ThermalRunaway said:I don't think that what you're suggesting is going to work in the way that you want it to. If I'm understanding your idea correctly, you want to design a voltage divider which splits your 5V supply into 10 equal parts, yes? That in itself is very easy - if you design a 10 resistor voltage divider all of which have the same value (let's say, 100K) then you'll get 10 equal divisions of the 5V available at each of the 10 divider output. In this case, you'll start with 5 at the top, 4.5V at the next resistor down, 4V at the one after that, and so on until you reach ground.
But you're saying you want to switch these resistors out of circuit one at a time, as the fluid level in the tank increases or decreases. So I assume you're just going to have one output from the voltage divider (probably the top one) and, as you switch each resistor out of circuit you want the voltage at the top-most ouput of the divider to change by an equal amount. This will work to some degree, but you won't get a linear change at the output and therefore the output won't change by the same amount each time a resistor is switched out of circuit. Instead you'll end up with more of an exponential effect, so that the first resistor you switch out will only cause a drop in output voltage of perhaps a 100mV. But as you get further up the chain the change would become more apparent until finally, you get down to your last two resistors, at which point the 5V supply would be spilt in half between the last two resistors. So basically, if I understand your idea correctly it's not going to work in the way that you want. By switching out the resistors in the divider network, you change the properties of it each time.
Brian
G'day Audio,audioguru said:Hi Len,
Thanks for the spreadsheet with exponential resistor values. Why did you select voltage levels at each step that are 0.25V too low?
I sketched the voltage reference setting and a few other things:
audioguru said:Hi Trevor,
Since you are using a supply of only 5V for the LM3914, its reference voltage must be set no higher than 3.5V, since its max dropout voltage is 1.5V. The closest that common resistors give is 3.16V which will work fine.
I have added everything to the circuit to make it work properly for you. :lol:
It's too bad that Len's sketch was full of dots.
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