So I want to design a circuit...

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But I have no clue where to start seeing as it's been a half decade since I took introductory EE. I'd like to have a timer, the duration of which is inversely proportional to a sum, which is determined by voltage from a throttle position sensor over some time. In other words, assuming it would trigger after 5s at .1V, if the input is at .2V I would like for it to trigger after 2.5s or is the input is .2V for 1s and .1V thereafter I would like for it to actuate at the appropriate time between 2.5s and 5s. Bonus points for being able to build this from scrap electronics like motherboards, power supplies, microwaves, etc... Any is very much appreciated.

P.S. I realize I could probably figure out what I need with enough RTFM'ing, but having someone point me in the right direction at the very least would probably cut down on that time significantly.

Make your logical diagram first. Just boxes where circuits should be. Draw lines to connect interrelated functions. Make your logic tables, replace with boolean, then replace with standard logic circuit. Replace your clock box with a standard clock circuit. Replace your counter box with a standard counter circuit. etc, etc.

you can do this very easily using a 555 timer chip. have a look there : http://www.uoguelph.ca/~antoon/circ/pulsetimer.html

Mudboy, the problem is I have no idea what circuit could count/add voltages and open when the sum is equal to some larger voltage in the first place, so when I get to that point I'm totally drawing a blank. coyote, in retrospect, I don't think I'll need a timer circuit, but thanks since I'll probably use that at some point later anyway.

Edit-Duh, all I need is an integrator circuit. It's been too long since signals and systems.

I am not sure what you are looking for.

What I understand is you want a delay before you send a trigger out. I imagine a pulse to flash an LED as a function check.

You need to fix your statement of needs:
1s at .1V (same as 5s?)
2.5s at .2V
5s at .1V (same as 1s?)
No pulse at ??V
Constant pulses at ??V
Then pulse between 2.5s and 5s depending on voltage, but never back to 1s pulses?
What duration is your pulse?

Coyote probably knows which chips to use for the DACs, Logic chips, logic inverters, and pulse generator. The 555 timer does have the delays you want.

Your throttle position sensor is your input.
Your input should feed a couple of analog to digital converters. (DAC)
The output of your DACs should go to your logic circuits
Your logic circuits should go to your timer circuits
Your timer circuits should all go to the pulse generator
Add logic inverters where circuits are doing opposite of what you want

I'm using a TPS and switch for the input. Ideally I...
-Hit the switch, which activates the integrator circuit and a motor.
-Circuit counts/integrates voltage from the TPS until it reaches some Voltage V, which corresponds to the TPS being in some positions over some time and consequently, a certain amount of fuel flow.
-Circuit hits V, which switches on a motor via another circuit.

The only part I'm not sure about is having the voltage level C activate another circuit and motor.

sorry, what is a TPS ? So if in understand well, the higher the input voltage, the less time before the output pulse right ? I'm having a hard time picturing what you need exactly, can you rephrase:

I don't know if it's my Aspie ears, but i can't make sens of that sentence...

An integrator circuit will give an output voltage proportionnal to the "area under the curve" of the "graphic" of the input voltage over time. In other terms, you'll get the input voltage TIMES time. I'll use non-true values only to illustrate the principle: if at the input you have a constant 1 volt, the output will slowly rise as the time passes (1V X 1sec = 1Vout, 1V X 2sec = 2Vout etc...), if your input is at 3 volts, the output will rise faster as time passes (3V X 1sec = 3Vout, 3V X 2sec = 6Vout etc...). The output voltage of that integrator (Vout) would trigger a pulse when it reaches a preset value using a comparator. All you need for this is a capacitor and a resistor (and the comparator, easily made with op-amps).

Fuel flow ? can i ask what are you working on ? I'm working on a rocket engine presently....

The sentence is just a linear'ish example, since I don't think fuel flow needs to be integrated in any strict sense, just approximated in a useful way. But, since integrators are so so easy to make, why not? The circuit is just so that when I switch fuel sources in my vehicle, I can have a delay between the line-in valve and line-out valve such that cross-contamination of fuel is minimized. Since I was going to use a TPS (throttle position sensor) and VSS (vehicle speed sensor) to calculate mileage anyway, I figured that I might also be able to use the TPS signal when switching fuels.

Cool. A real, live, rocket scientist...