1_ My Arduino Code: http://tinyurl.com/n7qdg2r

The whole circuit was wired together on a small prototype board from RadioShack, so it's nice and compact (relay is mounted to back of board). Unfortunately all of their enclosures are way too large to justify putting this in, and I didn't want to order one, so I covered the circuit in hot glue and wrapped it in electrical tape to seal out moisture/dirt and mounted it under the dash. The microcontroller is easily accessible and removable for reprogramming if needed (hence why it's missing in the image). I also added two LEDs to the circuit. One to indicate power, and the other replaced the relay protection diode so that I can see that the relay coil is functioning correctly (for debugging if needed). An enclosure would make it look much more pretty, but it works. In the image, the two sense wires haven't been soldered yet, but they would be attached to the left of the board.

Ill post if anything goes wrong or any modifications I do to the circuit if anyone is interested, and of course answer any questions I can that you guys may have.

[truckster]

Cool!

Can you make it flash like a '68 Cougar turn signal? (Just kidding)

[Step'67]

Haha, absolutely, if we had multiple tail lights that is...

[franken]

Interesting, but the flasher isn't a relay.

[Step'67]

Correct, it doesn't have an electromagnetic coil to move the contacts. This is accomplished thermally as it heats up and cools internally, but I still think of it as one for whatever reason.