ECE 2074 Lab 06 Schmitt Trigger PDF

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Lab 06 – Schmitt Trigger and Inverting Schmitt Trigger Name: Wayne Thompson By putting your name on this work sheet, you certify that you have neither given nor received unauthorized aid, and what is presented is entirely your own work. No name will result is a score of 0. Show all work and calculations. Include the units for all quantities. Make sure your plots and graphs have a white background. Why are you making me do this? Schmitt Trigger creates hysteresis to eliminate noise near switching point. This lab gives you experience designing a Schmitt Trigger (a comparator with hysteresis) and inverting Schmitt Trigger to solve noise problem. Schmitt triggers are commonly used in circuits that are intended to switch based on the value of a slowly varying analog voltage. Inverting Schmitt Trigger has inversed transfer function and will be used in future labs to create oscillators Design (Schmitt Trigger) 1. Use KCL to design a Schmitt Trigger to the following specs. VLH = 1.8V, VHL = 0.4V. Assume the Vsat = 3.3V and –Vsat = -4.5V. We will adjust the Vsat and –Vsat by change the +5V and -5V sources to satisfy the requirement in LTspice. See the diagram below for requirement. 2. Calculate the Ri, Rf and VO. The current flows through Ri or Rf should be less than or equal to 0.1mA. 3. Create a voltage divider RD1 and RD2 circuit for VO as shown below. The voltage divider branch should have about 0.5mA in current.

4. Design RL so the LED has about 5mA through it. 5. Verify your design (VLH and VHL) by substituting the Ri, Rf and Vo to the formula as discussed in class. 𝑅

𝑅

(𝑉𝑜 = 𝑉𝐿𝐻 𝑓 + 𝑉𝐿𝑂 𝑖 𝑅 +𝑅 𝑅 +𝑅 𝑖

𝑓

𝑖

𝑓

𝑅

𝑅

and 𝑉𝑜 = 𝑉𝐻𝐿 𝑓 + 𝑉𝐻𝐼 𝑖 , Note: VLO is –Vsat, VHI is +Vsat) 𝑅 +𝑅 𝑅 +𝑅 𝑖

𝑓

𝑖

𝑓

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Simulation 6. Create an LTspice schematic for your Schmitt Trigger circuit. Set two voltage sources V1 (for positive) and V2 (for negative) and connect them to form the bipolar power supplies to the LF356 as you did in the previous lab. (V1 connects to Vpos. V2 connects to Vneg). See the diagram below for reference. 7. Set V4 as shown to 0.2V (< VHL) and do a transient simulation to 100ms. If the calculations of your Ri and Rf are correct, the Vout should be at the negative saturation voltage. That is –Vsat. If –Vsat is not at -4.5V, adjust V2 and re-simulate the circuit several times to let –Vsat be about -4.5V. 8. Set V4 as shown to 3V (> VLH) and do a transient simulation to 1ms. The Vout should be at the positive saturation voltage. That is Vsat. If Vsat is not at 3.3V, adjust V1 and re-simulate the circuit several times to let Vsat be about 3.3V.

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9. Edit the simulation command and choose DC sweep. Enter V4 as the example above to fill the “Name of 1st source to sweep” space. Choose linear, start from 0 to 3V with 0.01V increment.

10. Add voltage probes on V4, V+, Vout and Vo. Verify VLH to see if it’s around 1.8V±0.1V as requested. If not, find out the reason. 11. Add cursors to label the voltages (VLH, VHL, Vo, Vsat and –Vsat). 12. Copy your simulation plots below.

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13. Repeat step 9 ~ 12 and sweep DC from 3 ~ 0V. Verify VHL to see if it’s around 0.4V±0.1V. If not, find out the reason.

Design (Inverting Schmitt Trigger)

14. Continue with the same saturation voltage settings (Vsat = 3.3V, –Vsat = -4.5V). Use KCL to design an inverting Schmitt Trigger to have the specs as below. VHL = 2.2V, VLH = 0.8V.

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15.Draw the transfer function to show and mark VHL, VLH, Vsat and –Vsat. 16.After you obtain the Ri and Rf as well as Vfixed, Verify your design (VLH and VHL) by substituting the Ri, Rf and Vfixed to the formula as discussed in class. (𝑉𝐻𝐿 = 𝑉𝐻𝐼

𝑅𝑖 𝑅𝑖 +𝑅𝑓

+ 𝑉𝑓

𝑅𝑓 𝑅𝑖 +𝑅𝑓

and 𝑉𝐿𝐻 = 𝑉𝐿𝑂

𝑅𝑖 𝑅𝑖 +𝑅𝑓

+ 𝑉𝑓

𝑅𝑓 𝑅𝑖 +𝑅𝑓

, Note: VLO is –Vsat, VHI is +Vsat)

17. Design a 5V voltage divider circuit to generate Vfixed. Vfixed will be the Thevenin’s equivalent voltage (VTH), and Ri will be the Thevenin’s equivalent resistance (RTH). The designed RTH should have the same value as Ri. Let’s follow the steps below. (a) Find the attenuation factor α. (2) Use Ri to divide α to find the first resistor. (3) Use Ri to divide (1- α) for the second resistor. Use these two resistor to create Vfixed. 18.Verify the created Vfixed is as expected by calculating the voltage divider output.

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19.Show your complete circuit below.

Simulation 20. Edit your design to LTspice. Connect a voltage source to Vin and use DC sweep simulation to ramp up from 0 ~ 3V and down from 3 ~ 0V on the Vin, similar to step 9 ~ 13. Copy the plots below to show Vin, V+ and Vout.

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21.Add cursors to label the voltages (VLH, VHL, Vsat and –Vsat). Build, Troubleshoot, and Measure 22.Build the Inverting Schmitt Trigger with a Thevenin’s equivalent voltage for Vfixed and Thevenin’s equivalent resistance for Ri, as well as an LED and a series resistor RL. The resistors should be combined with the standard resistors to achieve the desired values. For example, you should use up to three (3) resistor combos (in series) to create a desired value, such as 11.6kΩ. You can use 10kΩ, 1.5kΩ and 100Ω to form one.) . 23.Connect a 10kΩ pot to 5V and ground, so it can be adjusted between 0 ~ 5V. Connect the center pin of the pot to Vin (V-, pin 2 on LF356). Connect a DMM to measure the Vin voltage. 24.Because this is an inverting Schmitt Trigger, the LED should stay on when Vin is lower than 0.8V initially. Slowly adjust the pot to increase its voltage from 0V on Vin. When the LED turns off, record Vin. Verify Vin is around 2.2V. 25. Slowly adjust the pot to decrease its voltage from 5V on Vin. When the LED turns on, record Vin. Verify Vin is around 0.8V. Note: LED turned off at ~.2V, could not determine cause Page 8 of 10

26.Take pictures of your circuit and paste them below.

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Submit your Worksheet as a pdf and your LTspice schematic (.asc file) to Assignment 06 on Canvas.

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