Remote shift light help

[Paul Jones]

Hello all, 1st post so please be gentle
I've got a little project which i could do with some help on.

Trying to make a remote shift light for a (bike engined)Westfield,
I have a shift light on the bike clocks (single LED) which are mounted
in the car, but would like a large remote 12v lamp to light instead of the small
LED.

Would the below circuit be suitable? with a 5v trigger for the transistor
coming from the existing LED supply on the bike clocks.

Hope that makes sense, Thanks.

**broken link removed**

 

[marcbarker]

If you replace the resistor/transistor with an n-ch power mosfet, and replace the relay/prot diode with a light bulb, you'll have a simpler circuit that replicates the circuit you posted (assuming it drives a light bulb with one end connected to +12). If the Fet is chosen carefully, it won't need a heatsink, and the whole circuit can be made on a connecting block and have insulating tape wrapped around it.

THough having just said, the circuit you posted switches on when there is a voltage applied. You didn't describe the existing LED circuit you wish to interface to. Can you tell us more about the LED circuit? How is the LED driven?

 

[Paul Jones]

Thanks Marc,
The existing LED is mounted on the bike clocks main circuit board which also has all the other usual warning lights/tacho etc.
I've no idea about how the LED is driven, although i can test the voltage applied to it, when activated.
Are you thinking that the LED may be supplied with a permanent +supply, & uses a -switch to operate it?

 

[marcbarker]

The LED could be wired that way (low-side switched). You won't know until you measure it, or look at the bike schematic. The more that can be found out about the existing circuit, the simpler the modification will be.

If there isn't this kind of information then the modification design becomes a little more complicated because it has to cater for all possibilities. Some designers prefer to always cater for any possibilty (conservative) while other designers find out all they can, so as not to duplicate functionality (minimalist).

 

[Mr RB]

I agree with marc to use the transistor to drive th ebulb direct, but allso you might want to put a capacitor and a resistor (both in parallel) from the transistor base to ground, this will reduce chance of it turning on with spikes and will provide a more gentle turn on/off of the bulb to increase bulb life. Something like 3.3k and 1uF should work.

 

[marcbarker]

Light bulb thermal inertia is a good spike filter

Good point is raised about noise pickup.

It's the "done thing" to include a resistor (of a high-ish value, say 100K), between the gate and source, to prevent the Fet picking up signal when the input is unconnected floating. This would prevent the fet "half-turning on" (if the input was ever disconnected, the fet warming up and then melting the insulating tape wrapped around it and possibly burning a hole somewhere. The resistor push into the same conn-block holes as the fet pins.

Another 'done thing' (more like a ritual) is the adding of a resistor in series with gate pin, the idea being that it "prevents oscillations at RF frequencies". I would say omit this gate series resistor completely (using the 'KISS principle'), because you want to connect the FET's 3 pins directly into a connecting block, not have to build a full 'bells and whistles' circuit on a PCB!

What's missing now is details of the existing LED circuit, that this proposed circuit interfaces to.

 

[Paul Jones]

Thanks for the replies fella's.
Been out to test the shift light. Seems a +2v is applied
to the LED when it is lit, but that is about all the info I
can get about it (Circuit diagram for the bike just shows
a magic box for the clocks).
Had a look at Fets & was a bit lost as to which specs i
need to look at. Any pointers?
The light i need to use shouldn't draw any more than 2A.

 

[marcbarker]

Seems a +2v is applied to the LED when it is lit, but that is about all the info I can get about it

(sarcastic mode) Wow that's useful, I bet no one knew that before?

Seriously, you need to measure whether the LED is in the high side or low side. I mean what is it? Is it a neutral indicator? That's usually driven by a grounded contact closure. (This means the Light is "high side" connected)

A FET can be chosen relatively easily. There will be about 100 different ones and they'd all do the job more than OK.

 

[Paul Jones]

Seems a +2v is applied to the LED when it is lit, but that is about all the info I can get about it

(sarcastic mode) Wow that's useful, I bet no one knew that before?

I didn't, but i do now, so i guess it was useful to me

Seriously, you need to measure whether the LED is in the high side or low side. I mean what is it? Is it a neutral indicator? That's usually driven by a grounded contact closure. (This means the Light is "high side" connected)

Its a shift light as described earlier, and has a +2v applied to it. which I'm guessing would be described as "Low side".

A FET can be chosen relatively easily. There will be about 100 different ones and they'd all do the job more than OK.

"Wow that's useful" I'll just choose any random one then

 

[marcbarker]

Yes you could almost choose a n-channel mosfet at random and it's likely it'd work!

When someone says something is "in the high side" this means for example as applied to the good old fashioned Horn or Hooter circuit, where one side of the hooter is connected to +12. The hooter is in the high side. The other side of the hooter is wired to the button, which completes the circuit to Ground. The button would be described as in the "Low Side".

Really need to know more about how the LED is connected before can proceed, otherwise the circuit may get more complicated than it needs to be!

Take a different tack.... what drives the LED? Is it a Switch screwed into the Engine Block? If so how many wires does Switch have? Is one of these wires connect to ground? Or maybe one connects to + 12V?

Or does the LED get driven from an ECU? If so, could you measure the voltage of the LED, with respect to ground? Do this by clipping your negative probe to ground and probe either side of the LED with the red probe.