Solid state turn signal relay for motorcycle

[newbie666]

Hello all,

Old motorcycles used the mechanic relay to turn the turn signals on and off. This function of course can be achieved by much simpler means using - drum roll - electronics!
My first idea was to use good old 555 timer, but I need some help designing the circuit. First of all, couple of design requirements:

* Operating voltage: 6-12V
* Switching the load up to 20W
* Doesn't draw any power when the turn signals are off

So I went on the internet looking for similar, commercial products, and look what I found:
http://motogadget.com/en/electrics/digital-flasher-relay-m-flash/m-flash-blinkrelais.html

The second picture from the manufacturer site looks mighty interesting:


two resistors next to the 8pin chip + a big cap makes me think 555 The other surface mounted thing must be a transistor since 555 should not take 1.5A of current (max power assumed for the set of blinkers)

There's just one thing I fail to figure out. How do they manage to use only two wires? I can imagine a circuit where I have V+ and GND going to the 555 and output going to the blinkers through the blinker switch. But two wires only?

Here are instructions on how to connect their relay:
http://motogadget.com/media/downloads/manual/mflash_manual_en.pdf

Any idea how they power 555 so that they use only 2 wires for the entire thing?

[Zero999]

I've done a similar thing before, see attached. The 555 timer gets its power when the lamp is off, by charging a capacitor.

[newbie666]

Hi, thanks for the quick reply. Can you tell me how should I select the transistor? What parameters would matter the most?

M

[Seekonk]

Choose the FET carefully.  The LED acts as a zener and that can be over 9V, which can limit the drive voltage.  Adding a resistor across the LED will help in a pinch.

[Wilksey]

The image shows a bulb rather than an LED.

[newbie666]

it needs to work with both bulbs and leds

[Zero999]

If it's a bulb, it's fairly easy: the MOSFET just needs to be able to handle the desired current and voltage.

An LED would make things more difficult, not only because of the higher voltage drop and lower current draw, but the LED may not fully switch off due to the current drawn by the oscillator.

I'd probably use a Schottky for D1, a logic level MOSFET (<3V if possible but it needs to be able to tolerate 15V on the gate), a CMOS 555 (TS555 or ICM7555) and make the resistor values as high as possible, say 470k for R1 and 10M for R2 with just 100nF for C2, or use a CMOS IC such as the CD4011 and build an astable with that.  This will keep the current draw as low as possible and allow it to work down to 3V, which is important if the LEDs drop a lot of voltage.

[newbie666]

Hi Hero999, thanks for help! I'll order all the parts and get breadbording soon!

[sparx]

If it's a bulb, it's fairly easy: the MOSFET just needs to be able to handle the desired current and voltage.

An LED would make things more difficult, not only because of the higher voltage drop and lower current draw, but the LED may not fully switch off due to the current drawn by the oscillator.

I'd probably use a Schottky for D1, a logic level MOSFET (<3V if possible but it needs to be able to tolerate 15V on the gate), a CMOS 555 (TS555 or ICM7555) and make the resistor values as high as possible, say 470k for R1 and 10M for R2 with just 100nF for C2, or use a CMOS IC such as the CD4011 and build an astable with that.  This will keep the current draw as low as possible and allow it to work down to 3V, which is important if the LEDs drop a lot of voltage.

I imagine if he is thinking of turn signals he will be using automotive led bulbs, so they will already be capable of (or limited to) being operated at 6/12v?

[Wilksey]

LED's or replacement bulbs which are made up of LED's?

[Zero999]

The datasheet for the flasher relay says it has a minimum load requirement of 1W, which is 167mA at 6V or 83mA at 12V. If the LEDs don't use enough power then you need to connect a resistor in parallel with them.

[SeanB]

The ones I looked at earlier on eBay have a 240R SMD resistor mounted across the power inputs to do this.

[Wilksey]

As Hero999 has said, unless it is a 1W+ super bright LED or something then 1 LED won't cut the mustard, and a load resistor must be used.

[mikerj]

You can buy two pin LED flasher 'relays' for pennies.

[Zero999]

You can buy two pin LED flasher 'relays' for pennies.

That looks too cheap to be trusted. There's very little information about it.

"Fix flash rate problem, 2 pin relay With built-in resistor."

Built-in  resistor WTF? If a resistor is needed, in order to provide the minimum load, it needs to be connected in parallel with the load, so it's not possible to incorporate it into a two wire flash unit.

[Wilksey]

Bit like one of those "+25BHP horsepower gain for any car" mods, a resistor in a bit of heat-shrink, and if you are lucky, it will be connected to the leads!

[mikerj]

You can buy two pin LED flasher 'relays' for pennies.

That looks too cheap to be trusted. There's very little information about it.

"Fix flash rate problem, 2 pin relay With built-in resistor."

Built-in  resistor WTF? If a resistor is needed, in order to provide the minimum load, it needs to be connected in parallel with the load, so it's not possible to incorporate it into a two wire flash unit.

It's just badly phrased (Chinglish) in an attempt to help those with little understand of electrics.  You have to use load resistors if fitting LEDs indicators to a system with a standard relay to get the correct flash rate.  Rather than saying that this provides the correct flash rate without the load resistors they simply tell people it's "built in".

Obviously a two pin flasher relay can not have a load resistor 'built in'.

They do work however.

[Zero999]

You can buy two pin LED flasher 'relays' for pennies.

That looks too cheap to be trusted. There's very little information about it.

"Fix flash rate problem, 2 pin relay With built-in resistor."

Built-in  resistor WTF? If a resistor is needed, in order to provide the minimum load, it needs to be connected in parallel with the load, so it's not possible to incorporate it into a two wire flash unit.

It's just badly phrased (Chinglish) in an attempt to help those with little understand of electrics.  You have to use load resistors if fitting LEDs indicators to a system with a standard relay to get the correct flash rate.  Rather than saying that this provides the correct flash rate without the load resistors they simply tell people it's "built in".

Obviously a two pin flasher relay can not have a load resistor 'built in'.

They do work however.

Are they electronic or mechanical? They don't say. Have you personally bought from that seller before? If so, then good for you. I suppose at that price it's worth giving it a go.