SENSOR OUTPUT SIMULATOR
If you are going to do any work with displays, meters, and oxygen sensors, it is really helpful to have a device that will simulate the output of a sensor. The device should be able to simulate the output of a sensor through the operating range. Unless you use a pressure pot, you cannot check your displays above 100% oxygen. Hence, this project.
The device can be made in an hour or less after you pick up the parts at your local Radio Shack.
You will need the following items to build and use the sensor simulator:
A light weight soldering iron
Thin solder
A 1,000 ohm trimmer potentiometer
A 5.1 K resistor
A 1 K resistor
AA or AAA battery holder
Battery to match
Two small test leads (out of a pack)
Multimeter
This is optimized for a sensor that has a nominal 10 mv output in air. If your sensor has a 20mv nominal output in air, just use three 1 K ohm resistors in series instead of the two resistors shown.
The circuit is called a voltage divider. The whole circuit has a total resistance of 7.1 K. that drops the 1.5 volts of the battery. The pot is a fixed resistor with a center wipe that allows you to measure the voltage drop across a portion of it. The measured voltage across the portion is in the same ratio as the resistance ratio.
The picture below illustrates the location and connection of each of the parts. The output voltage is measured at the points indicated by the yellow lines.
I just soldered the components in series with the leads tight against the battery holder. Then I epoxied them in place to prevent breakage in use.
Check the voltage output of your real sensor in air with your voltmeter. Make sure that the appropriate dropping resistor is in place when you do this test. For my sensor the output must be read across a 10,000 ohm resistor. It is 10.4 millivolts. Now that you know the true output in millivolts of the sensor you can adjust the screw on the end of the pot to make the output 10.4 millivolts. Check the output or the simulator across the center pin of the pot and either end with a voltmeter. Just keep the positive lead closest to the positive battery terminal.To simulate other PPOs calculate the expected output by first finding the scaling factor SF.
.21SF = 10.4
SF = 49.52
Then multiply the desired PPO by the scaling factor to determine what the output should be and dial it in. For example if the desired PPO is for pure oxygen
1 x 49.52 = 49.52 millivolts
And a PPO of 1.6 would be
1.6 x 49.52 = 79.24 millivolts
Now you don't have to get your sensor out of your rebreather to check things and you can exercise your displays well over a simulated 1.6 PPO.
If you are going to build a display, this is a great tool for testing. Sure takes up less space on the bench than a rebreather inflated with pure O2 or your gas mix.
Now if I could just create a dive simulator, complete with the feeling of weightlessness, the sounds and feel of the water, the sights. I think I am going to immerse myself in the bathtub, close my eyes, and dream about being back in the sea.
Cheers,
Tom
