failure with project!

[4electros]

I've built this circuit as it's shown in the diagram but the led at the last stage is permanently on (unfortunately ) ... It must be on only when LDR is exposed no light and when we light it must be off that is not flash.!


**broken link removed**




what could be the reason?

need your opinions!

 

[Gorgon]

Hi,
You need to give some more info. What is the voltage levels on the opamp's inputs, pos(+) and neg(-)? Is your power supply, +/- 9V? What is the output voltage to the LED? Resistor R5?

Have you checked your circuit for oscillations?

TOK

 

[akg]

u need to trace the voltage levels of each stage to find out the problem .

 

[4electros]

Hi,
You need to give some more info. What is the voltage levels on the opamp's inputs, pos(+) and neg(-)? Is your power supply, +/- 9V? What is the output voltage to the LED? Resistor R5?

Have you checked your circuit for oscillations?

TOK

My friend! all info that you need exist in figure 7.0 of the URL i've posted above..

but i want to ask you about (oscillations!), what do you mean?!

thanks in advance!

 

[4electros]

u need to trace the voltage levels of each stage to find out the problem .

You're right! but i want to tell you something...I removed LDR from the circuit and the circuit is still working the same as the previous( LDR does nothing) and LED at the last stage is still ON all the time...sometimes it doesn't work at all( OFF always)... i don't know that to say more :?


I'm so confused about that circuit! it's working UP-TO-DOWN :shock:

 

[sarang1_in]

Check your circuit as follows. I hope all the values you used are as shown in the circuit. Instead of checking the whole circuit, check it step by step. Instead of LDR, you put a potentiomenter. Now vary the pot from 1K to 10K (Use 10K pot) and note down the output voltage, it must be negative. At 10K resistance, you need the output of stage 2 to be 0 volts. The gain of second stage is 2. So the input should vary from 0 V to 2.5V. Check this at '-ve' (INV) terminal of second stage by varying pot from 1K to 10K. If that happens correctly then you will get V1 ie the voltage given to the Display. Now about LED. I hope you have adjusted the +ve input of 3rd stage to 2V. Now imagine that your voltage is less than 2V, the output of 3rd stage will be +9V, as it is a simple comparator. If the output exceeds 2V, the LED will be off.
You need to understand the basics of the circuit. It is very simple and absolutely fine circuit. You can check it theorotically also, by calculating the voltages at each point.
BTW are you using the same 741 opamp. If so, I suggest you to use LM324.
Do some work with multimeter and come back here with the problems or success.
Remember V comp will always be either +9 V or -9V. So as the LED either ON or OFF.
Best of luck.

 

[4electros]

BTW are you using the same 741 opamp. If so, I suggest you to use LM324.

Best of luck.

first, what's the meaning of BTW? then why you suggest LM324 instead of 741 op-amp...could you clarify?

Many thanks for you!!

 

[Optikon]

BTW are you using the same 741 opamp. If so, I suggest you to use LM324.

Best of luck.

first, what's the meaning of BTW? then why you suggest LM324 instead of 741 op-amp...could you clarify?

Many thanks for you!!

BTW = "By the way"

I don't see any reason why the humble 741 cannot be used here.
Be sure to follow a good layout in the proto board. Make sure al connections are short (especially inverting terminals to the opamps)
I would also add a 0.1 uF ceramic capacitor across the power pins of the 741 physically located right at the part with short connections.

Double check your wiring job if it doesnt work, most errors are due to construction.

 

[Gorgon]

Hi,
You need to give some more info. What is the voltage levels on the opamp's inputs, pos(+) and neg(-)? Is your power supply, +/- 9V? What is the output voltage to the LED? Resistor R5?

Have you checked your circuit for oscillations?

TOK

My friend! all info that you need exist in figure 7.0 of the URL i've posted above..

but i want to ask you about (oscillations!), what do you mean?!

thanks in advance!

Hi,
Thanks for the quip answer
Since the circuit in the schematics probably works, I was thinking of your measurements on your actual circuit. I suppose you asked your question because the physical circuit failed, thats why I asked for the values from this, not some paper.

If a circuit for some reason is unstable, it tends to oscillate, and the output is an alternating signal and not a level. If you then feed the LED amp this oscillating signal the LED will seem to be on all the time, even if it is turned on and off at a fast pace. If you have removed the LDR, without replacing it with a pot, I would assume that your amp is oscillating or at least picking up electrical noise.

TOK

 

[4electros]

I would also add a 0.1 uF ceramic capacitor across the power pins of the 741 physically located right at the part with short connections.

why do you generaly use capacitor across the power pins of 741?is that for stabilizing supply voltage of the op-amp?!
Another thing do you mean that i should put the capacitor betwee ( pin 4 and pin 7 of 741)?!

 

[4electros]

If a circuit for some reason is unstable, it tends to oscillate, and the output is an alternating signal and not a level. If you then feed the LED amp this oscillating signal the LED will seem to be on all the time, even if it is turned on and off at a fast pace. If you have removed the LDR, without replacing it with a pot, I would assume that your amp is oscillating or at least picking up electrical noise.

TOK

ok! what could be the reason of oscillating or at least how can we eliminate it to have perfect performance from 741?

I'm so sorry my friends, but i really need your thoughts!

Many thanks for you all!!

 

[4electros]

Another thing! is it necessary to add (0.1uf) capacitor not any other value of capacitance ( such as 0.47uf or higher)?!

and what about the polarity of the capacitor , should i connect the (-) pole of it to the inverting terminal of 741 or as natural ( to connect (-) pole to the positive terminal of 741)

thanks!

 

[audioguru]

Let the old 741 opamps rest in peace! They're too slow to oscillate in this simple circuit. Just about any supply bypass capacitors can be used from 0.1uF to 1000uF, but connect polarized capacitors with the correct polarity.

The circuit only has 3 opamps, so measure the DC output voltage of each one and it should be obvious which variable resistor needs adjusting to make the circuit work properly.

 

[sarang1_in]

I don't see any reason why the humble 741 cannot be used here.
Be sure to follow a good layout in the proto board. Make sure al connections are short (especially inverting terminals to the opamps)
I would also add a 0.1 uF ceramic capacitor across the power pins of the 741 physically located right at the part with short connections.

The simple reason is 741 does not have any offset elimination inbuilt!

Do some work with multimeter I already said. It is just a matter of 5 mins. Check final output and crosscheck the early stages as per the calculation.

for bypass capacitor see this.

**broken link removed**

If you use LM324 or LM358 ICs you need not have to modify the resistor values.

 

[audioguru]

b]The simple reason is 741 does not have any offset elimination inbuilt![/b]

I don't think the 6mV max input offset voltage of an opamp in this simple circuit will upset it since the circuit has up to 7V swings from the outputs of the opamps and many adjustment pots.

 

[4electros]

b]The simple reason is 741 does not have any offset elimination inbuilt![/b]

I don't think the 6mV max input offset voltage of an opamp in this simple circuit will upset it since the circuit has up to 7V swings from the outputs of the opamps and many adjustment pots.

I think that you and have some different viewpoint about using 741 op amp, i don't know what to say, i'm a bit confused about that!!

The strange thing in this project is that it caused me to confuse and ask here about it although from the first look at it , i thought that it's very easy and not reserve such work..

any suggestion more will be appreciated!!

Your opinions are very important for me, thanks a lot!

 

[audioguru]

Just post the voltage measurements of the ouputs of the opamps and what settings are its pots and we can help you with it. :lol:

 

[Optikon]

b]The simple reason is 741 does not have any offset elimination inbuilt![/b]

I don't think the 6mV max input offset voltage of an opamp in this simple circuit will upset it since the circuit has up to 7V swings from the outputs of the opamps and many adjustment pots.

I think that you and have some different viewpoint about using 741 op amp, i don't know what to say, i'm a bit confused about that!!

The strange thing in this project is that it caused me to confuse and ask here about it although from the first look at it , i thought that it's very easy and not reserve such work..

any suggestion more will be appreciated!!

Your opinions are very important for me, thanks a lot!

Dont worry about the opamp! As already pointed out, this circuit wont mind offsets, doesnt have to be fast, will be tough to get to oscillate (although possible with lousy breadboard job). A bypass capacitor (value not too important) is needed if your power supply is far from the circuit. Try it without one, if you did a clean wire job, its probably OK. You see, we can't really see how you've constructed it. We dont know where your errors are if any at all.

The other advise is for improving the circuit by eliminating offsets etc.. how about just gettting it to work first? No need to overcomplicate things. If I were you, I would use the parts indicated, do a clean wiring job and check it over at least twice. If it doesnt work, get a scope out and start looking for problems. Take a photo and upload it. There is lots of help available.

 

[audioguru]

If it doesnt work, get a scope out and start looking for problems.

You don't need a 'scope when a voltmeter is all you need to analyse this DC circuit. :roll:

 

[panic mode]

it's true... well i still haven't seen confirmation that circuit is powered correctly. are you using dual voltage and not just one and same 9V battery? did you measure 9V between positive rail and ground? how about negative 9V rail and ground? try testing individual stages, like try removing R2 and use 22k or 33k resistor but instead of connecting left side to Vo (output of Stage1), connect it to -9V rail. this will form voltage divider with R4 so now you should be able to use R4 to test Stages 2 and 3. if you have meter, remove RT from the circuit and check it's resistance when exposed to light and when not. measure voltage of all op amp inputs and outputs relative to ground.