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Author Topic: Help-Light detecting with LDR or photodiode,etc(Simplest.i.e.without some chips)  (Read 5792 times)

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Offline zanynikTopic starter

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Hi.. Me from a mech. I have an idea for line following bot (not the one on the site)..
For it i need a  a circuit diag(wid 2 photodiodes/LDR's, and motor )
COndition - When BOTH the diodes/resistors detect light motor must stop.
When atleast one/none detects the motor must run.

Plz..suggest a simple Circuit fot the above conds..c'mon electrical guys...

Offline arixrobotics

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I've done something like this a few years ago...

I think all I did was to make a voltage divider with an LDR and a resistor, where they feed into the base of a transistor. The transistor acts as the motor driver. So when it turns on, the motor will spin.

Make two sets of these, put them side by side, and you'll get a simple light follower. Will work as a line follower too I think, but I've never tried that.

A better version (line follower without microcontroller) is the one by David Cook in his book Robot Building for Beginners. See http://www.robotroom.com/Sandwich.html for more info.

Offline zanynikTopic starter

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Hi... I appreciate ur reply..

But the one i want to make is based on "COMPLETELY DIFFERENT" idea.Once i suceced wid the bot .. i'll let you know wat it is.. but for now...

I want someone to PLZ  post the type circuit diag for BELOW GIVEN CONDITIONS (any rough dia) i think i'll be able to understand dia once i see it.. i.e

CONDITONS
1) Should be simple consisting 2 LDR's/photodiodes,etc..
I Need two diodes controlling single motor such that:
2)motor must stop When BOTH the diodes/resistors detect light
3)motor must run When one/none detects light.

help req..I'm not demanding here..but try to post the diag plz..
I have the basic knowlege of electronics..so wont take much time to understand once u post the dia... .

Offline chrischristian

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Hi zanynik !

This should do the job ,
For help 1
select resistance of R such that over all resistance ( total series resistance ) of all three ( LDRs and pot.) remains higher than 470 ohm ( that of Rs) even when ONE OF the LDRs detects light but as soon as both LDR detects light the total resistance fall below 470.


For help 2

It is just the same like help 1 but just one LDR is in parallel making it mor efective.

hope will help
« Last Edit: August 30, 2008, 08:55:15 AM by chrischristian »

Offline zanynikTopic starter

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Hi..

My thinking (1)
I think in the above circuits The motor doesnt stop completely when both detect light. Because some amout of current flows through motor though most of the current takes the less resistant path. I'm thinking that small amout of current through 470/motor route will still cause slight rotation..whereas I want it to completely stop when "both diodes/ldr's detect light".

Thinking (2)
I'm using stepper motor. i think ur circuit may work  cuz the 470 resistance may cause significant voltage drop across motor ,that it refuses to rotate(i feel stepper motors dont rotate with less voltages).So maybe it works

Could you tell me Which of the above two contradicting THINKINGS are right.And do u think I can use ur circuits?

One more thing..
Hey! by simple i meant dont use chips/drivers stuff....u can use transistors and all basic stuff plz.. as i understand the basic stuff...
THANX A LOT though..!!  ;D

Offline chrischristian

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I think it should work you may replace 470 ohm with some bigger value ( though it will slow down motor, but if that doesn't matter to you ).

NOTE :

Don't forget to put some resistance between battery and the ckt ( the joint where 470 and pot connects to each other ) or you may fried out the LDR.

Offline RobD

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Arixrobotics was onto something.  For example, take two of something like these:



...and feed them into an OR gate http://hyperphysics.phy-astr.gsu.edu/Hbase/Electronic/or.html#c2 

Offline zanynikTopic starter

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Hi.. Rob D..

I hope its the circuit i want i.e. The motor is on when no light detection(by one or bothe detectors) & off when both detectors detect light.

I thin i understand the circuit and or gate.Here is my understanding please correct me if i'm wrong.

Given D1 is emmitter.. Q1 is detector.When Q1 doesnt detect light - it is open, the transistor conducts current through R4.
When Q1 detects light(i.e. it is on white line) then Q1 is SHORT ..thus switching OFF the transistor Q2.. hence no Output at OV.

When OV is connected to OR gate and OR gate to motor then,
Motor runs for any BLACK line detection by Detector and Stops only if both detectors detect White line.

IS THAT RIGHT?? IS that what happens..
Today I'm goin to build the circuit HOPE it works.. If its not correct ..Plz inform me..!!
Than-Q.

Offline RobD

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Correct on the circuit.  When the IR is received the voltage drops from five to 0-ish (0.040 or so).   

I designed that circuit a few years ago and have since upgraded the 4k7 resistor to a 10k pot so it could be tuned.  A resistor value for R2 greater than 5K will make it behave more like a logic gate...less than 5K and it behaves more like an analog sensor.  Using the Radio Shack IR emitter and photoresistor pair it has about a 5 or 6 inch range if the diodes are mounted side-by-side about 1/4 inch apart. (I just built another one this weekend for a new robot project.)

Using two of these circuits fed to an OR gate:

The OR gate:

A     B     OUT
0v   0v    0v   - both diodes detect light
0v   5v    5v
5v   5v    5v
5v   0v    5v

So, according to this:

Quote
I Need two diodes controlling single motor such that:
2)motor must stop When BOTH the diodes/resistors detect light
3)motor must run When one/none detects light.

For a motor control circuit that switches motors on when the input is high:
Condition 2 is met
Condition 3 is met

Happy building!

Rob

Offline zanynikTopic starter

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Hi robD.. I think i can Use two IR Detectors in series instead of Two of those circuits n OR gate.

The series connection allows current only then both detect light n Motor STOPS..
What say..

I'm having some problem building the circuit..didnt work...maybe i've burnt a transistor..and i'm using BC548..no probs wid that rite..
new to soldering.. Now i'm thinking of trying it on bread board first..

Can u explain me the concept of why the transistor doesnt conduct when IR detector is conducting... I mean a small current could go to base and make the transistor conducting... I need some clarity on it..!!!
« Last Edit: September 03, 2008, 07:53:46 PM by zanynik »

Offline RobD

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I say, give it a try! 

I breadboard just about every circuit I intend to build first.  First of all, resistor values (when using the non precision type) can vary quite a bit. This way I know the components are all working together.  Secondly, I have on occasion gotten ahold of some bad info on the pinout of general purpose transistors for example.  This is especially important when using substitutions.  Last, but not least, I tend to get a feel for how I might lay the circuit out when I solder it all together.  etc... etc... etc...

The output transistor does turn on when the IR detector is conducting. The output is held high by a resistor (R4) until the transistor turns on - which pulls the output signal low.

Double check your E-B-C pinouts on the BC548 if you decide to try it again.  You can test the transistor using a multimeter on the ohms setting much like you would test a diode.       

 

 

 

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Offline zanynikTopic starter

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Ya..somewhat like that..
 but can that similar light detecting circuit be made without a L293D driver..?

 


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