Hi,

Ok I've been wanting to do this for a while now, so thought I'd try to kick things off by designing the circuit here and working on it a bit during my lunch breaks.

I wish to make a device that takes a pulse from my motorbike/car tacho and lights up a single or gang of LEDs at a given RPM.

Anyway the brains of the operation will be this baby...

http://www.national.com/pf/LM/LM2917.html

Here are some diagrams for some application ideas
http://www.national.com/an/AN/AN-162.pdf

It's frequency to voltage converter, seems widely used and about £1.80 from RS (http://rswww.com).

Here are a few things I've found on the subject...

Shiftlight wiring diagram (linked from 911 forum)
http://innoves.com/pmaehling/20050817_ShiftLight.pdf
911 forum, bit of discussion
http://forums.pelicanparts.com/showt...0&pagenumber=1

Guy designing his own shift light
http://www.electro-tech-online.com/e...-problems.html

Bar graph shift light
http://www.horrible.demon.co.uk/bike...ev_counter.htm
and wiring diagram for it
http://members.tripod.com/~foz11/DIY...al_speedo.html

Now the LM2907 has a built in op-amp comparator, so I guess I can use this to either power my led or more likely control a relay for them.

Now I'm an electronics novice so please take it easy on me

 

You don't ask any questions, so perhaps general observations are appropriate. Your choice of IC, the 2917 looks very good. Of course, you can expect to hear suggestions from other posters that this can be built with a small PIC or AVR chip instead, which is true, but that would be more work for little gain. Anyways, the application problems include how you will interface your vehicle to this chip and how you want the output to behave. You need to decide how many LEDs you want to come on as this will determine your current requirement at the output and thus answer the question of whether you need an external relay/transistor or not. As for the input, I see there are a few application examples in the data sheet and app note, so just pick the one that matches your plan and copy it. To make the shift point adjustable, you will need to use a variable resistor for R.

As mentioned in that one referenced thread, make sure you use a good input voltage filter on your 12V power supply from the vehicle. I often suggest that you can't really go too far with this, even to consider adding an additional linear voltage regulator and some additional RC filtering.

__________________
RadioRon

 

Thank you very much for that Ron. Yes the 2907 looks good to me, I don't want to invest in any PIC stuff either.

I'm at a bit of a loss starting hence no questions yet, just thought if I post my thoughts on what I'm up to others can hopefully steer me away from obvious errors

I don't want any sequential lighting or any flashing etc. Just an bright light to come on when a given RPM is exceeded, and off again when it drops below. This RPM will need to be variable, I guess a pot somewhere or maybe dip switches could be used.

Ok I'll work backwards,

Firstly get a decent power supply from my bike, I think I will go to town on this as I might (if this goes well) build other projects in the future which could utilise this. I will get a wiring diagram for my bike, there maybe already something suitable that is used for the ECU or similar? The come up with that, I have a little book on designing power supplies that might help me.

Secondly do the fun bit of building the LED holder and that'll dictate some of my circuit

Thanks for your help.