Hi all:

i have been advised to uses a Rail to Rail op-amp to build a IR distance detection sensor.

The amplitude of the IR signal will be inputed to a ADC port and a microprocessor is going to calculate the distance.

But could anyone please explain to me that why a rail to rail op-amp is an ideal option?

PS: the diode is represents a photodiode, i couldnt find a symbol for it so i just used a diode instead

 

While sometimes you can get by with a non rail-to-rail opamp, by using a rail-to-rail opamp you will be able to get the maximum useable signal.

Provide a schematic so that we may better understand your application.

hgmjr

 

While sometimes you can get by with a non rail-to-rail opamp, by using a rail-to-rail opamp you will be able to get the maximum useable signal.

Provide a schematic so that we may better understand your application.

hgmjr

thanks

i have provide it

 

What is the function of diode D2?

 

I would be interested in a bit more details concerning how you plan to use the IR detector to measure distance. Your schematic seems to be missing an IR-detector.

hgmjr

 

I would be interested in a bit more details concerning how you plan to use the IR detector to measure distance. Your schematic seems to be missing an IR-detector.

hgmjr

Hi,
sorry, the diode is infact a photodiode,
i couldnt find a symbol for it so i just used a normal diode to present it.

A infrared light source will shine on this photodiode
the current pass through the high pass filter resistor to create a voltage and send it to be amplifed

 

It seems as though an ADC is not much use to you. The photodetector circuit you have is blocking dc so it appears that your IR transmitter is being pulsed.

hgmjr

 

It seems as though an ADC is not much use to you. The photodetector circuit you have is blocking dc so it appears that your IR transmitter is being pulsed.

hgmjr

yes, a 5% , 2Khz will pulse the emitter
and the high pass filter to get rid of ambient light

 

I am still having problems envisioning how you are using this to measure distance. The speed of light is approximately 186,000 miles per second. Exactly what technique are you using to determine distance?

hgmjr

 

It appears you will be using non-modulated (i.e., continuous) IR and basing your distance on a decrease in intensity of the reflected beam.

I do not think that method will work very well. What accuracy do you need? Over what distance will you be measuring?

Some of the reasons are: 1) Continuous IR will experience lots of interference from other sources; 2) Range will be very limited; 3) Intensity of reflected beam will depend on several factors besides distance, e.g., reflectivity, angle, dust and other interferences.

I suggest you consider either triangulation or time of flight. There are packaged IR devices that use triangulation, such as the Sharp GP2D12 to GP2D15 series of sensors.

John

Edit: Above was written before your comment about a modulated beam appeared.

 

I am still having problems envisioning how you are using this to measure distance. The speed of light is approximately 186,000 miles per second. Exactly what technique are you using to determine distance?

hgmjr

i will pulse the IR signal, sends a pulse for about 20us long, every 1ms

ie, enable the emitter the for 20us, measure the output
switch of the emitter, and stop measuing



so the detector will detect the pulse

 

It appears you will be using non-modulated (i.e., continuous) IR and basing your distance on a decrease in intensity of the reflected beam.

I do not think that method will work very well. What accuracy do you need? Over what distance will you be measuring?

Some of the reasons are: 1) Continuous IR will experience lots of interference from other sources; 2) Range will be very limited; 3) Intensity of reflected beam will depend on several factors besides distance, e.g., reflectivity, angle, dust and other interferences.

I suggest you consider either triangulation or time of flight. There are packaged IR devices that use triangulation, such as the Sharp GP2D12 to GP2D15 series of sensors.

John

Edit: Above was written before your comment about a modulated beam appeared.

Thanks

the detection range is only up to 6 cm

 

At 6 cm, triangulation would be by far the easiest and probably most accurate.

John

 

Time of flight for light traveling 6cm assuming that the total distance traveled is 12cm (6cm out and 6cm back) is on the order of 0.4 nanoseconds. At that speed I am not sure I see how your IR detection scheme would work.

hgmjr

 

Time of flight for light traveling 6cm assuming that the total distance traveled is 12cm (6cm out and 6cm back) is on the order of 0.4 nanoseconds. At that speed I am not sure I see how your IR detection scheme would work.

hgmjr

I meant switch the emitter on for 20us

so the detector can measure it while the emitter is switched on

here is the output from the scope

 

At 6 cm, triangulation would be by far the easiest and probably most accurate.

John

Thanks

i have used both my self made ones and sharp series for longer range.

due to a bugget issue really

i am just trying to understand why a rail to rail is perfered

 

Does the scope you are using have the ability to measure on the order a few picoseconds?

hgmjr

 

It seems to me like the challenge of your project has less to do with whether you use a rail-to-rail opamp or not. The real challenge is to get picosecond resolution from a circuit that has device propagation times on the order of nanosecond.

hgmjr

 

Does the scope you are using have the ability to measure on the order a few picoseconds?

hgmjr

from the screenshot

you can see the divison is 5us

so the length of the pulse is about 15us

what i am saying is that

switch on the light for 20us

and the detector could detect pulse because the light source is switched on for 20 us

 

So you are intending to use "time of flight"?

hgmjr