relay not working in comparator circuit

[gurjant.sandhu33]

i have made a light dependent circuit using ic 741 as comparator but when i connect relay to the output the voltage drops to 1.5 v otherwise the output voltage is abuot 9.5 v
please suggest something.
i have uploaded the circuit which i used . just replaced the buzzer with relay.

 

[duke37]

The 741 can not output a high voltage at a high current.
The output voltage will be the battery voltage less the drop in the 741, less the drop in the base resistor,less the drop in the 2N2222.
To help things, put the buzzer/relay in the collector of the transistor with the emitter connected to 0V.
Use a diode across the relay coil to protect the transistor from voltage spikes due to the inductance.

 

[gurjant.sandhu33]

thanx.....sir

 

[BobK]

You probably need to lower the 10K base resistor as well. How much current does the relay coil take?

Bob

 

[gurjant.sandhu33]

it takes about 350 mA

 

[gorgon]

Due to the resistor divider you'll only have around 2V or less over the relay. To get a proper drive from the transistor you should move the relay to between the collector and the +9V, connecting the emitter to 0V.
To drive 350 mA you'll need a heavier drive Circuit.
The hFE of the transistor is way too low to drive 350mA from the hundreds of uA you put into the transistor.

 

[BobK]

A '741 can provide at least 10mA of current. The PN2222 has a minimum gain of 40 at up to 500mA collector current. So it would be borderline. Realistically, you are going to get only about 5V out at 10mA of the 741 off a single 9V supply. So, the base resistor calculation is:

V = I R
(5 - 0.7) = 10 * R

R = 430 Ohms.

I would try it with a 330 Ohm resistor. It might work.

But why use an ancient 741 for a task it was not designed for? A modern rail to rail op amp or comparator would work for sure.


Bob

 

[Arouse1973]

You shouldn't really use op-amps as comparators, I know many people do. The reason is op-amps are designed to work with feedback to control their very high gain. Without this they can oscillate due to stray capacitances on the PCB. Also most op-amps apart from over the top types can't handle large differential voltage across their inputs. A comparator in this instance is what you need to use as you are not worried about CMR and offset voltages, comparators are also much faster.
Adam

 

[gurjant.sandhu33]

thanx.... everyone i will try every possible technique you all provided me
regards

 

[gurjant.sandhu33]

please can any one name a suitable comparator ic for my use

 

[duke37]

The LM393N is shown in a catalog that I have but if you have a 741 I would stick with that.

A small amount of positive feedback (pin 2 to pin 6), maybe 10M resistor will give a dead band to stop jitters.

 

[gurjant.sandhu33]

thanx....sir...i will try