0
\$\begingroup\$

I'm nowhere near savvy in the electronic engineering department, so despite having read up on op amps/buffers/voltage followers, I'm still just as confused.

I need a pure DC circuit with 2 inputs and 1 output. A 5V input, a variable .2 to 4.2 volt input signal, and the same .2 to 4.2 volt as an output (Having them match up identically is not required, just need to be somewhat close).

Current draw is very small; in the order of .5 mA. This is for a car that has a seperate computer for the 4WD function. Normally the car recieves a voltage signal (throttle position sensor) from a follower inside the main engine computer, but I installed a new computer that no longer has a built in follower. For now, I simply spliced into the TPS voltage signal, which works fine, however the extra current draw for the 4WD computer throws off the 0-100% scale.

Can anyone point me in the right direction as to what I would need to make this circuit? I'm presuming it can be done with a basic BJT or similar?

Thanks for help in advance!

\$\endgroup\$
2
  • 1
    \$\begingroup\$ I wish I knew more about cars. I don't, though. I gather you are intending to still use the TPS voltage signal, but that you cannot afford to have the 4WD computer loading down the TPS signal. So you just need a high impedance input, to low impedance output, voltage follower. I gather you also have 5 V power available? \$\endgroup\$
    – jonk
    Commented Apr 23, 2017 at 7:42
  • \$\begingroup\$ you already have the op-amp tag – and that's the right way to go; wikipedia's article on "operational amplifier applications" has a section on the voltage follower. Also, on non-inverting amplifiers with adjustable in/output relation, in case you need that. \$\endgroup\$
    – Marcus Müller
    Commented Apr 23, 2017 at 7:43

2 Answers 2

Reset to default
2
\$\begingroup\$

The question you are asking is very easy to answer. In order to make a voltage follower, you need a single opamp connected in "voltage follower" configuration.

enter image description here

Yous hould have a 5V line available for your TPS, you can use it to power the opamp. Make sure you use a rail-to-rail opamp so that you can go as low as 0.2V.

You will most likely need 2 opamp as most throttle pedal have 2 output for redundancy.

Last thing, what is the resistance of your pedal ? And what is the input impedance of your computer analog input ? Maybe there's another solution than buffering the signal.

Good luck

\$\endgroup\$
4
  • 2
    \$\begingroup\$ Better point out to use a R2R amp, and not something stupid like a 741 \$\endgroup\$
    – Neil_UK
    Commented Apr 23, 2017 at 8:49
  • \$\begingroup\$ That's the whole problem... there's thousands of different IC's and transistors to choose from, I don't know what would be a good choice. Not knowing any better, 741 would have seemed like a decent enough choice. \$\endgroup\$
    – Bungly
    Commented Apr 23, 2017 at 9:12
  • \$\begingroup\$ Also, the TPS resistance is 2 kohm at closed throttle (.2 volts), and 10 kohm at full throttle (4.2 volts). \$\endgroup\$
    – Bungly
    Commented Apr 23, 2017 at 9:13
  • \$\begingroup\$ Of course, there is an enormous amount of opamp out there. But we have already defined some constainst that will help select a device. 1) Rail to Rail, 2) 5V supply, 3) Minimum output current of 5V/<Input impedance of ADC>. 4) Number of circuit 2, 5) Form factor that suits your desing. You will proably want a DIP or SOIC package. 6) Your application is low frequency so we don't really care about Gain-bandwidth product. 7) Automotive grade. With these constaints I narrowed a set of 33K available opamp to about 20 on a electronic part provider website (Digikey). Take the cheapest \$\endgroup\$
    – user109388
    Commented Apr 23, 2017 at 9:26
1
\$\begingroup\$

Here is a simple two transistor circuit that should be good enough for what you want to do; schematic A quick simulation shows he sensor voltage at both ends; simulation This will get the load on the throttle pot down to a few uA.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.