Comparator circuit + OP Amp 741 help - Electronic Circuits Projects Diagrams Free

Hussain · 15th January 2009, 10:54 AM

Hi guys ...

This is my first post , and I seek some help ....

I'm building a comparator circuit , and it's working perfect . The signal of the of the output goes to trigger a coil of relay. Unfortunately , the signal is not enough to energize the coil !!!!

I have been advised to install another circuit as OP amplifier with BC 337 connected with a resistor 100Ω . I tried it last night , but it still won't work .>>>>

What shall I do bro ...

Waiting u ........

**Nigel Goodwin** · 15th January 2009, 11:08 AM

Check my PIC tutorials under 'Hardware extras'

Nigel's PIC Tutorial Hardware Extras

It shows how to switch a relay.

Also check here

741 Op-Amp Tutorial, op-amps, Operational Amplifier

Which even uses 741's, and has exactly what you want.

Hussain · 15th January 2009, 04:25 PM

Thanx a lot bro ...

I'll try it tonight and I'll let u know with the result

Hussain · 16th January 2009, 11:41 AM

I tried the circuit , but it did not work as i want ....

The comparator circuit works fine with the set point of potentiometer , however, if i connect the OP / AMP 337 , the coil is energized as soon as switch on the circuit .....and it can not be controlled with the pot.

notice that the LED still showing that it varies proportionally with the pot.

How can I control the energization of the coil with my pot.?????

See the attached picture , it shows the connection of my circuit ....

Please help me ASAP

aljamri · 16th January 2009, 12:48 PM

If you are doing this circuit for demo, connect an LED (Lower load). and if it is made to drive the Relay coil, no need for another 741, a proper transistor will do the job. In my case i'm using 2n2222a.

**Nigel Goodwin** · 16th January 2009, 01:42 PM

As you're using a split supply, the emitter of the transistor needs to go to the negative supply, check the link I posted which has exactly what you're trying to do.

It's also absolutely essential to have a diode across the relay.

Hussain · 16th January 2009, 03:32 PM

aljamri , thanks for ur commens .... yest I'm using the circuit for to drive a relay coil .

Mr. Nigel . . .

I connect the circuit as shown in the link ....except the diode that I'm going to install tomorrow ....

You have mentioned that the emettier has to be connected in the negative supply !! I connect it to the GND !! Isn't that enough ???

Is negative -12 Vdc?? or the GND can be considered as Lower potential source??

**Nigel Goodwin** · 16th January 2009, 05:22 PM

Like I said check the second link I posted - for exactly what you want.

And no, you can't use ground, you need to use -12V.

aljamri · 17th January 2009, 10:23 AM

Quote:

Originally Posted by Nigel Goodwin

Also check here

741 Op-Amp Tutorial, op-amps, Operational Amplifier

In this link, the AUTO FAN circuit is exactly simmilar to the circuit you are looking for.

audioguru · 17th January 2009, 06:20 PM

The 741 opamp is shown as an inverter. So its output will always be negative which will always turn off the transistor.

I think the transistor is shorted because it powered the relay coil without the protection diode.

Hussain · 22nd January 2009, 12:50 PM

Mr. Nigel ... Thanx alot ... it works now !!

Mr. Audiofuru .... You are 100 %right , when I troubleshooted the circuit , I found that the transistor was shorted .....!!

Now .... is working fine .....

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15th January 2009, 10:54 AM	#1
Hussain	Comparator circuit + OP Amp 741 help Hi guys ... This is my first post , and I seek some help .... I'm building a comparator circuit , and it's working perfect . The signal of the of the output goes to trigger a coil of relay. Unfortunately , the signal is not enough to energize the coil !!!! I have been advised to install another circuit as OP amplifier with BC 337 connected with a resistor 100Ω . I tried it last night , but it still won't work .>>>> What shall I do bro ... Waiting u ........

15th January 2009, 11:08 AM	#2
Nigel Goodwin	Check my PIC tutorials under 'Hardware extras' Nigel's PIC Tutorial Hardware Extras It shows how to switch a relay. Also check here 741 Op-Amp Tutorial, op-amps, Operational Amplifier Which even uses 741's, and has exactly what you want. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

15th January 2009, 04:25 PM	#3
Hussain	Thanx a lot bro ... I'll try it tonight and I'll let u know with the result

16th January 2009, 11:41 AM	#4
Hussain	I tried the circuit , but it did not work as i want .... The comparator circuit works fine with the set point of potentiometer , however, if i connect the OP / AMP 337 , the coil is energized as soon as switch on the circuit .....and it can not be controlled with the pot. notice that the LED still showing that it varies proportionally with the pot. How can I control the energization of the coil with my pot.????? See the attached picture , it shows the connection of my circuit .... Please help me ASAP Attached Thumbnails

16th January 2009, 12:48 PM	#5
aljamri	If you are doing this circuit for demo, connect an LED (Lower load). and if it is made to drive the Relay coil, no need for another 741, a proper transistor will do the job. In my case i'm using 2n2222a. __________________ Its what your friend in Your mind, what you in your friends mind

16th January 2009, 01:42 PM	#6
Nigel Goodwin	As you're using a split supply, the emitter of the transistor needs to go to the negative supply, check the link I posted which has exactly what you're trying to do. It's also absolutely essential to have a diode across the relay. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

16th January 2009, 03:32 PM	#7
Hussain	aljamri , thanks for ur commens .... yest I'm using the circuit for to drive a relay coil . Mr. Nigel . . . I connect the circuit as shown in the link ....except the diode that I'm going to install tomorrow .... You have mentioned that the emettier has to be connected in the negative supply !! I connect it to the GND !! Isn't that enough ??? Is negative -12 Vdc?? or the GND can be considered as Lower potential source??

16th January 2009, 05:22 PM	#8
Nigel Goodwin	Like I said check the second link I posted - for exactly what you want. And no, you can't use ground, you need to use -12V. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

17th January 2009, 06:20 PM	#10
audioguru	The 741 opamp is shown as an inverter. So its output will always be negative which will always turn off the transistor. I think the transistor is shorted because it powered the relay coil without the protection diode. __________________ Uncle $crooge

22nd January 2009, 12:50 PM	#11
Hussain	Mr. Nigel ... Thanx alot ... it works now !! Mr. Audiofuru .... You are 100 %right , when I troubleshooted the circuit , I found that the transistor was shorted .....!! Now .... is working fine .....