The reason I purchased these engine oil pressure and temperature gauge was to get the corresponding sensors. The gauges were so cheap ordered directly from China so I thought the purchase would be a good hedge against my homemade multi-function display project should I decided against it. I would have the option to install these two low cost, yet relatively unobtrusive gauges.
both gauges for under $25 shipped
the oil temperature gauge was dead on arrival and it has a small dent on the trim ring
I contacted the seller site and they responded promptly that they will send a replacement one. I decided to open up the gauge to see what makes it tick, and try to repair it. Initially I thought the trim ring is made of plastic that is heat-shrunk onto the gauge. As I tried to ply it off I discovered it is actually made of thin aluminum that is crimped onto the gauge housing to form a trim ring.
I used a flat blade screwdriver to "uncrimp" the aluminum trim ring
a bit of patience and I was able to remove the trim ring assembly with the glass cover
the next challenge is to disassemble the gauge - hopefully without breaking it; here you can see the heat weld with plastic at 3 and 9 o'clock that hold the gauge dial in place; I used an x-Acto knife to cut out the plastic weld bead
surprise, surprise - it is a analog volt meter of sort, but very crudely constructed at the expense of energy inefficiency; note that there is no permanent magnet because it is substituted by energizing the field coil
a demonstration analog voltmeter; note the permanent magnet
once the voltmeter movement is removed one can see why the blue LED back light does not illuminate - because of a broken solder joint
fixed the LED back light wiring - note there are 5 blue LEDs
note the current limiting resistor - as I would discover the sensor resistance is very low
I tack the dial face in place with dabs of plastic adhesive
I was able to reassemble the gauge back together by re-crimping the aluminum trim ring back in place; I rotate the dent to the 6 o'clock position so it is less noticeable
I discover the temperature sensor has a very long latency responding to temperature change - it is an evidence that the low resistance NTC thermistor must had been potted in epoxy like compound; here I heat it up with a soldering iron in order to exercise the needle of the gauge
as the temperature rose the dial moved; the movement is jerky due to the low quality of the voltmeter
The purchase of these two cheap gauge gave me a lot of insight of aftermarket automotive gauges. Most of them are no less better than typical consumer products that are value-engineered to be the lowest production cost. If you expect accuracy you are a fool, notwithstanding if they are the leading name brand or cut rate products like the ones I bought and play with. There is very little information as to the characteristics of the sensors - until you dig and read between the lines. Apparently most of these sensors and gauges are built to be like those from VDO. The sensors typically are in the resistance range of no more than a few hundred ohms.
ignoring the power consumption overhead of the LED back lights the temperature sensor is about 1mA per degree F
As things look right now, I see no reason that I will use these oil pressure and temperature gauges. I am have happy with the multi-function display. I have been spending some spare time to develop the code the multi-function display. I implemented the code to use the joystick for user interface. Initially I just poll the joystick in the main loop of the parameter update, but knowing that I would enhance it later by the use of timer interrupt to make it more responsive. It turns out adding the timer interrupt is relatively painless. There are three timers in the uC. It is important to know that timer 0 is used by many of the Arduino library functions. I chose to use timer 1 for this.
Now the polling of the joystick is handled by a timer interrupt routine. With this the response is immediate. The update rate of the parameters can be set for say once per second.
the mock display for the data logger page navigated here by the use of the joystick
I bought one of this neat USB power meter for just a few dollars - it can be handy for checking the power consumption of mobile devices
more fun Arduino modules arrived from China; some took almost a month
I unwittingly purchase a blue one which I dislike because the text is inverted from the blue black ground; fortunately I realize it and immediately order one with green LED backlight
it is just the similar LCD but with a I2C to parallel bus backpack board
my makeshift software development lab
The benefit of the LCD with I2C interface is it requires only 4 wires instead of about 10. Additionally, the backlight can be turned on or off by software. As I proceed to swap in the new LCD with I2C interface, I realized I have to sacrifice 2 analog pins because they are pin-share with the SDA and SCK for the I2C. Now I am 1 analog pin short and I have to forgo measuring the starting battery voltage.
Switching to the I2C interface was not as straight forward as I expected. It took a bit of work and I eventually got it to work. One down side about the I2C based LCD is it is much slower. This requires more care in how I change the characters on the display as they are updated to minimize the flicker.
While I am now short of one analog input, there is a simple way to add more. There is a 4 channel ADC module with I2C interface for just a few dollars.
4 channel ADC with I2C interface
here is the mock data logger page
I also received the miniature 128x64 pixel OLED display. It is tiny. I purchased the one with yellow and blue as I want to highlight the oil pressure and temperature. Initially it seems that the display is too small to be useful for used as a multifunction display. As I mock up the display screens it is actually not that bad. The best thing about this display is it is very easy to mount and stay discrete.
the mock multifunction display using the OLED display
the mock data logger screen
I am really warming up to to the tiny OLED display; these are the only two modules that need to be mounted onto the accessory track in Isolde
the accessory track that I designed for Isolde
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