Hi guys. I need some help - can someone design and whip up a couple of circuit schematics for me? Whilst I'm quite at home constructing them and know which end of soldering iron is which, I'm hoplessly lost when it comes to the design aspect of electronics - and my friendly neighbourhood ex-electronics engineer is too busy with work and his family at the moment to help me out. What I'm after is a low fuel warning light, and a shift light. I've googled around and found various schematics for both, so here's the info I've gathered so far: LOW FUEL: I was planning on basing the circuit on this design here: http://users.telenet.be/miata/english/misc/lowfuel.htm But I'd like to do the additional step of adding the delay to the light coming on (as per the authors suggestion) to prevent false readings due to spirited driving. I don't know how to do this - My friend intimated that it would be fairly easy, but left it at that. I'd like to be able to set the threshold at which the light turns on to a set level in the tank, but i'll need to be able to vary it in case I find that I've set it too low or too high once the car is on the road. Other information: 12V supply, Negative earth. The light will be an incandesent bulb on the dash, not an LED. The fuel gauge is discussed here in the manufacturer FAQ: http://www.etbinstruments.com/FAQ_Gauges.htm Fuel guage installation: http://www.etbinstruments.com/ETB_Gauge_Wiring_PDF/52wire3.pdf Sender ‘Full’ = 14 Ohms (It fluctuates a little, as the full movement is actually above the position of the top of the tank, so 20 Ohms from the FAQ is approx. correct) Sender ‘Empty’ = 264 Ohms Resistance across gauge = 142 Ohms SHIFT LIGHT Again, I've googled on this, and pretty much know that the heart of it will be an LM2917 Frequency to Voltage converter IC http://www.national.com/ds/LM/LM2907.pdf It shows a couple of Tach based schematics in the typical uses section of the datasheet. I need it to turn on a light for a preset number of revs. The tach pulse will be from the back of the rev counter, and that is fed from the neg. side of the coil. As with the low fuel light, I'd like to be able to set the threshold at which the light turns on. It'll come on once a set RPM is reached, but i'll need to be able to vary it in case I find that I've set it too low or too high once the car is on the road. I'm aware that I can buy shift lights, but they're all very expensive for what they do, and for the most part look a little to fast and to furious for my tastes. Other information: 12V supply, Negative earth. The light will be an incandesent bulb on the dash, not an LED. Don't know if this is useful, but the relationship between Ignition Frequency (number of pulses from the coil) & RPM is RPM = (60 * IF * 2) / C Where 60 is number of seconds in a minute IF is the ignition frequency 2 is the number of revolutions in a complete ignition cycle in a 4 stroke engine C is the number of Cylinders (6 in my case) If you can help me out, you'll have my undying gratitude and a mention in dispatches on my build blog. Maybe a pint as well. Ta GOO.
they way to make any of them adjustable is to use a comparator. one input gets whatever you're measuring, the other input would get an adjustable voltage reference. then your output toggles when the 2 points cross. b/c the reference is adjustable, it makes the trip point adjustable.
Thanks Jake ...but I need a lot more detail than that. As I mentioned in the original post, I'm not an expert with electronics at all. Constructing circuits from schematics I can do (just about! ) I really need a lot more hand holding, and a sanity check on the googled circuits I've found to ensure they're fit for purpose and the values on the components are correct for my application. Ta, GOO
Bada-bump. MG Man has offered to help me out, and is looking into it - but only after posting about it on a different forum
Okay. Lets start with the gas gauge light. The 339 comparator with a 555 timer would be the way to go here, but if you have already the other thing working, adding a 555 one minute timer on the output stage would work too. That's easy, but the tach is a little harder. There are two ways you could do this, and it depends on your budget and patience. The inexpensive way would be to use a 556 timer IC (2x 555's in one chip). First define a minimum time T between ignition events (ex, 6 cyl @ 6000 rpm is 8.33 sparks/second or .12 seconds between sparks), and set up the first 555 in mono-stable mode (plenty of these on google) to pulse for T seconds. This is typically adjusted using a pot for R1 or R2. When an ignition signal comes in, the timer is triggered, and the 555 output goes from low to high. A N type MOSFET transistor, with the source connected to the 555 output and the drain connected to a pull-up resistor and the input of the second 555, has it's gate connected to the ignition signal as well. Should a second ignition pulse come in while the first timer is active, the second timer will trip and trigger the light. This would involve a second MOSFET and a delay of about a second because an incandescent lamp provides no useful feedback with any less. The tricky parts are 1) setting the timing is arbitrary 2) getting a clean input signal is the really tricky. If you'd like, I can provide a circuit. If you require more accuracy the second option is to set up a F-V converter and comparator with a 339, but in all honesty you are better off (time, money, frustration) picking up a kit like the JayCar CAT. NO. KC5290 which provides a precision digital output that can be set to trigger the shift light whenever you want. EDIT: If you want a circuit for the 339/555 let me know.
Update: I no longer require the Tach circuit for the shift light, as I've got an Omex box'o'tricks to do this now. I do still need a little help with the low fuel light though. Rob, my ex electronics engineer friend has drafted a first version of a low fuel light circuit for me - he's not specified some of the values on the components, and I'm not sure when he'll get time to do so for me as his Mum had been taken seriously Ill and his time is obviously and understandably been spent with her (Fingers crossed she gets well soon) Anyway, I'm posting the transcribed circuit (Rob is a pencil and paper guy) here for you chaps to mull over, and possibly suggest values for those components without them. I'm sure a bunch of clever guys like you would be able to fill in the blanks! If you'd like resistance values for the fuel sender and fuel gauge, they're in the in the original post. Ta, GOO.
I'll take a bash for you. I'm tempted to start with a clean slate though and use methods that I know myself though of course you're welcome to run it past your mate to get a second opinion. Moriquendi
Not yet I'm afraid, been busy over xmas and my main computer has died. Now that the post is working again I should have my comp up and running again soon and then I can make a start. Moriquendi
