I debated posting this in the electronics board. I figure it will get adequate attention here as well. I want to make a TPS Sensing Switch for 4.0 Volts or Higher at wide open throttle (by tapping into the middle TPS wire). I want it to be basicly identical to the Nitrous Express $122 switch box. Can anybody think up a wiring diagrm for this logic circut? It needs to complete a circut if it senses a >=4V on the input. It also cannot interrupt the TPS signal to the computer. Any ideas? I like doing things myself like most of you probably do.. Thanks! edit: The purpose of this type of logic switch is to eliminate an external indication that nitrous is being used. Both for aesthetic purposes or for a hidden nitrous setup if that is what you desire. If somebody comes up with a working circut, I will make sure it is posted here. Thanks! -Jerod

Take an opamp and set it up as a comparator. using a variable resistor for feedback you can vari what voltage will trip the output, to trigger a transistor, or small relay. Sorry I don't have a schematic, but it shouldn't take to much to find one. cheers, Bob

Edit:

The Operational Amplifier ( Op Amp ) can be used in many different ways. The Op-amp has two inputs an inverting input ( - ) and a non-inverting input ( + ) and one output. A signal applied to the inverting input will have its polarity reversed on the output. A signal applied to the non-inverting input will retain its polarity on the output. The gain or amplification of the signal is determined by a feedback resistor that feeds some of the output signal back to the inverting input. The smaller the resistor, the lower the gain. Some typical op-amp pin outs are shown below. The most common are the 741 and 1458 dual op-amp.

Another common way that opamps are used are in comparator circuits. A comparator circuit will compare the voltage on the two inputs and then making the output high or low. This is accomplished by having one input the voltage reference (Vref) and the other input is the voltage input (Vin).



With this circuit you would not need R1, instead this would be your output from the TPS, Using +5V (regulated)for VCC and adjusting R2 to +4V would make it trip where you want it, or wherever you decide to set it. Your output would be between the Emitter of the transistor and the LED. In fact I beleive you could get the +5v and ground via the TPS as the 741 has very little current draw by design. HTH Bob

Info from:
http://home.maine.rr.com/randylinscott/learn.htm

Dang, that's perfect! Thanks for the info. I really need to go back and review my electrical science class. I specificly remember failing the op amp question on my final! Now's the perfect time to learn. Thanks for your help. I will let you know what part numbers I can come up with to fit the criteria and the prices. Surely, this can be a cost effective project. Thanks again!:hail:

Just one other thing to add to what Saturn5 showed you. If you are planning to use this to trigger nitrous or some other high current load, you will have to use this op-amp to turn on a FET or Bipolar transistor(which can handle multiple amps). The schematic shows the op-amp turning on and off an LED, which probably will draw something like 10 milliAmps. To properly use this comparator to trigger a switch and not overload the TPS reference voltage, the safest thing would be to use an opto-coupler, to isolate the reference and not overload it. the opto-isolator is wired similar to the transistor, but the 'base' is actually an LED. If you do a search on the internet, you can find all the datasheets you need, as well as some application circuits. Just be careful, since the +5V reference can't handle a large load, and it would suck to have to replace it in the ECM. Hope that helps.

Here's a schematic using an LM339 quad comparator to do the level sensing for a few WOT detect circuits - there's also an O2 bargraph circuit and a TPS brgraph circuit in the diagram, as well.

Sheet 2 of the schematic......

Thank you all for your input. I attached a copy of a circut that came to my head after reading all of your input. Will this design work? I am not sure how to calculate what resistors I will be needing in the circuit and if they will be generating too much heat. I'm also not sure if the op amp will be able to practically trip the 25amp relay and if the LED will light correctly. I would greatly appreciate your comments!

The 339 is actually a comparator - special case op amp - and it has an open collector output, so it can only sink current (be grounded) when the output is "low" - or be hi impedance (open circuit) when the output is high. You would also need a second resistor from the "-" input to ground to form a voltage divider. Look at the "NOS ENable @ WOT" circuit on the first sheet of my scheatic for a reference.

Tim, there's only one sheet in that zip file! Thanks!

This should work.



If you don't need too much curent you can skip the relay and use the wire going from the transistor to the relay instead.

Woo hoo! Thanks!