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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. 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.

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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And set a ref voltage on pin 4.
 
Xmas was great, How about you? Oh, I thought the drawing said 1-5 volts. After zooming in I see you are correct. Oops.
 
hi Mike, Hope you are enjoying your Xmas...;)

The OP has +1.5V on pin#4.

My wife bought me a workout machine. Do you think she is trying to tell me something? :eek:
 
My wife bought me a workout machine. Do you think she is trying to tell me something? :eek:

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

or build you up, after all it is Xmas.:rolleyes:

Does the machine have a PIC .?
 
Does the machine have a PIC .?

If it did, then I might be able to crack it and say, see honey, 500 reps :)
 
Oh, that, never tried before. Does not work with Ie, tried with Firefox but everyone left.
 
On the chat now... Anyone?
 
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