horist

well... my fuel pressure gauge is one of the older autometer full sweep gauges w/the black box...

As such my only reference voltage is 0-12 volts... anyone know how to design a cricuit that would scale the output voltage in 1/2 ?(0-6 volts)

Then I'd also need something so if voltage goes over 5 volts it'd not pass the extra voltage on (Diode?)

ArKay99

iTrader: (1)

Go to Radio Shack. And buy either an LM117 or an LM317 three terminal 5volt regulator. I'm not sure off the top of my head which one is positive and which is negative, so get the positive one. There will be a pinout diagram on the back of the package. I believe pin 1 is input (hook your 12v line to that), pin 2 is ground (hook your ground to that), and pin 3 is output (hook that to the meter). They are 1 amp shunt regulators, so if the current requirements of the gauge are above 300ma you may want tohook a heat sink to it. There is a hole in the tab to do this. They are about $2-$3 for the regulator.

Dooo, you said reference voltage. The input impedance is a factor but since it is a reference it may be high. So connect 2 4.7k 1/2 watt resistors end to end. On one end connect to the voltage source, the other end connect to ground. The junction of the 2 resistors will be 6 volts +- the tolerance of the resistors. I don't know if there are 5v zener diodes, but there is a 5.1v. Connect the side with the band to the junction of the 2 resistors and the other side to ground. That will shunt any voltage over the zener threshold to ground and clamp the voltage, sort of, at +5.1v.

Brains

iTrader: (7)

This one's pretty easy... Pick up a 741 (or 4558, or any other popular) op-amp from radio shack. Wire it up as follows:

Brains

iTrader: (7)

Actually the more I think about it, you can likely just use a pair of resistors in a voltage divider without having the buffer of the amp.. Basically just take the two resistors you see in front of the op amp and forget about the chip. You'll want a 1:2 ratio on the resistors -- the one going to ground should be 1/2 the value of the one inline. Use something like 10k and 20k ..

M_T_0

the voltage devider should be 2 resistors of the same value:
12V
I
\
/ r1=1k - 10k depending on power req.
\
/
I
o--------------o---- 6v
I I
\ I
/ -- r2 = r1
\ /\ 5v crowbar diode circuit in parellel
/ --
I I
Gnd. Gnd.

the resistor values should be 10x or so of the impedance of the guage.

More Than Zero

ps sorry this does not do graphics lol

y2khawk

iTrader: (5)

are you trying to modify the sender output from the autometer sender, or make the 5V supply for it?

horist

Modify the sender output so that I can input it safely into my EIO HP Tuners cable...

y2khawk

iTrader: (5)

umm, the autometer senders are 5V out. I ran a second one for quite awhile to montior fuel pressure via the AC pressure input to the PCM. 5V supply to the sender, 5V out.

Where are you getting the 12V from?? You didn't feed 12V into the sender did you?

horist

hmmm... I'll have to look at it again... When we were testing a month back or so we couldn't find anything that seemed to be 5 volts ... and I thought I had read somewhere that the older full sweep autometer fuel pressure gauges were 12 volt from the black box to the gauge... I'll have to investigate once it warms up...

Thanks everyone for the input

y2khawk

iTrader: (5)

Well, i had one of those box style ones a few years ago. I've had 3 different styles of senders from autometer, and bought an extra directly from them. All have been 5V.