Help: Opamp Comparator Circuit

[neoshinzo]

hey guys, i was hoping someone could help me to understand how the below circuit works. i understand the operating principle, but i wanted to know what the parts do and how i can analyze it mathematically. i'm not an electronics student, i'm just playing around with multisim and i have read through some of my older brother's electronics text books.

 

[wingerr]

The only math involved may be the resistor dividers being used to set up the inputs to the comparator. When the input to the (+) pin is higher than the (-) pin, the output goes high, turning on the transistor and lighting up the LED.
In the other diagram, the input is tapped off a different point in the divider, so the + input is lower in voltage than the -, so the output goes low.

 

[audioguru]

You need to learn about electronics' basics to see how that simple circuit works.
Some people here will teach a NOOB.

 

[audioguru]

I have a cheap DMM and a Fluke DMM.
On DC they read the same.
But on AC their readings are completely different.
If i leave my Fluke turned on then it turns itself off.
If I leave My cheap one turned on then its dead battery leaks all over the place.

 

[wingerr]

what they need is a battery condom to keep all their innards in, like I've seen on some devices- maybe some Saran wrap as a retrofit fix.

 

[neoshinzo]

The only math involved may be the resistor dividers being used to set up the inputs to the comparator. When the input to the (+) pin is higher than the (-) pin, the output goes high, turning on the transistor and lighting up the LED.
In the other diagram, the input is tapped off a different point in the divider, so the + input is lower in voltage than the -, so the output goes low.

that much i get.

how does the transistor turn on the LED? i read about biasing transistors, but i'm not sure how that works.

really what i was trying to do was figure out how the circuit in this link works:
https://www.redcircuits.com/Page115.htm

 

[ericgibbs]

that much i get.

how does the transistor turn on the LED? i read about biasing transistors, but i'm not sure how that works.

really what i was trying to do was figure out how the circuit in this link works:
https://www.redcircuits.com/Page115.htm

hi,
The 741 OPA is able to source and sink current at its output pin.

When the output is high, ideally it would be +15V, but the 741 has a poor specification, I would expect about +13Vout at best.
Consider the transistor requires a forward base/emitter voltage of approx +0.7V to turn it on, this gives a driving voltage of 13V - 0.7V = +12.3V.

From ohms law 12.3/100K = 0.000123A, as the LED in the transistor requires say, 20mA to glow, would mean that the gain of the transistor would have to be about 160.

I would advise that you reduce the value of the 100K to some value that will give a base current of about 1 to 2mA.

When the output of the 741 swings to about -13V, the diode after the 100K becomes forward biased and conducts, this clamps the reverse Vbe voltage to -0.7V and protects the transistor from the excessive reverse voltage.

For a comparator, use a more modern, improved spec OPA in place of the 741, low cost OPAs LM358 etc.

I would recommend a LM393 comparator if you can acquire one.

Do you follow this,?

EDIT: this text refers to your original post circuit.

 

[neoshinzo]

yes, thanks for explaining. i follow this.

for that link, i simulate the circuit and i have question...when i want to change the voltage i click on the battery, is there way i can change the voltage without keep changing on the battery?

i'm also unsure of how this circuit works. if anyone can explain it and what the function of the diodes are would be appreciated.

 

[ericgibbs]

yes, thanks for explaining. i follow this.

for that link, i simulate the circuit and i have question...when i want to change the voltage i click on the battery, is there way i can change the voltage without keep changing on the battery?

i'm also unsure of how this circuit works. if anyone can explain it and what the function of the diodes are would be appreciated.

hi,
1. If V1 is greater than voltage required to start D8,D9,D10 conducting then Q3 is switched ON and the GREEN LED is ON.

2. As Q3 is ON, D3 is connected to the +V rail, so the ORANGE LED is shorted out and is OFF.

3. If V1 is greater than the voltage required to start D4,5,6 conducting then the ORANGE LED would glow, BUT as Q3 is ON, the ORANGE LED is OFF.

4. As Q2 is ON then Q1 is OFF, so the RED LED is OFF.

5. If V1 now falls to where Q3 is OFF, but is greater than voltage to make D4,5,6 conduct the ORANGE LED is ON and the GREEN LED is OFF. [the RED LED is still OFF]

6. If V1 falls below the conduction voltage of D4,5,6, then Q2 switches OFF and the ORANGE LED goes OFF.

7. As Q2 is OFF, then Q1 switches ON and the RED LED is ON.

Its a V1 voltage status indicator..

Do you follow this.?

 

[neoshinzo]

yes, thanks i understand.

 

[neoshinzo]

hi,
1. If V1 is greater than voltage required to start D8,D9,D10 conducting then Q3 is switched ON and the GREEN LED is ON.

2. As Q3 is ON, D3 is connected to the +V rail, so the ORANGE LED is shorted out and is OFF.

3. If V1 is greater than the voltage required to start D4,5,6 conducting then the ORANGE LED would glow, BUT as Q3 is ON, the ORANGE LED is OFF.

4. As Q2 is ON then Q1 is OFF, so the RED LED is OFF.

5. If V1 now falls to where Q3 is OFF, but is greater than voltage to make D4,5,6 conduct the ORANGE LED is ON and the GREEN LED is OFF. [the RED LED is still OFF]

6. If V1 falls below the conduction voltage of D4,5,6, then Q2 switches OFF and the ORANGE LED goes OFF.

7. As Q2 is OFF, then Q1 switches ON and the RED LED is ON.

Its a V1 voltage status indicator..

Do you follow this.?

hi, what voltage is required to conduct the diodes? i put a voltmeter to measure it but i'm unsure of why its that value and how it turns on the transistor.

 

[ericgibbs]

hi, what voltage is required to conduct the diodes? i put a voltmeter to measure it but i'm unsure of why its that value and how it turns on the transistor

hi,
D10 is a 12V zener diode and D6 is a 10V zener diode.

The other diodes 1Nxxx series have forward voltage drop of about 0.7V.

Some meters have a diode test function.

Typically a 1Nxxx series diode will measure between 500R and 1000R when the DMM leads are connected across the diode and megaohms when the DMM the leads are reversed.
Remember the diode is designed to conduct current in one direction only, thats when the Anode voltage is greater than +0.7V than the Cathode.

The zener diode gives resistance values the same as a 1Nxxx diode.
but where a 1Nxxx diode blocks reverse current. [upto its maximum Reverse voltage], the zener diode will start to conduct when the Reverse voltage reaches it 'zener operating voltage'.
eg: D10 zener will start to conduct current when the Cathode is +12V with respect to its Anode.

In the case of your circuit, the current flowing thru the zeners, at their breakdown voltage, flows thru resistors causing a voltage drop across the resistor, which drives current into the transistor and starts it conducting.

Do you follow this.?

 

[neoshinzo]

what is the voltage required to make the string of leds conduct?

how does d3 short the orange led?

 

[Styx]

all I am going to add to this is

OPAMP's and COMPARATORS shouldn't not be interchanged

 

[ericgibbs]

what is the voltage required to make the string of leds conduct?

how does d3 short the orange led?

hi,
Look at this drawing.

The Orange LED requires about 2V forward in order to conduct [ON]
when D3 is conducting there is only 0.7V across the LED so it cannot light.

Do you follow this.?

 

[ericgibbs]

hi styx,
Copied useful pdf, but the OP is not using any OPA's.

 

[neoshinzo]

ok i see, thank you

 

[neoshinzo]

what about the transistor conducting voltage? is it the input voltage plus the conducting voltage of the diodes?

 

[ericgibbs]

what about the transistor conducting voltage? is it the input voltage plus the conducting voltage of the diodes?

hi,
When the zener paths conduct they create a voltage drop across R5 which switches Q2 ON, the other zener path current creates a voltage drop across R8 which switches Q3 ON.

Does this answer your question, its not clear what you mean.?

Is this a homework project.?

 

[neoshinzo]

no its not for homework, but i understand thanks.