# Schmitt trigger

Discussion in 'Homework Help' started by mrkbuddy, May 2, 2013.

1. ### mrkbuddy Thread Starter New Member

Oct 3, 2012
6
0

Hello guys
can any one answer me the the questions below with respect to the circuit diagram. thanks in advance.

a) When Vin= -12 the green LED glows. What is the output voltage polarity and magnitude approx. and why does the red LED not glow?

b) if Vin is now altered from -12V towards +12V at what value of Vin will the output voltage alter in polarity to make the red LED glow

c) Explain how the 7k and 6k resistors provide feedback to control the values of input voltage necessary to alternate illumination of the LED's

Last edited: May 2, 2013
2. ### #12 Expert

Nov 30, 2010
16,026
6,540
The first question is asking whether the output of the amplifier will be positive or negative (compared to the common point, ground) when the input is negative 12 volts, and why only one LED is on at that polarity.

The second question is asking you at what point of moving the input to a more positive voltage will the amplifier switch its output to the other polarity.

The third question is asking how the feedback resistors divide the voltage at the output of the amplifier to create the point where the polarity of the output switches in question 2.

Last edited: May 2, 2013

Oct 3, 2012
6
0

4. ### #12 Expert

Nov 30, 2010
16,026
6,540
This is YOUR homework. We don't do your homework for you. You have to try. Then we would direct you where you have a misconception.

5. ### mrkbuddy Thread Starter New Member

Oct 3, 2012
6
0
a) When Vin= -12 the green LED glows. What is the output voltage polarity and magnitude approx. and why does the red LED not glow?

Output voltage polarity is + 9.41v. I checked the output voltage polarity by using a multi meter to measure the voltage.
The amplifier is connected in inverting mode this changes the polarity from  to + thats why only the green LED glows because the green LED is connected in forward bias mode. Whilst the amplifier saturates it has to consume some of the voltage for it to operate correctly.

b) if Vin is now altered from -12V towards +12V at what value of Vin will the output voltage alter in polarity to make the red LED glow

Vin is 12V and I have tested the output voltage using a multi-meter and the voltage polarity shows  9.21 V. the red LED only lights up because its connected in reverse bias in which the amplifier is connected in the inverting mode so this changes the polarity from + to  Value of Vin is One of the characteristic of a diode it has large input impedance only very little current is used for the op amp to work

c) Explain how the 7k and 6k resistors provide feedback to control the values of input voltage necessary to alternate illumination of the LED's

7k and 6k resistors provide negative feedback to prevent switching back to the other state until the input passes through a lower threshold voltage, thus stabilizing the switching against rapid triggering by noise as it passes the trigger point.

i am not sure am i going on the right direction or not.

6. ### WBahn Moderator

Mar 31, 2012
17,446
4,699
Well, at least you're more honest than many and are willing to state that you just want to cheat and have someone else do your homework for you.

Should you decide that you want to actually learn something, then post your best attempt to solve YOUR homework and we will then try to guide you along a path by which you are able to solve it mostly on your own, which is how you will learn something.

7. ### mrkbuddy Thread Starter New Member

Oct 3, 2012
6
0
i have already tried but i wasn't confident about my work. so asked for some help, i have posted my answers above. and my croclips measurements for each LED.

8. ### WBahn Moderator

Mar 31, 2012
17,446
4,699
Thanks for posting the attempt. I have got to crawl into bed, but if no one else has already done so, I will look things over and provide some feedback tomorrow.

mrkbuddy likes this.