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# Activity: Temperature Control using Window Comparator

## Objective

The objective of this lab activity is to use two high speed voltage comparators as a Window-Comparator and program the TMP01 Low Power Programmable Temperature Controller using this approach.

A Window-Comparator is a circuit configuration, usually consisting of a pair of voltage comparators (inverting and non-inverting), in which the output indicates whether an input signal is within the voltage range bounded by two different thresholds. One which triggers an op-amp comparator on detection of some upper voltage threshold, VREF(HIGH) and one which triggers an op-amp comparator on detection of a lower voltage threshold level, VREF(LOW). The voltage levels between these two upper and lower reference voltages is called the “window”.

## Materials

Solder-less breadboard, and jumper wire kit
1 – 2N3904 NPN transistor
2 – 1N914 small signal diodes
1 – LED ( any color )
3 – 10KΩ resistor
1 – 20KΩ resistor
1 – 470Ω resistor

## Window Comparator

### Background

Consider the circuit presented in Figure 1.

Figure 1 Window Comparator

The circuit uses a voltage divider network, formed of three equal value resistors R1 = R2 = R3. The voltage drops across each resistor will also be equal at one-third of the reference voltage (VREF). Therefore, the upper reference (VREF(HIGH)) is set to 2/3VREF and the lower reference to 1/3VREF.

Considering that we use the When VIN is below the lower voltage level, (VREF(LOW)) which equates to 1/3VREF, the output will be HIGH and D2 will be forward biased. Due to the positive voltage at base the npn transistor, Q1 moves into the saturation. Thus, the output voltage is zero, and the supply voltage will drop on R5 and D3, turning the LED on.

When VIN exceeds this 1/3VREF lower voltage level and it is below 2/3VREF (VREF(HIGH)), both comparators' outputs will be LOW and the diodes reverse-biased. No voltage is applied to the base of Q1,the transistor is in cut-off and no collector current flows through R6 or R5, D3. The output voltage is the supply voltage V+.

When VIN is above the upper voltage level, (VREF(HIGH)) which equates to 2/3VREF, the output will be HIGH and D1 will be forward biased. Due to the positive voltage at base the npn transistor, Q1 moves into the saturation. Thus, the output voltage is zero, and the supply voltage will drop on R5 and D3, turning the LED on.

### Hardware Setup

Build the following breadboard circuit for the window comparator circuit.

Figure 2. Window Comparator breadboard circuit

### Procedure

Use the first waveform generator (W1) as source to provide a Triangular signal with 5V amplitude, 100Hz frequency and 2.5V offset.

Use the second waveform generator (W2) as 5V constant reference voltage.

Supply the circuit using the 5V power supply.

Configure the scope so that output signal is displayed on channel 2 and the input signal is displayed on channel 1.

A plot example is presented in Figure 3.

Figure 3. Window Comparator waveforms

On the plot the “windows” can be noticed when the input voltage is between the upper and the lower voltage references. 