PC/CP200 Electronics Laboratory I

Resistive Sensors and Bridge Circuits

Objectives

1. To measure the resistance of a strain gauge over its range of operation.
2. To measure the resistance of a photoresistor over its range of operation.
3. To set up and test a photoresistor using a voltage divider circuit.
4. To introduce the concept of null measurement.
5. To set up and test a photoresistor using a Wheatstone bridge circuit.

Equipment

• digital multimeter, bench power, dual supply
• breadboard
• various resistors, variable resistor
• to make the strain gauge: overhead transparency, sandpaper, pencil, a pair of alligator clips
  or
  use an existing strain gauge on a transparency
• photoresistor
  
  [yours may differ slightly]
  

Procedure

1. A strain gauge is a resistive sensor where the electrical resistance changes according to the expansion or contraction of the sensor. You will make the strain gauge (or use an existing one). If you are going to make one, you will need a regular overhead transparency that has been roughed up with sandpaper and cut into pieces the size of a business card (5cm by 8 to 9 cm). Draw a symmetric pattern with a graphite pencil on the roughed up side of the card, examples shown below.
   
   
   
   Use alligator clips attached to the ends of the strain gauge as shown above to connect to the probes of a multimeter, so that you can measure the resistance across the gauge with a meter.
   • Measure the resistance of strain gauge at rest on the bench.
   • Carefully pick up the strain gauge by holding the alligator clip leads close to the clips attached to the gauge. Do not touch the exposed metal of the clip. Measure the resistance of the strain gauge with the gauge in a neutral position, flexed in, and flexed out. Make sure to note which side of the gauge is the rough side.
   • What is the relationship between the three positions of the strain gauge -- compression, rest, tension -- and the resistance you measured?
   • Carefully pick up the strain gauge by holding the alligator clip leads close to the clips attached to the gauge. Do not touch the exposed metal of the clip. Measure the resistance of the strain gauge as you gently pull on the leads to put strain on the gauge. As the strain increases does the resistance of the strain gauge increase or decrease?
   • Remove the strain gauge and clip the two alligator clips to each other. What is the resistance of the leads?
     When you gently pull on the leads does the resistance change?
     
     Demonstrate your strain gauge to the lab staff.
     
     
2. Whenever you use a sensor, the first thing you have to do is calibrate the sensor. Since the photoresistor is a resistive sensor, you will measure the resistance when there is no light shining on the photoresistor and when there is no obstruction to light shining on the photoresistor (beware of shadows). These measurements should provide the minimum and maximum resistance under regular light conditions.
   
   As the light increases does the resistance of the sensor increase or decrease?
   
   
3. For this question, use the 5V supply for V_s.
   As discussed in class, sensors can be used in a voltage divider circuit. For the photoresistor, as the light changes, the resistance R_sensor changes and therefore V_out changes. Given your measurements in the previous question and given the voltage divider circuit configured as shown below, as the light increases does V_out increase or decrease? Note: you should answer this without constructing the circuit.
   
4. How would the relationship between increasing the light and V_out change if you switched the locations of R₁ and R_sensor? [Hint: write out the equations for V_out in terms of V_s, R₁, and R_sensor under both configurations.]
   
   
5. Normally, R₁ is selected to give the greatest difference between the output voltages when R_sensor is at its maximum (R_max) and at its minimum (R_min). Theoretically, this is calculated using the equation R₁ = √(R_min x R_max).
   
   Construct a voltage divider circuit for the photoresistor so that V_out increases as the light hitting the photoresistor increases. Decide which configuration of the voltage divider circuit is appropriate to meet the specifications. Measure V_out for the two lighting extremes. How does this compare with your expected results?
   
   
6. In digital logic circuits, anything above 3.5 volts is a logical '1' and anything below 1.5 volts is sensed as a logical '0'. If this circuit was intended to drive a digital logic subcircuit with no light corresponding to logical '0' and full light corresponding to logical '1', can you get reliable operation using this voltage divider circuit? Explain.
   
   Demonstrate your voltage divider circuit to the lab staff.
   
   
7. For this step, you must use the external voltage supply at your station.
   It may look like this:
   
   Or like this:
   
   If you don't have one at your station, temporarily move to a station that has one. The internal bench supplies cannot be substituted for the external supply.
   • Turn the supply on.
   • Adjust the voltage knob on the variable supply to 5 volts.
   • Is the variable supply equal to the fixed 5V supply? Put a voltmeter across the terminals for each supply and fine tune the variable supply voltage to the measured voltage of the fixed supply. Do not touch the metal part of the meter probe with your hands!
     

   Are they really equal? Although both supplies have been set to the same voltage, one supply will be set higher than the other. Set up a null measurement circuit (shown below) by inserting a volt meter between the two positive power supply terminals.
   
   

   • Are the supplies 'balanced'?
   • Which one is higher?
   • Can you balance the supplies more accurately using the null measurement circuit configuration?
     
     
   Demonstrate the concept of null measurement to the lab staff.
   
   
   
8. One application of the Wheatstone bridge circuit is to measure an unknown resistance in terms of three known resistances. The same circuit can be used to obtain a precise measurement of a resistance. In fact, meters use an internal Wheatstone bridge to measure the value of a resistor in an electric circuit.
   
   The bridge shown below has two resistor branches: the left branch with resistors R₁ and R₂ is the reference branch and the right branch with R₃ and an unknown resistor R_x is the evaluation branch. [Normally, for this application the resistor R_x would not be measurable with a normal meter. Since we are interested in investigating the characteristics of the bridge circuit, all resistances will be measurable with a normal meter.]
   
   As discussed in class, if the bridge circuit is in 'balance', the voltage between the midpoints of the two branches is zero (at the point noted as 'Meter') and the branches are voltage dividers where V₂ = V_x.
   
   Construct a Wheatstone bridge with four resistors of the same magnitude but different values. Use a variable resistor for R₃ and use it to balance the circuit (voltmeter at location 'Meter' should indicate 0 volts).
   • Measure the resistances of R₁, R₂, and R₃.
   • Calculate R_x. [Hint: what is the relationship between the resistances when the bridge is balanced?]
   • Measure R_x to verify.
   
   The Wheatstone bridge can be direct-reading, i.e. the value given by R₃ is the same as R_x allowing the value of R_x to the read directly from R₃. If the Wheatstone bridge is to be direct-reading, what values are required for R₁ and R₂?
   
   Demonstrate your Wheatstone bridge to the lab staff. Explain how the concept of null measurement from the previous step is the basis of the operation of the bridge circuit.
   
   
9. Construct a Wheatstone bridge circuit to compare two photoresistors at no light and full light conditions. For optimal operation keep all resistances of the same order of magnitude.

• Where in the bridge circuit will you locate your photoresistors?
• What do the other two resistors have to be to satisfy null measurement conditions? For optimum operation, the two resistors in a branch should be of the same order of magnitude.
• Are the photoresistors identical (hardly likely)?
• Is the range of one shifted from the range of the other?
• Is the range of one contained within the range of the other? (In other words, is the minimum of one higher than the minimum of the other while its maximum is lower than the maximum of the other?)

Demonstrate your Wheatstone bridge to the lab staff. Explain the relationship between the two photoresistors and demonstrate your experimental procedure.

Before you leave the lab, have the lab supervisor sign your lab notebook immediately after your last entry.