|Tech Article Title
|Digital numeric LED
Well, I finally got around building and installing the boost/vacuum gauge in the car. Here are the final
pics for the impatient. Readings are in mbar (mili bar). Sorry don't have a picture while under boost since it's hard to drive and snap at the same time. Construction details are next.
The bezel is held in place by little (but strong) rare earth magnet so I can swap it our and put the original cup holder face back if desired to give the car interior a stock look. And yes, the tray still slides in and out just like the original cup holder (good for showing off the circuit board and my handy
For the interested, here are the specs and circuit details.
I used the Motorola MPX4250AP sensor. This one is a single ported transducer reading absolute pressure from 20kPa to 250 kPa (that's 20 mbar to 2500 mbar or 2.9 - 36.2 PSIA). Now to get it to read in gauge pressure I cheat and electronically subtract the atmospheric pressure from the reading. So the final gauge has a range of about -800 mbar to +1500 mbar (21.7 PSIG). This could be too restrictive for some of you mad booster, in which case you can use another sensor from Motorola.
OK parts list:
From Newark electronics www.newark.com
MPX4250AP Pressure sensor part# 07F9899 $18.19
LM10CLN OP-amp and voltage reference part# 07B5851 $ 2.75
PT5101A DC-DC converter part# 35C0569 $11.61
also order the various capacitors and resistor/potentiometers, switches from Newark. I recommend getting a precision 10 or more turns pots (should be ~ 2$ each).
From All Electronics www.allcorp.com
PM-102B 3 ½ digit LED panel meter part# PM-102B $14.95
7 pins jumper strip part# RJ-57 $ 0.50 each
PC-3 perf board part# PC-3 $ 1.50 each
Rare earth magnets your choice $ cheap
You may want to get several jumper strips and several perf boards. They are cheap anyway. I had to splice together 2 jumper strips to get one with sufficient length.
Your total should be less than 80$.
I salvaged some ribbon cables and other parts (including the front bezel) off a old broken CD changer I had laying around.
Here is the wiring diagram with pin number for the components. You can download data sheets for the components from the vendors yourself.
The Motorola MPX4250 series pressure transducers are very nice. Relatively cheap, rugged, temperature compensated and pre-calibrated. This means that you can mount them in the engine compartment using a minimum length of tubing. The signal output will be directly proportional to the pressure and can be computed via a formula given in the data sheet. I got this model which reads absolute pressure so I could also measure vacuum (relative to ambient).
The PT5101A is a nice little DC-DC converter with a wide input voltage (9-38 VDC) and a regulated 5 V output with a max. load of 1 A, more than sufficient for our purpose. This is required to power both the pressure transducer and the panel meter. I chose to use a converter because of efficiency (low heating) and simplicity. You can save a few $$ by using a power transistor and biasing it but that would increase the number of components.
LM10CLN is a nice little op-amp with wide input voltage and capable of running on single rail power. In addition it has a 200 mV voltage reference built in, which is used to zero the gauge. I chose to use +5V to power this chip, though you could power directly from the 12 V rail.
PM-102B panel meter with differential input. I found this cheap panel meter that will complement the interior of our cars. Obviously this is too big to fit though the opening of the cup holder so I had to hack it by disordering the LEDs display from the meter board. This could be a little tricky if you don't have good soldering skills. I would recommend getting a de-soldering pump (about 6$ from All Electronic, not as nice as the 30$ ones but works OK). The meter board can then be mounted flat on the modified cup holder sliding tray. I simply resoldered the LEDs onto a perf board and re-attached them to the meter using ribbon cables, matching the original pin configuration.
How the circuit works
The DC-DC converter provides a regulated 5V supply to all components. The pressure sensor is mounted inside the black box housing the ECU box. Signal and power from and to the sensor is carried via a 3 conductor wire (5V, Ground, Signal). The signal is reduced in the first voltage divider (R1 and R2) so as not to saturate the op-amp. I configured the LM10 as a simple voltage follower feeding into the precision potentiometer. The sweeping output of the pot is fed into the positing signal input of the panel meter. The 200 mV voltage reference output of the LM10 is feed into another pot and then to the negative signal input of the panel meter. This is how I cheated in order to display vacuum (negative signal) without using a dual rail power supply.
Zeroing and setting the gain can be done in several ways. Here how I did it:
Make sure the transducer is under ambient pressure.
a) First I tuned P2 to read zero on the display.
b) Then I connected a syringe to the pressure transducer via a short piece of tubing and applied and held pressure/vacuum and measured the voltage output of the transducer. Since the transducer is calibrated I calculated what the pressure should be and tuned P1 to reflect the proper value on the display. Here you can tune it to read PSI instead of mbar. There is a way to set the decimal point on the panel meter via jumpers as well.
Because I cheated to get gauge pressure, upon removal of the syringe from the transducer, the display will not read zero. Thus you will have to repeat steps a) and b) several times until the display reads correctly. Each time you'll be making smaller adjustment until the errors become negligible. If you don't have a way to apply pressure/vacuum then simply disconnect the sensor and apply a voltage with a couple AA bateries (like #V or 4.5 V) and proceed as in b).
I then mounted the circuit into the cup holder. The LEDs were attached to a piece of plastic via screws so as to allow adjustments for tilt, in and out and up and down. You can see the board with a minimum number of component. I like simple.
I made the front bezel out of a piece of plastic removed from an old CD changer. It's held on by magnets. After mounting the assembly into the dash I adjusted the orientation of the LED display so as to be flush with the dash once the bezel is in place. Electrical and pneumatic connections were done based on the instructions given on the AWE tuning boost gauge install document (available on their web site).
Here's a picture of the sensor assembly in the ECU compartment:
That's it. It was a pain to do at times but it turned out pretty nice. The red display goes well with the stock look and it is very readable even in the day. The meter is rated at 2 refresh per second. I don't find it too distracting even if the display is constantly changing. Besides, I can always turn it off.