help with LDR circuit line sensor

[aruna1]

hi, i'm going to make a LDR based line follower robot.before make the line sensor i want to clarify something.circuit diagram is attached below.my question is will this circuit work? because all ldrs are in parrellel and that will reduce the total resistance.specially when one sensor is on the line and others are outside. thanks

[arixrobotics]

i think the circuit will work.

i'm assuming that you'll be connecting out1-4 to some uC's ADC?

the changes in the LDRs' resistance will affect the others since they are connected, but only the current will change. the voltage should stay the same i believe (since its in parallel). so if the outputs are to be connected to something that measures voltage (and not current), i think it should be ok.

try it out and let us know.

[Soeren]

Hi,

[...] will this circuit work? because all ldrs are in parrellel and that will reduce the total resistance.specially when one sensor is on the line and others are outside. thanks

Yes, if eg. the upper trimmer (don't see a use for it) is at 1M and the lower trimmers are at say 10k, with the LDRs going to 100 Ohm, just one of them seeing light will make the common node (where the 4 x 47k trimmers connects to the 1M trimmer) go down to:
5V * (10k + 100) / (1M + 1k +10k +100) = 50mV

To remedy the situation, just use 1M (or whatever you find needed) for the lower trimmers and connect them through a 1k resistor directly to the 5V rail, then they cannot interfere with each other.
If you need a global adjustment, make a variable (fairly low impedance) voltage that you regulate (instead of the 5V rail), but there's no real need for that unless you need it to run in various dissimilar environments.


Whether you measure voltage or current, Ohms Law still stands! 

[aruna1]

hi, i just want to make an analogue sensor which i can use in various environments with different intensities of ambient light (eg. indoor or bright outdoor) just by adjusting a single trimmer. i thought 1M trimmer as global adjusment and 47k trimmers as fine tuners.can you post a circuit which will fit to my description?
thanks

[Soeren]

Hi,

hi, i just want to make an analogue sensor which i can use in various environments with different intensities of ambient light (eg. indoor or bright outdoor) just by adjusting a single trimmer. i thought 1M trimmer as global adjusment and 47k trimmers as fine tuners.

OK, but that won't quite cut it, as I wrote.

can you post a circuit which will fit to my description?

I would like to see you do a bit on your 'bot yourself, so here's a few suggestions.

Let the 'bot begin measuring the environment. If your A/D-C is 0..5V, shoot for 2.5V on a medium "color".
If it reads lower, raise the voltage at the top and vv.
The active element could be one or two transistors, driven from the controller. It would have to be working OK from max voltage (close to the 5V) in full sun.
For the drive, either use D/A-c or integrated PWM.

[aruna1]

Hi soren, i didnt get your method.can you explain it in more simpler manner

[zabuza™]

I dont understand why you are using a trimmer? i see this in a lot of diagrams why is this

[aruna1]

TRIMMER IS USED TO ADJUST SENSITIVITY

[zabuza™]

surely you would want it to be constant right?

[aruna1]

Constant for a given light condition.
as light condition change,need to adjust trimmers to set new sensitivity levels