At least two of the original gauges in the Spitfire are not instruments. Namely the fuel gauge and the temperature gauge.
They are far from it. A bi-metal strip with a heater serving as a gauge, powered by a pulsed 12 volt source of some unknown duty cycle is not an instrument.
A gauge with no calibration points is not an instrument. Unless it has a know reference, it is not an instrument. Even if you power these two
gauges with a known, stable 10 volt source, they are not instruments.
The addition of mechanical gauges gets us closer to an instrument. The addition of some warning indicators with a known calibration point
helps us to get closer to an instrument.
Two lamps were added to the Smiths combination oil pressure / temperature gauge. Right red for oil pressure and bright yellow for temperature.
These are driven by a calibrated oil pressure switch, and an added thermal switch on the radiator. Both switches have known trip points.
A low fuel warning lamp was added to the fuel gauge. This lets some people know that a likely reason for the car sputtering
to the side of the road just might be that it is out of fuel. That is if the brilliant blue LED gets anyone's attention..
It is set for two gallons in the tank before the engine quits.
An over voltage indicator was placed in the volt meter. I guess.. just because... no one looks at that gauge either...
So why not make it trip at a known voltage point, currently 15 volts. It can be made to trip at most any value under that. I suppose
low voltage is indicated by the "ignition" (alternator light) or by the car refusing to operate.
Each circuit is latching, so that once tripped, it requires turning the ignition off and then back on to reset the indicators.
These are simple analog comparator circuits and are adjustable. The circuit design and boards were done "in house". They are mounted to
the gauges and match the profile of the gauges. As the wiring for the car is a complete custom install, all interconnections are
done with multi pin connectors.
The fuel gauge has a built in voltage stabilizer. The stabilizer on the rear of the speedometer is no longer used.
Gauge illumination has been converted to LED.
This is what the fuel gauge and voltmeter look like illuminated.
The rear of the fuel gauge.
The rear of the voltmeter.
Solid State Gauge Voltage Stabilizer
The original gauge voltage stabilizer was or is an electro mechanical device. It pulsed the voltage to the gauges so that the average voltage
was around 10 volts. The gauges, fuel and temperature are not actual instruments as they are not calibrated to anything specific. They are
also a simple bi-metal resistive heater unit. They would not care or give a stuff it the voltage that drives them is AC or DC. It simply does not matter
to the gauges. (AC however would give approximately a 30 % lower reading.) The point is that they are a resistive heating element, not a proper meter movement
or meter motor as we have in more modern vehicles. Their one attribute is that they are reasonably rugged and do hold up to open top spirited driving.
However they do require a somewhat stable voltage source. The original units relied on the fact that the "gauges" were slow to react, so a pulsed 12 volt
source approximating 10 volts average seemed to work OK. After all these are not instruments, just relative readings that an operator could get accustomed to.
However the voltage stabilizers over time do fail.
One could gut the original voltage stabilizer and insert a simple 7810 regulator. The specs and wiring can be found in many places on the web.
Pin #1 is input, pin #2 is ground, Pin #3 is output. If you have a positive ground car use a 7910. Feel free to jamb capacitors in there if you feel you must.
They are not required in this application. Those anal enough to take everything in the application notes as gods word will want capacitors. By all means jamb those in there
if you can. It gives a warm and fuzzy feeling I'm sure.
Or you can simply buy one of these. Note the heavy use of capacitors in the commercial units sold.
I won't go into all the reasoning behind this. Having an A/F meter seems prudent at least for initial tuning if nothing else.
I didn't like any of the commercial ones I saw and didn't care for the price either. Somehow I don't see the attraction to all those blue LED's.
I wanted more of an analog meter with some calibration points. Something to give some clue, with a toe hold based in reality and not look like
some off shore addition to the Star Ship Enterprise.
I first started out with a narrow band design, but soon abandoned that. They call it narrow band for a reason.
Researching wide band theory and controllers, I tried to come up with something I liked at a reasonable cost. Also the readings had to be meaningful,
not just a bunch of pretty LED's. I found schematics of analog controllers, but didn't want to build one and then have no way
to calibrate it to make it an actual instrument.
I came across a product by 14Point7. They make a wideband controller module as an OEM evaluation kit. It includes everything one needs
to install a DIY wide band O2 sensor controller. It provides both wide and narrow output to inter face to an existing ECU or in this case simply drive a meter.
They are at: http://www.14point7.com/products/sigma-lambda-controller-oem-eval-kit Frankly I don't know of, and didn't find anywhere else that had this value.
This is truly a kit. You have to assemble components on the carrier board, so a very good soldering pencil or a good soldering station
and soldering skills are a must. (I have the Weller 3000 series re-work station.) This is a photo of the kit as it was just started to be assembled. I'd put only two capacitors on it.
Here it is assembled. When this photo was taken, I'd only attached two temporary wires on it to power it for test.
This will output 0 to 5 volts corresponding to 10 to 20 A/F ratio. So I still need a meter, and a way to turn this on only
after the engine is running. There is concern that if the O2 sensor is hot before the engine is running, moisture in the exhaust could stress crack the sensor.
A time delay based on engine RPM controls power to the controller.
The meter is a two phase movement with a series of 6 range LED's at various A/F ratio points.
The basic components are.
1: The O2 controller
2: The engine run detector
3: The meter motor driver board
4: The LED driver board
5: The meter motor / illumination board
6: The meter face circuit board
7: The meter case and connector that houses items 3 through 6
The 52mm gauge face currently looks like this. There is no LED at 14.7. LED's extinguish in a band centered around 14.7.
This is also a reverse meter movement, as the O2 controller outputs a higher voltage for a lower A/F ratio or richer mixture.
(And I happen to have a few.)
Meter portion on prototype board. I need to invent a different pointer.
The power control / timer board. Not part of the actual gauge. Used to determine that the engine is running before
applying power to the O2 sensor heater control.
Some layers of the gauge....
If I can get the face decal to lie flat on the edges.......
The pointer is from an old pressure gauge. It is painted white. The center is bushed with a section of #20
stainless hypodermic needle to fit the gauge motor.
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