2 pin flasher relay circuit suggestions

[int2str]

My motorcycle (like many others) has a 2-pin flasher relay to make the turn signals work.

Essentially:
12V --> SWITCH --> RELAY --> TURN SIGNALS --> GND

The relay has no other pins. Apparently the actual flashing is often accomplished with a bi-metal strip that disconnects the turn signal temporarily when it's heating up and then closes again etc. This is load dependent and thus falls apart when you switch from incandescent bulbs to LEDs.

Now, I know there are multiple "cheats" to fix this:
1. Add ballast resistors to the LEDs (lame)
2. Wire in an "LED relay", which usually means it needs a separate ground pin, aka 3 wires

What I'm trying to figure out is a way to do this without the 3rd wire.

Ideas that came to mind:
1. Use a micro controller (ex. ATTiny85) to handle the flashing and use a large enough capacitor to keep the chip running in the "off" period. Use a mosfet to control power. I'll have to do the math on this if I can get enough charge in 1s or so "on" period to survive a 1s or so "off" period.

2. Use a 555 timer and again, a large enough capacitor to do the same thing as in 1). Using a 555 makes the circuit more complicated, but it can run at 12V. But gain, need to figure out how to keep it running in the off period.

3. Come up with a completely discrete charge + flip/flop? circuit that does this. Problem here is that I would have to de-couple the discharge timing from the load itself. In other words, I'd have to find a way to discharge a capacitor through an arbitrary load while retaining constant timing.

I'm strongly leaning 1), but any tips/tricks welcome!

Thanks!

[Zero999]

It's fairly simple, as you guessed, using a 555 timer.

[Ian.M]

While that circuit would work with an incandescent bulb load, good luck running a 555 off the couple of volts that will be left after the Vf drop of a string of 5 or 6 Amber LEDs!  You'll either need a micropower circuit that that will run off as little as 2V and a boost circuit for the FET gate drive, or a reasonably low value resistor across the LED bulb load.

[Zero999]

While that circuit would work with an incandescent bulb load, good luck running a 555 off the couple of volts that will be left after the Vf drop of a string of 5 or 6 Amber LEDs!  You'll either need a micropower circuit that that will run off as little as 2V and a boost circuit for the FET gate drive, or a reasonably low value resistor across the LED bulb load.

Use a CMOS version such as the TLC555, TS555, ICM7555 etc. (these draw less power and work down to lower voltages than the traditional NE555) and a MOSFET with a very low threshold voltage (such as the PHD38N02 or FDN337) or a BJT (such as the ZTX690 or ZTX1053) with a high beta to drive the LEDs.

https://www.fairchildsemi.com/datasheets/FD/FDN337N.pdf
http://www.nxp.com/documents/data_sheet/PHD38N02LT.pdf
http://www.diodes.com/_files/datasheets/ZTX690B.pdf
http://www.farnell.com/datasheets/1643255.pdf
http://www.st.com/web/en/resource/technical/document/datasheet/CD00000893.pdf
http://www.ti.com.cn/cn/lit/ds/symlink/tlc555.pdf
http://www.st.com/web/en/resource/technical/document/datasheet/CD00000893.pdf

[Ian.M]

That may do the trick.  Better use a Schottky diode for D1 and *DON'T* even *THINK* of fitting it to an old kickstart bike with an AC lighting system!

[int2str]

Wow, you guys are rockstars!! 

I was looking for inspiration, not a complete circuit, but that's even better!
I'll whip it up and see if I can get it to work!

Thanks so much!

 

[Ian.M]

Try it on a breadboard on the bench, with the supply set to whatever voltage your bike's electrical system runs at (probably around 14V) with the headlight on full beam at idle, and one of the LED bulbs you want to use.  If it craps out on the road,  while you are waiting to turn at a busy junction, it could get you killed, and if it doesn't meet your local regs for flash rate etc. it could get you a traffic ticket.

[int2str]

Ian, of course
I've also been riding for 20+ years and a misbehaving turn signal is very far down the list of things that will get me killed on a motorcycle

[Ian.M]

Once you have a handful of years riding under your belt, the experience you gain doesn't make you much safer due to the other bloody half-blind idiots competing for the same patch of Tarmac.   I quit riding anything two wheeled a good many years ago.  I *used* to have a couple of mates who didn't quit . . . .

[Zero999]

Personally I ride a motorcycle and don't drive a car (even though I have a licence and can afford to do so) because I feel safer on a bike.

I have replaced a bike's indicators with LEDs before. It worked quite well and passed the MOT (for non-UK readers, this is a compulsory annual road worthiness test). I added resistors though and didn't bother with replacing the flasher relay with an electronic one.

An electronic flasher should be more reliable and a mechanical one. If you're worried about over voltage transients then connect a 16V zener diode in reverse parallel with the flasher.

[vk6zgo]

I've not had a lot to do with bike flashers,but I did look into replacing my car one with a solid state circuit,years ago.
It turns out that the  bi-metal device is a pretty hard act to follow.

If you lose a lamp,the flasher click rate increases audibly,giving you a warning.
Trying to do anything like that with Electronic components gets very messy,very quickly.

I know most cars use solid state flashers now,but it looks like they've just given up on trying to reproduce the electromechanical device's speed increase with reduced load.

Of course,with LEDs the chance of losing a "lamp" is very much reduced.

[SeanB]

Most cars use an electronic flasher which senses current flow, and flashes faster if there is a blown bulb dropping the load below the expected one of 21W+21W+5W+3W. Newer vehicles with a body control module use a high side switch for the indicator lamps, and do current sensing in the module to indicate a blown bulb on the driver display. The ticking sound is from a small speaker inside the dashboard, fed with a sound file that emulates the relay clicking.

[vk6zgo]

Most cars use an electronic flasher which senses current flow, and flashes faster if there is a blown bulb dropping the load below the expected one of 21W+21W+5W+3W. Newer vehicles with a body control module use a high side switch for the indicator lamps, and do current sensing in the module to indicate a blown bulb on the driver display. The ticking sound is from a small speaker inside the dashboard, fed with a sound file that emulates the relay clicking.

In other words,a complex circuit to do what a bi-metal switch did!
A factory made unit can be more complex than the homebrew one I was trying to design 30--odd years ago

[Seekonk]

I worked on a friends motorcycle after his step brother wire up LED turn signal so they only worked when the motor was off.  It was a two wire electronic flasher that worked fine with LED.

[SeanB]

Car manufacturers also use an ASIC design, which pays for itself over many units as the lamp load is pretty much constant across vehicles. I had to replace a 2 pin flasher, so went to the dealer spares and they gave me a 3 pin one, along with a small wiring loom with a 2 pin connector to fit the original socket and a wire to connect to a specified bolt on the frame as the ground connector. Old bimetal flasher got too expensive to make as it is a mostly hand assembled unit.