Analog Function

[MrJammin]

Is there any circuit system where the voltage vs. time function is 1/x; at least after the first few milliseconds(I don't need the part of the function that goes to infinity). The only function close to this that I can think of is that of a discharging capacitor which is k*e^(-t/RC).

 

[Hero999]

An integrator?

I don't really understand what you're talking about.

 

[JimB]

A capacitor discharging through a constant current circuit will give a voltage across the capacitor which varies as 1/t (within appropriate limits).

JimB

 

[Speakerguy]

Use an op amp log generator, then a differentiator. So basically you'd be taking the derivative of ln(x), which is 1/x. There are log generator op amp ckts out there, very old-school stuff. Differentiators very basic and common. X would be a voltage that you would ramp from 0 to Vmax that would correspond to time. If I've got my thinking hat on straight.

 

[MrJammin]

Thanks for the replies, I think I will take speakerguy's advice and try and find a op-amp circuit that generates a logarithmic function and then differentiate it.
While I'm at it, I might as well explain my project and you can tell me if it seems like it would work. I am attempting to build a bicycle speedometer. I will have a magnet on the bike wheel and a sensor that pulses on each pass of the magnet. The length before the pulses tells me the elapsed time and the velocity can be calculated from that. Since v=d/t, velocity is inversely proportionate to time, I needed a analog circuit that produced such a relationship. The analog voltage from that circuit is run through a analog-to-digital converter and run to a digital display.
That's the general description, but I'm struggling to find a op-amp log generator circuit that is not too monstrous.

 

[ericgibbs]

While I'm at it, I might as well explain my project and you can tell me if it seems like it would work. I am attempting to build a bicycle speedometer. I will have a magnet on the bike wheel and a sensor that pulses on each pass of the magnet. The length before the pulses tells me the elapsed time and the velocity can be calculated from that. Since v=d/t, velocity is inversely proportionate to time, I needed a analog circuit that produced such a relationship. The analog voltage from that circuit is run through a analog-to-digital converter and run to a digital display.
That's the general description, but I'm struggling to find a op-amp log generator circuit that is not too monstrous.

As you are going to use an ADC, a digital display and require some method of applying a timed period, why dont you use a PIC.
Count the interval between the magnet/coil pulses, using the PIC, over a known time period, using the PIC
and drive the digital display, using the PIC.

I think your analog approach while interesting, is the hard way to display the bike's speed.

Also all the components for the PIC method are off the shelf, just need a short PIC program.

 

[MrJammin]

Your absolutely correct, using a PIC would be significantly easier. I don't have any experience with microprocessors but have started reading about them and will most likely use them in my future projects. However, I kinda like the challenge provided by using a more analog approach and see if I can get it to work on this project.

 

[Speakerguy]

Couldn't you use a PIC with the pulses sent to the external interrupt trigger pin along with in internal timer to get an instantaneous speed?