I'm new to electronics. I've been into programming and computers for a long time though. What I need to figure out is how this IC is configured. My signal (tachometer wire from a honda) is anywhere from 0-26volts according to my multimeter... thats the analog signal, the DC is constantly 12.9. My friend told me that I'm going to need a constant power source for this (as in something that will not vary at all). My questions are, is that true? And do I need to get the signal in between certain values (like 0-5v) for this to work correctly? I see lots of diagrams, but I just need this put into lamens terms. Any help in this area is greatly appreciated. Thanks!
Welcome, andera! I'm not sure what you mean. Does the reading jump about between 0 & 26V at steady revs? What DC - the battery? You can run the LM3914 no problem off a battery/alternator supply that varies say 11-15V, there's a built-in fixed reference voltage that scales the signal accurately. The signal can be any voltage up to 2V less than the minimum supply voltage, so you may need to divide it down to suit with 2 resistors or a preset potentiometer. The same pot can be used to calibrate the scale. If the tacho signal is a series of pulses at a frequency proportional to speed, which is usual, you'll need to convert to a voltage proportional to speed first, LM2917 is the best IC for that, followed by the LM3914 circuit.
I only vaguely know what I'm talking about so take this with a grain of salt. I have a tach project pending and it's my understanding that the tach is a pulsed output and you'll need a frequency to voltage converter to measure it. A common chip for this is a LM2917 If you look at this datasheet (pdf), you find towards the end an automotive tach circuit for 4,6 and 8 cylinder vehicles.
Good deal I'm already learning a bit aobut it . So the 0-26V is 0V at 0rpm and 26V at the redline...the cars rpm sensor takes the pulses and outputs a voltage so as far as i know, i will not need a 2917. So If I'm only gett 13V from the power source I'm just going to wanna setup a pot (ordered a few just in case ) to restrict the input signal? or should I use an LM317 to do that?
If it's a DC voltage output no problem, the easy way is to use the basic 3914 circuit, which is in the datasheet or like this one with R2 changed to a link between Rlo (pin#4) and ground to read zero at zero revs. (Or you may want to use a trimpot for R2 as a variable resistor to get an expanded scale with just the bottom led lit at tickover revs). The bargraph full-scale is about 1.2V, so the 26V needs reducing. A 47k resistor in series with a 4k7 trimpot to ground, taking the 3914 input from the wiper, should be about right. Forget the 317, that would give the same reading at any RPM. Motors are electrically noisy, so don't forget the capacitors.
how do i know if it's DC? the signal from the tach reads 12.9V constantly when my multimeter is set to DC, when it's set to AC it reads anywhere from 0-26V. Will that work with the 3914?
You said But what you're saying now doesn't sound like a DC voltage proportional to speed. And a 3914 won't report if it's variable frequency.
