Interfierence with infinite gain op amp

[uart]

can you explain this circuit please?

is that LED used n photo current mode? and is resistors big like 47M exist? (never heard before)
and is there anyway to add a pot here so i can chage sensitivity-threshhold value?and can yu read my previous post about my objective of this circuit and give a suggetion?
thanks

Yes it's in photocurrent mode. You can get resistors above 10 Meg but some places may only stock up to about 10M. Try it with about 10Meg first up if you like, it might drive it ok. This is the part that's hard to design unless you know the characteristics of your particular LED - just how much photo-current can it supply when reverse biased and illuminated?

Edit : Actually the LM324 bias currents might a bit on the high side if you're using too higher value resitors. Try 10 Meg and if that still has enough sensitivity then you might even be able to go a bit lower. With this circuit you'd probably be better off with a FET input opamp like you had before.

well actually I'm making this as a sensor for line follower robot.my objective is to make a sensor which has less effect on ambient lighting and have high stability.since LDR and photodiodes are too ensitive I though this would work.

Not sure about this. I assume that what you need are illumination and sensor LED's that are narrow band to minimize the influence of ambient light. I'm not very familiar with the use of a LED as a photo-detector. I'd guess that as a sensor it would respond most strongly at the wavelength that it emits in normal LED mode, but I'm not 100% clued on this aspect.

 

[aruna1]

thanks

 

[vk6kro]

Looking at your second circuit again, the 47K pot would never be able to give a negative output, so both states of the LED should be more negative or equal to the output of the pot.
So, it shouldn't be switching the opamp used as a comparator.

It is probably just working because of a lucky input offset voltage.
Even using a different opamp on the same chip may not give the same result.
So, this is a risky circuit as it stands.
The first circuit, though would be OK if you can get enough sensitivity.

With your line follower, do you have 3 sensors so that if the robot goes off line, the robot controller can work out which way to turn to get back on track?

You can get better rejection of ambient light if you use a pulsed light source.
You can get 38 KHz receiver chips intended for TV remote controls. These give a low output if a signal of pulsed Infra Red light is received but high output otherwise.
They are good chips and easy to use.
http://au.mouser.com/Search/Refine.aspx?Ne=254016&N=1323038+11326342+4294930559
A 555 can drive an Infra Red LED at 38 KHz.

 

[aruna1]

Looking at your second circuit again, the 47K pot would never be able to give a negative output, so both states of the LED should be more negative or equal to the output of the pot.
So, it shouldn't be switching the opamp used as a comparator.

It is probably just working because of a lucky input offset voltage.
Even using a different opamp on the same chip may not give the same result.
So, this is a risky circuit as it stands.

that explains why it is changing time to time.

The first circuit, though would be OK if you can get enough sensitivity.

how to increase sensitivity?

With your line follower, do you have 3 sensors so that if the robot goes off line, the robot controller can work out which way to turn to get back on track?

i have 8 sensors in array

You can get better rejection of ambient light if you use a pulsed light source.
You can get 38 KHz receiver chips intended for TV remote controls. These give a low output if a signal of pulsed Infra Red light is received but high output otherwise.
They are good chips and easy to use.
http://au.mouser.com/Search/Refine.aspx?Ne=254016&N=1323038+11326342+4294930559
A 555 can drive an Infra Red LED at 38 KHz.

well then i have to send bursts of 38KHz IR signal right? that means 2 555 ICs per sensor (-->2*8=16 555 for complete array)
and with it how can i adjust sensitivity?


cant we use LM567 tone decoder with IR receiver diode?

 

[vk6kro]

The first circuit, though would be OK if you can get enough sensitivity.
how to increase sensitivity?

The obvious way is to get more light on the white line. There are some very bright LEDs available. I would try that before anything else. If you get a brighter subject and more output from your detector, the whole thing will be less noisy and more predictable.



You can get better rejection of ambient light if you use a pulsed light source.
You can get 38 KHz receiver chips intended for TV remote controls. These give a low output if a signal of pulsed Infra Red light is received but high output otherwise.
They are good chips and easy to use.
http://au.mouser.com/Search/Refine.a...342+4294930559
A 555 can drive an Infra Red LED at 38 KHz.

well then i have to send bursts of 38KHz IR signal right? that means 2 555 ICs per sensor (-->2*8=16 555 for complete array)
and with it how can i adjust sensitivity?

You probably only need one 555 and a bunch of switching transistors to turn the IR LEDs on and off at 38 KHz.
You don't need bursts of 38 KHz. Each LED should be just switching on and off at 38 KHz.


cant we use LM567 tone decoder with IR receiver diode?
Sure, but 567s are not as sensitive as these receiver chips. I tried one with a TV remote control and I couldn't get far enough away from it in the room to stop it receiving the signal.
These receivers have nice filters and amplifiers in them so they are fairly immune to ambient light. They also include the diode while a 567 would need you to supply one.

Not sure why you want to reduce sensitivity.
You can put up a shield so that the line does not get sun on it where it is being viewed by the sensors.
You can have each LED and each receiver looking down a black tube so it only covers a small area at the end of the tube.

 

[aruna1]

well how can i adjust sensitivity i.e where to add POT to adjust if i use these 3 pin IR modules?

 

[vk6kro]

If you want to reduce sensitivity, you could just do it by reducing the power to the light source.

Or, you could actually detune the light pulse frequency from 38 KHz. These devices have a tent-shaped response, so they would work fine, but at reduced sensitivity, if you tune the input off frequency.

Or, you could do as I suggested above. Get the receivers looking down black tubes, so they get light from only one direction.

Also, some of these devices operate on 32 KHz. If you had each second one on 32 Khz and the others on 38 KHz, this would give some more immunity from adjacent sensor pickup.

 

[aruna1]

ok,i'll chk, thanks