I am modifying my motorcycle's rear turn signal housings so that they contain both amber turn signal bulbs and red brake light bulbs that have two brightness levels. I want to connect the brake light bulbs so that they sync with the brake pedal, and I want to build a circuit that can break the brake pedal's signal when the turn signal flashes.

The bike has the following outputs available ("on" is +14.4V when the bike is running):
-Running brake light (always on)
-Running turn signal light (on, when turn signal is activated it does not provide enough power to light even a LED bulb, but when it is not loaded it does float between 0V and around 4V, so this might not be an ideal lead to use for turning a circuit on or off.)
-Turn signal light (flashes on/off w/ turn signal, floats when turn signal is off - LED turn signal bulb may turn this into a virtual ground)
-Brake light (on when brake lever/pedal is pushed in, I believe it floats when the pedal is not pushed - it is 2001 circuitry)
-Ground

I'll have two bulbs in the turn signal housings and 3 positive terminals. The positive terminals will be:
-Amber signal light
-Low red running light
-High red brake light

I would like to connect these like this:
Turn signal running output - Low red running light
Turn signal output - amber signal light
Brake light - "Cut out circuit" - high red running light

My goal is to create a "cut out circuit" that will connect to the brake light output and the turn signal output, and then power the high red light only when the brake light output is on and the turn signal output is off. I want this circuit to cut power to the bright light when the turn signal is on, and I need to make sure it won't turn the bright light on if the turn signal is on and the brake output is not on.

I am not sure where to start when designing a circuit like this, any suggestions would be very helpful, and if anyone feels like devoting the time to drawing up a circuit diagram this would be infinitely appreciated!



The bike's circuit is made to power incandescent bulbs, and I have replaced the relay with one that does not need the load of the incandescent bulbs to flash at a normal rate, but it can handle the load of incandescent bulbs.

Thank you in advance!

 

Do you have access to the turn signals straight from the turn switch before they are pulsed signals ?

Les.

 

Okay. A little two much info.
Let me try to summarize:
Amber Light - Operated by turn signal
Low Red Light- Always on
High Red Light - On only for brake and no turn signal
Sound correct?
If so, is it okay for the brake light to blink opposite the turn signal, i.e. when the turn signal is on, the brake light is off and vice versa?

 

Do you have access to the turn signals straight from the turn switch before they are pulsed signals ?

Les.

I'm not sure exactly where I'd access that, but I do have a circuit diagram of the entire wiring harness in the service manual. However, I am trying to keep the main wire harness un-modified.

Okay. A little two much info.
Let me try to summarize:
Amber Light - Operated by turn signal
Low Red Light- Always on
High Red Light - On only for brake and no turn signal
Sound correct?
If so, is it okay for the brake light to blink opposite the turn signal, i.e. when the turn signal is on, the brake light is off and vice versa?

Yes, everything you've said is correct! And that is how I want the turn signal and High brake light to operate (blink opposite each other). I want to hook up the low brake light to the turn signal running light though, so that when the turn signal is on it blinks amber and off when I'm not on the brakes, and amber and red when I'm on the brakes.

 

I misunderstiood your requirements I thought you wanted the brake light to be off for both the on time and the off time of the turn signal. The way you want it to work means it can be done just with the pulsed turn signal. All you need to do is put the NC contacts of a relay in series with the brake light. One end of its coil will connect to ground. Each turn signal will be connected to the other end of the coil via a diode. The positive end of the diodes to the coil. The relay will pull in when either turn signal is illuminated which will cut the power to the brake light.

Les.

 

The relay will pull in when either turn signal is illuminated which will cut the power to the brake light.

I believe you need two relays, one for each side, since I think you want the brake-light opposite the turn-signal to stay on.

Questions for OP:
Is a two relay solution okay, or would you rather have a transistor solution?
How much current do the brake-lights take?

 

I believe you need two relays, one for each side, since I think you want the brake-light opposite the turn-signal to stay on.

Questions for OP:
Is a two relay solution okay, or would you rather have a transistor solution?
How much current do the brake-lights take?

Awesome, glad you asked. I was hoping for a transistor solution because that would be silent and most likely use the least current. These LED bulbs draw 0.6A, so any transistor circuit that can handle 1-2A should work!


LesJones, thank you for the help!

 

I think this circuit should work OK instead of a relay.

I have assumed that the battery is 12 volts.

Les.

 

I think this circuit should work OK instead of a relay.
I have assumed that the battery is 12 volts.

Les.

Ok, that looks easy enough, how would it change if I wanted to do it per turn signal? (disable right brake light only when right turn signal is on, same for let)

I'll have to look up what the one symbol is. Is one diode in there a Zener diode?

Also would it work at 14.4V? That's the typical voltage when the bike is running. It might be good to set it up so that the max voltage is 16V, but so that it still works at 10V in case I stall and my battery is old (it's new right now but) and the radiator fan is on too.

 

If you have right an left brake lights you would need to duplicate the circuit. You would only need one 1n4148 diode per circuit. The zener diode is to ensure that transients do not exceed the gate source volltage rating of the mosfet.It should work OK between 10 and 16 volts. If it had been a 6 volt battery then It may have been necessary to choose a different mosfet.

Les.

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I'm looking up these parts and I want to purchase them on digikey today, but I'm curious about the Zener diode. Wouldn't I want a regular diode there? I'm just confused as to why I would want to allow a small amount of current to flow towards the gate. Couldn't that result in unwanted turning-off of the mosfet?

 

The zener diode will clamp any transient spikes that may occure so thet the gate can not become more negative than -15 volts with respect to the source. It will also prevent the gate from becoming more positve than about 0.7 volts with respect to the source. The zener diode will not prevent the mosfet turning fully on. The mosfet will be fully on by the time the gate is 10 volts negative of the source.

Les.

 

Oh that makes sense! Thank you so much! I'm going to order these parts during my lunch break.

Thanks for your help!

 

Hey LesJones, I have one more question. I'm having some trouble sorting through Digikey's vast results to find a suitable Zener diode. What specs should I look for? (Bidirectional channels, reverse standoff voltage, breakdown voltage, clamping voltage, peak pulse current, etc.)

I'm not sure which of those three voltages should be 15V, and I don't know what other specs it should have either.

The resistors and mosfet and other diodes seem a lot more straight forward.

 

Any 15V 1W zener will do. Try 1N4744a

http://www.digikey.co.uk/products/en?keywords=1n4744a

 

Any 15V 1W zener will do. Try 1N4744a

http://www.digikey.co.uk/products/en?keywords=1n4744a

Awesome thanks man, I just finished ordering everything!

 

If you have right an left brake lights you would need to duplicate the circuit. You would only need one 1n4148 diode per circuit

Why would need the diode if you connect only one turn signal to its respective brake light?

 

Hi crutschow, You are correct no input diode would be needed with just one turn signal per circuit.

Les.

 

Ok now I have a problem. I received the components today, and assembled the circuit LesJones posted above on a breadboard, and it doesn't work. Current seems to always be able to flow from source to drain. Even when I apply +12V directly to the gate, with no resistor, the light will still turn on (being powered through the mosfet drain).

Also, when the Zener diode is connected the way you have it in the diagram, and I apply a bias to the gate through the 1K ohm resistor, the light turns on slightly.

I have a medicine cup painted black over the light so that it doesn't white-out the photos

The circuit is set up like above, and the turn signal is on, but the brake light still turns on:


The light turns on slightly when the gate is biased but the drain is not given power:


Even with 12V applied directly to the gate with no resistor, the light is still powered.



The MOSFET I purchased is a International Rectifier IRF9540NPbF

The light will be on (powered through drain -> source -> light) no matter what I do to the gate. I suppose the only thing left to do would be apply greater than 12v to the gate? But I'm not sure how I could do this while it's on my bike.

 

You have the source and drain pins crossed over. The middle pin is the drain and the right hand pin is the source.

Les.