Tempertaure Detector help

[srimannarayanakarthik]

hi.
My previous problem of the voltage conversion is solved. But i have an another problem. The circuit is giving output for all values. i need the circuit to operate for 12V 50degrees. but the LED is glowing for all the values even for the room tepmerature. Can anyone help me out. I am attaching the schematic here

 

[eng1]

I see two problems with your circuit:
1- You're using the op amp in an open loop configuration. If you want to use it as a comparator with hysteresis, you may add a proper feed-back resistor.
2- The op amp you chose can't swing to ground and its output always biases the transistor (it is always ON)

 

[dch222]

I agree with eng1. The feedback resistor needs to go between the output and the + input - try 1Mohm.

You should be able to get around the swinging to ground thing by putting a diode (or two) between emitter and 0V with anode to emitter. This will cause the transistor to turn off at a higher base voltage.

 

[srimannarayanakarthik]

I see two problems with your circuit:
1- You're using the op amp in an open loop configuration. If you want to use it as a comparator with hysteresis, you may add a proper feed-back resistor.
2- The op amp you chose can't swing to ground and its output always biases the transistor (it is always ON)

Then how can i make that circuit work

 

[FRIED]

Use a comparator where you need a comparator, not an opamp.

 

[srimannarayanakarthik]

I agree with eng1. The feedback resistor needs to go between the output and the + input - try 1Mohm.

You should be able to get around the swinging to ground thing by putting a diode (or two) between emitter and 0V with anode to emitter. This will cause the transistor to turn off at a higher base voltage.

ok ill try them

 

[srimannarayanakarthik]

Use a comparator where you need a comparator, not an opamp.

The above ciruit also does the same job as that of a comparator them what the difference of both. Both should perform the same

 

[ericgibbs]

hi,

When the LED is supposed to be OFF, measure the output voltage of the 741.

If over +1V, as it most likely is,,, say you measure +2V when its supposed to be low, [ideally 0v],
then connect two low cost diodes in series with the 2K2 base drive resistor, cathodes towards the transistor base.

In this way you will not switch ON the LED when the 741 is less than 2.5V as the diodes will drop about 1.4V.

Fit the 1megohm resistor, this will give a little hysteresis, so the 741 dosn't dither ON/OFF at switch over.

Eric

 

[srimannarayanakarthik]

hi,

When the LED is supposed to be OFF, measure the output voltage of the 741.

If over +1V, as it most likely is,,, say you measure +2V when its supposed to be low, [ideally 0v],
then connect two low cost diodes in series with the 2K2 base drive resistor, cathodes towards the transistor base.

In this way you will not switch ON the LED when the 741 is less than 2.5V as the diodes will drop about 1.4V.

Fit the 1megohm resistor, this will give a little hysteresis, so the 741 dosn't dither ON/OFF at switch over.

Eric

i have measured the voltage across the output terminal and the GND. The voltage is 8.45volts. i am shocked see it. what might have gone wrong. the variable resistor is mentioned in the circuit as 0-47k. i have kept the max value but still why the voltage is so high

 

[ericgibbs]

Its possible that the 741 is faulty. You may have exceeded the input voltage range of the 741.

If possible, unplug the 741 and measure the voltages on the 741 socket.
Also check the connections are going where they are supposed to go.

While the 741 is unplugged, measure the voltage on the two input pins 2 and 3, does the variable resistor change the voltage value?

I would place a 4K7 in series with variable resistor to the +12v rail.

Post the results of your test.

Eric

 

[srimannarayanakarthik]

voltage on pin 2 is 9.67 and pin 7 is 9.75 when the opamp is removed. the rest all pins are Zero

 

[srimannarayanakarthik]

and another doubt. how to give the negative supply for ua741. i have grounded is as shown in the circuit

 

[ericgibbs]

hi,

It looks as though the 741 has failed.

Try to find a LM393 dual compartor, they work OK on a single +V supply.

To get a -V from a +V rail, use a 7660 ic, take care not to exceed the +V input.

Eric.

RonH: I suspect his +12v rail is not 12V but +9.7V.

 

[Roff]

voltage on pin 2 is 9.67 and pin 7 is 9.75 when the opamp is removed. the rest all pins are Zero

On a 8 pin package, pin 7 is VCC.

 

[srimannarayanakarthik]

On a 8 pin package, pin 7 is VCC.

The voltages what i gave are the vakues of the voltages what i got when i removed the IC from its bed and the terminal voltages are measures.

 

[srimannarayanakarthik]

hi,

It looks as though the 741 has failed.

Try to find a LM393 dual compartor, they work OK on a single +V supply.

To get a -V from a +V rail, use a 7660 ic, take care not to exceed the +V input.

Eric.

RonH: I suspect his +12v rail is not 12V but +9.7V.

Can i use LM358 instead of lm393. i have it right now with me

 

[ericgibbs]

hi,
A LM358 is a dual op amp, designed for a single supply.

You will have to rewire your circuit to suit the pin out.

Eric

 

[Roff]

RonH: I suspect his +12v rail is not 12V but +9.7V.

If this is the case, pin 2 is outside the common mode range. It is connected to a pot, which needs to be turned down to 7 volts or less.

 

[srimannarayanakarthik]

how to do that. how to get down the value of voltage. instead i tried it out with LM358 also. The voltage at the output pin of it i mean the pin1 of it is coming as 7.25. i connecte d a feed back of 1 mega ohm. but still it sthe value. what might have went wrong

 

[ericgibbs]

What is the actual value of the +12V rail???

Did you add a resistor to the 47K pot as RonH advised??

What is the voltage at the junction of the thermistor and resistor?

Eric