failure with project!

[4electros]

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?

I powered the circuit in this way:

 

[Optikon]

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?

I powered the circuit in this way:

That is fine. Are you still stuck?

 

[4electros]

That is fine. Are you still stuck?

yes..

the output voltage of stage2 is 5.20 v
the output voltage of stage 1 is 5.22
it must be V2=2*V1

anyway , even these values are always changing when i touch a resistance a bit or any other component in the circuit, it must be that the board i use it is not good at all!

my board is called "bread board".

please give me any suggestion , thanks!

 

[Gorgon]

That is fine. Are you still stuck?

yes..

the output voltage of stage2 is 5.20 v
the output voltage of stage 1 is 5.22
it must be V2=2*V1

anyway , even these values are always changing when i touch a resistance a bit or any other component in the circuit, it must be that the board i use it is not good at all!

my board is called "bread board".

please give me any suggestion , thanks!

Hi,
The voltages you give is not logical if you have wired the circuit correct. Check the pin configuration of your opamp and make sure that the +9/-9volts are placed on the correct pins. You also need to check that the (+) and (-) inputs are connected correct. If you want a guess from me, I would assume that something is floating(not connected properly or correct) or that you have managed to toast the opamps. Since both opamps gives the same output I would assume a systematic fault.

Study the datasheet and schematic and rewire your circuit from start, is my advice.

TOK

 

[audioguru]

Isn't R1 connected to the positive 9V supply?
Isn't the output of the 1st stage negative 5.20V?
Isn't the output of the 2nd stage positive 5.22V?
Is your Light Dependant Resistor shorted?

Are your 9V batteries nearly dead at about 6V?

 

[sarang1_in]

What happened at last???

 

[4electros]

I've made my circuit as the figure shown below.
V1=3.29 v
v2=6.17 v

and other values
v1=3.11 v
v2=5.81 v

about the batteries I used , I measured its voltage with a voltmeter, one is 7.38v , the other's 8.93v

 

[Nigel Goodwin]

Your circuit makes no sense!.

Your first amp has an inverting gain of more than two times, and you are feeding it with a +9V input, so the output should be greater than -18V, this isn't possible, as your supplies are only +/-9V.

Likewise, your second amp has an inverting gain of about the same, so fed from the -9V odd from the first amp, it should output more than +18V, again impossible!.

 

[4electros]

Your circuit makes no sense!.

Your first amp has an inverting gain of more than two times, and you are feeding it with a +9V input, so the output should be greater than -18V, this isn't possible, as your supplies are only +/-9V.

Likewise, your second amp has an inverting gain of about the same, so fed from the -9V odd from the first amp, it should output more than +18V, again impossible!.

you're right!! what could be the reason?! what would you advise me at this case?

best regards!

 

[Nigel Goodwin]

Your circuit makes no sense!.

Your first amp has an inverting gain of more than two times, and you are feeding it with a +9V input, so the output should be greater than -18V, this isn't possible, as your supplies are only +/-9V.

Likewise, your second amp has an inverting gain of about the same, so fed from the -9V odd from the first amp, it should output more than +18V, again impossible!.

you're right!! what could be the reason?! what would you advise me at this case?

Try using values that work!.

Where did you get the value of the first feedback resistor?, how about calculating it as the original website gives the formula for?.

 

[4electros]

The first amplifier should work as inverting amplifier, the website gave me the value of feedback resistor, it's variable resistor of 1k but i used 2.2k , why not?!

The important thing in the first stage is the law of inverting amplifier:
Vout=-Rf/Rin * Vin

Rin is the resistor connected to the inverting terminal.

 

[Nigel Goodwin]

The first amplifier should work as inverting amplifier, the website gave me the value of feedback resistor, it's variable resistor of 1k but i used 2.2k , why not?!

So why did you double it?, giving twice the gain, and at the same time removed the LDR (minimum 1K) in series with Rin (giving 1K instead of a minimum of 2K, and maximum of 11K), doubling the gain again.

So your unwarranted changes give the circuit four times the gain it should have, rendering it useless!.

If you make Rin the 2k2, and Rf the 1K, it should then fall within working limits

The important thing in the first stage is the law of inverting amplifier:
Vout=-Rf/Rin * Vin

Yes, so why didn't you use it?.

 

[audioguru]

It looks like a couple of saturated inverters to me.

 

[4electros]

So why did you double it?, giving twice the gain, and at the same time removed the LDR (minimum 1K) in series with Rin (giving 1K instead of a minimum of 2K, and maximum of 11K), doubling the gain again.

The LDR I use it doesn't change it's value from 1K to 10K sometimes it reaches 30k or even 50k.... and after many tests I made on my circuit The LDR is no longer working as previous that is it gives false values completely different from the values it should be..

for the resistanse 2.2k I will replace it with 1k var-resistor and post back the new voltages

It looks like a couple of saturated inverters to me.

what does that mean? You meant that the wrong voltage values are resulted due to that..

 

[audioguru]

It looks like a couple of saturated inverters to me.

what does that mean? You meant that the wrong voltage values are resulted due to that..

1) You posted four V1 and V2 voltages but didn't say what they were measuring.

2) The voltages you posted don't say positive or negative.

3) You posted three sets of opamp output voltages. Are the batteries so weak that you can see their voltage dropping?

4) Your batteries are dead if a saturated 741 can't give its output about 1.4V less than the supply voltage. Measure the battery voltage when it is powering the circuit. I bet one battery is at about only 5V and the other is about 7V.

 

[4electros]

1) You posted four V1 and V2 voltages but didn't say what they were measuring.

V1 is the output voltage of stage1(inverting amplifier that connects to LDR)
and V2 is the ourput voltage of stage 2 ( the second amplifier)

2) The voltages you posted don't say positive or negative.

V1 is negative but V2 is positive due to inverting terminals

3) You posted three sets of opamp output voltages. Are the batteries so weak that you can see their voltage dropping?

yes, abit , but the changes is about 0.5v or 0.7v...maximum!

4) Your batteries are dead if a saturated 741 can't give its output about 1.4V less than the supply voltage. Measure the battery voltage when it is powering the circuit. I bet one battery is at about only 5V and the other is about 7V

one is 5.75v the other is 8.52v
[/quote]

 

[4electros]

sorry , some errors in quoting....
could you tell me how use more than once for the same person?

thanks a lot!

 

[audioguru]

Your weakest battery is past being dead. The strongest one has a few hours of life remaining.

Why not change the resistors to be the correct values then try it with new alkaline batteries?

 

[4electros]

Your weakest battery is past being dead. The strongest one has a few hours of life remaining.

Why not change the resistors to be the correct values then try it with new alkaline batteries?

ok. but the weakest battery is new, i bought it few months ago but i haven't used it until this project, so i'm surprised for that weakness, what should i do now?

 

[audioguru]

but the weakest battery is new, i bought it few months ago but i haven't used it until this project, so i'm surprised for that weakness, what should i do now?

Duh! My Energizer alkaline 9V batteries are stamped "best before 2011" and always work well. Maybe yours expired in 1991. The Chinese batteries at the dollar store are leaking still in their packages. I wonder how old they are. I wonder why people buy that junk.

Did you load it with anything that got hot or made the battery hot?
The cells inside a little 9V battery are skinny AAAA size and can't provide much power for very long. Anything that causes heat uses a lot of power.