comparator help

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darsh

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hi . i am using LM339 as a comparator. in + terminal i provide a voltage which is coming from the emitter of transistor BC548.the - terminal has 5 V. but this circuit is given error.when i use a multimeter and test the voltage between + terminal and gnd, the output becomes low. so anyone please help.
 
Hi darsh,

a schematic or at least a wiring diagram would be helpful.

BTW measuring at the emitter of a BC548 I only expect logic zero (L).

Boncuk
 
schematics

hi. i have attached the circuit. sorry i have not made it properly.
here my idea is that when the light of led does not fall on the LDR , the emitter output is high(2.5 V). so comparator will compare it with 1.5 V and result is Vcc. now when light falls on LDR, its resistance is decreased and emitter output is low(0.7 V). so my comparator will give 0 V as output.
now the problem is that when the +ve and -ve wires of multimeter are touched to output pin 2 and gnd to measure voltage, the output changes suddenly.the high output becomes low.
 

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hi,
Connect a resistor from the transistor emitter to ground [0v]
A value of about 1K thru 4K7 should give a acceptable voltage change at +Vcomp.
 

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Last edited:
Xmas was great, How about you? Oh, I thought the drawing said 1-5 volts. After zooming in I see you are correct. Oops.
 
Mikebits said:
My wife bought me a workout machine. Do you think she is trying to tell me something?
Click to expand...

There are two possibilities,
she is either trying to tire you out ..

or build you up, after all it is Xmas.

Does the machine have a PIC .?
 
thank you ericgibbs.yes the comparator worked finally !!!! and it is giving the desired output .
 
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