Oxygen Sensor Write-Up

Parts:

• 1 x 14 pin LM324 Op-Amp.
• Resistors = 1KR 0.5W (R2,R3,R4), 390R 0.5W (R5), 10KR 0.5W(R6), 270R 0.5W (R7) and 470R 0.5W (R8).
• 2 x Capacitors (C1 + C2, Monolythic Dip C5K 1uF)
• 3 x 1.8V LED (Green, Yellow and Red)
• 3 x 1N4001 0.7V Diodes (D2, D3 and D4)
  
• Zener Diode 9V1 (D1)
• Vero Board 19x25
• Jumper Wires
• Power Wires = 12V Red, Earth Black and 1V Red

What it Does: Works as a tester for an oxygen sensor.

Calculations: Resistor Calculations. R2, R3, R4 = 12V-1.8V(LED) - 0.7V(Diode) = 9.5V/0.0095A(LED) = 1KR
R5 = 12-9.1V(Zener)/0.0095A = 390R (prefered value. answer was 305 ohms)
R6 = 10KR as stated in diagram (we use this to find the current for R7 and R8: 9.1V/10K ohms =
0.00091A)
R7 = 0.4V / 0.00091 = 470R prefered value (440 ohms)
R8 = 0.23V / 0.00091 = 270R prefered value (250 ohms)


How it Works: Connect the positive of a 12V Vs to the 12V Red wire at the top right of the board and the negative wire of the Vs to the Earth Black wire of the board. Also connect a seperate 1V Vs to the 1V Red wire at the bottom right corner of the board and the negative to the Earth Black wire. Pin 4 of the IC is connected to 12V and Pin 11 is Earthed. Pins 2, 5, 10 and 13 are connected inbetween resistors R6 and R7 leading to Earth.

At first the current passes through the Zener Diode, Capacitor and R5 resistor limiting voltage to 9.1V at the positive rail. This then goes down to each LED having passed through 3 more resistors limiting current at the IC. These all lead to each Output pin of the IC. A Sensor Input that can be adjusted to delover 0V -1V is connected to terminals 3, 6, 9 and 12. When initially hooked up the Green LED will light up. When adjusted to 0.3V the Yellow LED lights up. Then when 0.6V is put through, the Red LED lights up.

Testing the Circuit: Using a Voltmeter.
Each LED should read 9.9V when off.
Pins 2 and 5 should read 0.64V
Pins 10 and 13 should read 0.23V
Pin 11 should read near 0V as it is Earth.
Pin 4 should read 11.2V

Problems: As with everyone that everntually got this circuit to work I had not bridged Pins 2 and 5, and Pins 10 and 13 properly and not at the right points inbetween resistors. Once this problem was rectified my circuit still would not work apart from the LED's lighting up but not in correct sequence. This was because a jumper wire I had hooked up was touching another jumper wire. When I made sure both were no longer touching my circuit then worked as it was supposed to.

Reflection: If I were to repeat this task I would be much more careful with my jumper wire placement (less would be good aswell) and pay closer attention to the diagram for my wire placement with the resistors in mind.