Day 9 - Oxygen sensor tester circuit, DC motor

Theory
Op-amp as a comparator
Sometimes we want to compare one voltage with another to see which is larger. A comparator is similar to an op amp because it has two input voltages and one output voltage. But it differs from a linear op-amp circuit because it has a two state output, either a low or a high voltage.
Some examples are as follows -

IF V1 > V2 then Vout=-VEE
IF V1 < V2 then Vout=+VCC

Ref: http://www.youtube.com/watch?v=icFo5Zeydqg
http://www.youtube.com/watch?v=y0Q0ERSP24A&feature=related


DC motor control by using relays

Measure available voltages at each position

When a switch is at the position of N:
A(0V) B(0V) C(12V) D(12V)
When a switch is at the position of F:
A(12V) B(0V) C(0V) D(12V)
When a switch is at the position of R:
A(0V) B(12V) C(12V) D(0V)








H bridge DC motor control
H bridge is commonly used to drive motors. As shown below circuit, two of four transistors are selectively enabled to control current flow through a motor.
An opposite pair of transistors(Tr1 and Tr3) is enabled, allowing current to flow through the motor. The other pair is disabled and can be thought of as out of circuit.
By determining which part of transistors is enabled, current can made to flow in either of the two directions through the motor.

Practical
Oxygen sensor tester circuit

Component lists

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Name	Qty	Unit Price	Details
Resistor R5, R8	2	$0.55	470R, 1/4W, 5% tolerance
Resistor R6	1	$0.55	10K, 1/4W, 5% tolerance
Resisotr R7	1	$0.55	270R, 1/4W, 5% tolerance
Resistor R2,R3,R4	3	$0.55	1K,1/4W, 5% tolerance
Op-amp	1	$2.50	LM324, Low power op-amp wide power supply range 3-32V
Diode D2,D3,D4	3	$0.60	1N4001, VRRM=50V, VF=1.1V IF=1A,IFSM=30A,PD=3W
Zener diode D1	1	$0.66	1N4739, 9.1V, PD=1W @25°C VF=1.2V,IzRm=5.6mA
LED	1	$0.30	5mm Red,VF=2.0V,IF=20mA
LED	1	$0.30	5mm Yellow, VF=2.0V,IF=20mA
LED	1	$0.30	5mm Green, VF=2.0V,IF=20mA
Capacitor	2	$0.54	0.1uF 50V Electrolytic
Connectors	1	$1.50	3pin

<Pricelist from Jaycar Electronics>

Calculations
To calculate the resistance of R5, we assume IzRm=5.6mA
By ohm's law, R5= V/I =(11.4-9.1)/5.6mA = 410.71Ω(nearest value 470R)

To calculate the resistance of R7 and R8, firstly we calculate IT.
IT = (9.1-0.63) / 10K = 847uA
Then by ohm's law, we calculate the resistance of R8 and R7.

R8 = (0.63-0.23) / 847uA = 472.25Ω(nearest value 470R)
R7 = 0.23 / 847uA = 271.55Ω(nearest value 270R)

To confirm the result of the calculations, the voltage divider method can be used.

Firstly we calculate the total resistance
RT = R6+R7+R8
RT = 10743Ω
By using the voltage divider,
Vout1 = VIN x R7/RT = 9.1 x 271/10743 = 0.23V
Vout2 = VIN x (R7+R8) / RT = 9.1 x 743/10743 = 0.64V











To calculate the resistance of R2, R3 and R4, we assume the current of LED is 9.5mA.
By ohm's law, R2 = (12-0.6-1.8) / 9.5mA = 1010.53Ω(nearest value 1K)
Same as R2, R4 = (12-0.6-1.8) / 9.5mA = 1010.53Ω(nearest value 1K)
R3 = (12-0.6-1.8-0.6) / 9.5mA = 947.37Ω(nearest value 1K)

Components layout on the board

How the circuit works

Zener diode provides a stable reference voltage which is 9.1V
Capacitor is used for filtering.

To prevent some risks from connecting power supplies in a wrong terminal, a diode D2 is used.
Capacitor is used from filtering.

R6, R7 and R8 is used for voltage divider which provides the reference voltages to the inputs of the quad op-amp.
R5 is used for limiting current through that node.
R2, R3 and R4 is used for limiting current to each leds.
When the sensor input voltage is greater than 0.63V and if there is no connection between a diode D3 and a resistor R3, a yellow led and a green led can be lit on together.






Quad op-amp is used as a comparator.
If the sensor input voltage is less than 0.23V, a red led lights on.
If the sensor input voltage is greater than 0.63V, a green led lights on.
If the sensor input voltage is between 0.23V and 0.63V, a yellow led lights on.

Test Procedure

1. Connect 0.151V to the node no.1 and Check the available voltages at the nodes 2-16 as shown above diagram

2. Connect 0.540V to the node no.1 and Check the available voltages at the nodes 2-16 as shown above diagram

3. Connect 0.780V to the node no.1 and Check the available voltages at the nodes 2-16 as shown above diagram

Availabe voltages at the nodes 2-16

	Sensor input voltage
Node number	0.151V	0.540V	0.780V
2	0.944V	10.14V	10.11V
3	9.25V	10.14V	10.11V
4	11.29V	11.44V	11.42V
5	11.29V	11.44V	11.42V
6	9.43V	0.93V	0.941V
7	9.99V	8.84V	9.12V
8	0.052V	10.75V	10.85V
9	11.29V	11.44V	11.42V
10	9.99V	10.14V	0.946V
11	9.99V	10.14V	9.28V
12	11.29V	11.44V	11.42V
13	0.005V	0V	9.81V
14	9.99V	8.84V	9.12V
15	9.12V	9.12V	9.12V
16	11.29V	11.44V	11.42V

As shown above video, if  the wire at the node no.7 and no.14 is connected, the yellow led lights on.

Wire the circuit to an engine which goes into closed loop.(Number 8 engine in 108-1066)

Problems

1. Once the yellow led lights on, it continues lighting even the input sensor voltage reaches 0.63volts. Therefore, I traced the available voltages at node no.7 , 13, and 14. The problem was that the wire at node no.7 was connected to the node no.5. After reconnecting the wire to the right place, the yellow lights off when the input sensor voltage reaches 0.63volts.
2. When I check the voltage drop acorss diode D2, the voltage was 0.05volts. To check the condition of the diode, I turned the power off and measured the voltage by dialing the multimeter to the "diode" position. The voltage was still 0.05volts. So the diode D2 was replaced by new working diode. Then the voltage drop across the diode was 0.7volts.

Reflection

I have learned the following things as I build and test the O2 sensor tester circuit.
1. Zener diode can be used for the voltage reference.
2. Diode can be used for protecting the circuit from the back EMF.
3. To get the different voltages other than the power supply voltage, a voltage divider can be used by connecting the resistors in series.
4. When connecting LEDs in the circuit, a resistor should be connected to the LED(current limiting purpose).Otherwise, the LEDs can be blown out.
5. ICs pins(eg LM324) should not be touched by fingers. ICs should be kept in an anti statistc bag. When handling the ICs, wear an anti-statice wrist band. If there is no anti-static wrist band, try to touch the case rather than ICs pins.