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Old 06-15-2013, 03:12 AM   #61 (permalink)
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Originally Posted by Occasionally6 View Post
I don't think you are too far off, no. I wouldn't like to pick one out though. I think that should be your decision (sorry).
So, of that list, I won't have any problems (at least for this project)? Some DMMs I have found say 20M ohms, is that going to cause a problem? You mentioned with the voltage divider, having more than 10M ohms could cause an issue with the circuit being too resistive. Would this be the same problem with a 20M ohm DMM?

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Originally Posted by Occasionally6 View Post
You'll need to put together a schematic design and work out what they should be first.
See attachment. I think if you open it in a new window, it might be a little easier to see. Let me know if any of it is wrong. It assumes a Mega2560 board (more about that later, see below )

Quote:
Originally Posted by Occasionally6 View Post
Do you still need the Mega when using I2C, given that I2C will only need two digital pins rather than 6 analog pins?
I am going back and forth between the Mega and Uno. I am trying to figure out if all of my future projects will take up all the pins of the Uno, thus necessitating the Mega. The other reason why I would want the Mega is processing power/memory, being that it has more of it. I am also worried about having too much total current, thus possibly necessitating 2 or 3 Uno boards. This page says the Mega can handle 800mA, but I'm not so sure about that. Total Current (The Uno is on top, the Mega is at the bottom). It's possible I am just misreading it, but it seems to be able to handle more amps. If the total amps is indeed 800mA, then I might go with the Mega, just for the reason of not overpowering the board.

Quote:
Originally Posted by Occasionally6 View Post
Not really. Maybe there's some benefit to keeping the unit isolated from other circuits in the car to avoid noise. The current it draws (it needs to be under 200mA total to protect the Arduino) won't be enough to require a dedicated circuit and it can be connected into an ignition switched supply. I'd (will) try it without one first.
I have a cigarette lighter USB adapter. If I power the Arduino through that, will that be good enough to supply power without over-supplying it? (i.e. 7-12V and <200mA). That would certainly be a lot easier than any other option.

Quote:
Originally Posted by Occasionally6 View Post
I can't recall the last time I bought a relay anyway. They're usually free from the junkyard, especially if purchasing something else.
Would a junk relay work with the Arduino? Would a typical car relay be able to be switched from an Arduino without overpowering an I/O pin (i.e. 40mA to make the connection switch)? I was thinking something like this, if need be:Arduino Relay
_____________________________________
Are voltage drop and forward voltage the same thing? I can't seem to find the answer anywhere.

I've been watching a few of these videos (just to get the basics) Youtube Arduino Basics Luckily for me he reviews DMMs too

On the voltage divider, I'm thinking I might use a 10M resistor and a 5M resistor, that way if the ECM is supplying 14V, it will be less than 5V (so I don't destroy the Arduino) and if it is supplying 11V, it will be more than 3.3V (which will mean it has enough voltage to read properly).

This page helped me with the transistors and associated resistors. Transistor Diagram He mentioned using a 4.7k resistor, but I am still a little confused about how I figure out what I would need for my application.

Alas, I stop here feeling like I have forgotten something...

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Old 06-16-2013, 04:20 AM   #62 (permalink)
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Quote:
Originally Posted by jeff88 View Post
So, of that list, I won't have any problems (at least for this project)? Some DMMs I have found say 20M ohms, is that going to cause a problem? You mentioned with the voltage divider, having more than 10M ohms could cause an issue with the circuit being too resistive. Would this be the same problem with a 20M ohm DMM?
Part of the reason I don't want to say one is "good" or not is that I would have to wade through the DMM specs. to form an opinion. That takes too long for me to want to do it.

As long as it is greater than 10M Ohms (which is a recommended/required DMM spec. for automotive/electronics use I have seen enough times to be confident of it being a useful guideline) it's OK. It will be described somewhere as "impedance". Often with cheaper meters it is 1M.

My understanding is that it is due to the way in which DMMs measure voltage i.e by placing a resistance - a shunt - in parallel with the circuit being measured and measuring the current through the shunt to determine the voltage.

At the extreme, with very low resistance, the shunt would be a short around the load that the voltage drop you are trying to measure is occurring across. That alters the voltage dropped through the load and increases the current flowing through the rest of the circuit, to at least some degree. The higher the shunt resistance (impedance) the less like a short the shunt is.

Quote:
See attachment. I think if you open it in a new window, it might be a little easier to see. Let me know if any of it is wrong. It assumes a Mega2560 board (more about that later, see below )
Without checking the currents that circuit looks OK to me.

Quote:
I am going back and forth between the Mega and Uno. I am trying to figure out if all of my future projects will take up all the pins of the Uno, thus necessitating the Mega. The other reason why I would want the Mega is processing power/memory, being that it has more of it. I am also worried about having too much total current, thus possibly necessitating 2 or 3 Uno boards. This page says the Mega can handle 800mA, but I'm not so sure about that. Total Current (The Uno is on top, the Mega is at the bottom). It's possible I am just misreading it, but it seems to be able to handle more amps. If the total amps is indeed 800mA, then I might go with the Mega, just for the reason of not overpowering the board.
Have you tried playing with a few sketches in the Arduino IDE? That will give you some idea of the size of the sketch. (It's displayed at the bottom of the IDE window after compiling.)

There is some scope to reduce the size of the sketch with sensible programming, for example, by not doing things like declaring float variables(which take up a lot of memory) globally and by reusing the same variable for intermediate calcs., unlike in the really rough sketch I posted previously.

The current limit will be the lowest of individual pin, total or component current limits so maybe you will have a better idea if you look at what you have already. If you use 10mA as an acceptable supply limit for each Arduino pin there will be a possible 80mA on the 7-segment display (all 7 + dp lit); 20mA per is 160mA.

If you use the 4511 to switch the 7-segment, in addition to using 4 vs 8 Arduino pins, the current for the 7-segment is supplied by the 4511 (although still through the 5V regulator on the Arduino board - unless you connected it to an external 5V supply).

I guess if there's any doubt then the Mega leaves more margin for not very much more money.

Quote:
I have a cigarette lighter USB adapter. If I power the Arduino through that, will that be good enough to supply power without over-supplying it? (i.e. 7-12V and <200mA). That would certainly be a lot easier than any other option.
That is probably what I will do also, possibly gutted, possibly simply plugged into a new cigarette lighter socket wired in parallel with the OEM one.

Quote:
Would a junk relay work with the Arduino? Would a typical car relay be able to be switched from an Arduino without overpowering an I/O pin (i.e. 40mA to make the connection switch)? I was thinking something like this, if need be:Arduino Relay
I haven't measured the current through an automotive style relay for some time but I think it's around 100-150mA, so switching it directly with the Arduino won't work.

It can be switched using a transistor (plus a diode - like a 1N004 - across the relay coil, to prevent a voltage spike into the Arduino as the relay coil current stops) in the same way as the 7-segment cathodes. That might be useful for a grille block actuator.

What I had in mind for the Arduino power though is to switch the relay from an ignition switched source and use new wiring through the relay from the battery and to ground to provide power to the Arduino. I don't think it will be necessary to do that though as the USB power from the cigarette lighter socket wiring is easier.
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Quote:
Are voltage drop and forward voltage the same thing? I can't seem to find the answer anywhere.
Forward voltage is the voltage difference across a semi-conductor component (so diode, LED or transistor - all basically forms of diodes) required for it to conduct. It is also a voltage drop, which more generally also refers to the voltage difference across a resistance or other load.

If you connect an LED* to an AA battery (nominally 1.5V for an alkaline dry cell, a little lower, more like 1.3-1.4V for a Ni based rechargeable) it won't light.

*Any of them for this but LED's of various types, particularly colors, vary in their required Vf. A red LED might have a Vf of 1.7V; a blue LED a Vf of 3.0V.

It requires a voltage higher than Vf, like two or more of the AA cells connected in series, to get sufficient voltage for it to conduct and light (and maybe some resistance to limit the current due to any residual difference between supply and Vf voltages).

Quote:
On the voltage divider, I'm thinking I might use a 10M resistor and a 5M resistor, that way if the ECM is supplying 14V, it will be less than 5V (so I don't destroy the Arduino) and if it is supplying 11V, it will be more than 3.3V (which will mean it has enough voltage to read properly).
Sounds good. There's no reason to push the limits, either high or low, of the voltage input.

Quote:
This page helped me with the transistors and associated resistors. Transistor Diagram He mentioned using a 4.7k resistor, but I am still a little confused about how I figure out what I would need for my application.
Hfe is the ratio of current through B-E vs that through C-E. It varies with the current and temperature and is actually a range. You need to calculate the current that the transistor is controlling, find the Hfe for the transistor at that current and use Hfe to determine the current into the base (B-E) that is sufficient to fully switch on the transistor.

If you find the data sheet for the BC337 you should find a plot of Hfe vs current (and the range is often quoted numerically). For the BC337 I have it as 100-630 and I've measured it (DMM test) as ~400.

As long as the current flow through B-E is greater than 1/100 (or down to as little as 1/630 if you were to be precise and use the plot to find the Hfe at the desired current flow) of that through C-E, the transistor will be fully switched on.

A lower current through B-E than that will limit the current flow through C-E, to less than that which is desired, maintaining the Hfe ratio to that determined by the transistor characteristics. You don't really need to know Hfe exactly unless you are operating close to the limits.

You will be switching up to 8 x LED's so the maximum current C-E will be higher than the 20mA he is using for one LED. That means you might want a lower resistance than 4.7k on the current into 'B' i.e. through B-E, to maintain the 100 ratio.

If you use 20mA per segment, 8 x 20 = 160mA. Dividing that by an Hfe of 100 gives 1.6mA minimum required into 'B'. Note that while the maximum current limit for the BC337 is 800mA, the continuous current limit is 100mA. Since you're going to be switching the transistors on and off for half the time 160mA is OK.

With a 5V supply on the switching pin, minus 0.7V (the Vf for that part of the transistor) the current limiting resistor needs to drop 4.3V and limit the current to no less than 1.6mA (but no more than 20mA - without checking again, I think that is the limit for the Arduino to supply per pin; that guy does state 40mA though. I think it will sink that amount of current).

Keep the current into 'B' low so that the heat to be dissipated in the transistor (across the 0.7V drop) is low. I have the spec. as max. 625mW total i.e. including the load current through a ~0.3V voltage drop C-E.

Maybe @ 2.16mA with 2k? That might be a bit low (too close to 1.6mA). Because the transistor will have a duty cycle of 50% you could use a higher current. I think 1.5k (2.87mA) would be OK.

If the current into 'B' is a bit too low, you might get an '8' being displayed as dimmer than say a '1'.

It's sensible to bench test it before soldering anything. A bread board is easiest to do that on but, if you are careful not to short anything, I can't see any reason not to improvise and use the veroboard or prototype shield as a temporary platform, making the links using twist and tape, to test the 7-segment part before connecting it to the Arduino.

Masking tape is cheaper than electrical tape and OK for this. Wipe any glue residue off the component leads with isopropyl alcohol before soldering.

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Alas, I stop here feeling like I have forgotten something...
The circuit is pretty simple. The fun starts in the sketch.
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Old 06-18-2013, 09:01 PM   #63 (permalink)
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Something like this Android app for a phone or tablet?
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Old 06-19-2013, 04:29 AM   #64 (permalink)
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That's cool but if you read back about 5 or 6 pages you will see the discussion of phone apps. and why they're not a solution for measuring slope in a moving vehicle.

What I have done/am doing is not difficult (despite 7 pages discussing it), I'm just messing about with it, rather than finishing it, and then only when I feel like doing so.

Edit: I suggested above prototyping on the shield or veroboard thinking I was the only one crazy/stupid enough to do it on cardboard instead but no, someone else has also done it:

http://lab.guilhermemartins.net/2009...rduino-prints/

Last edited by Occasionally6; 06-19-2013 at 01:11 PM..
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Old 06-25-2013, 07:51 PM   #65 (permalink)
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After watching a few videos about DMMs (reviews, safety, how-to's, etc.), I think I have narrowed it down to the one. Craftsman 82337

Some of the things I'm looking for (based off of the videos and your input) are:
  • 10Mohm or more impedance (which is apparently different from the resistance it can measure, took me a while to figure that out!)
  • Separate inputs for high amp, low amp and voltage readings (for safety, less likely for me to make a mistake)
  • Input error alarm (if I put the lead into the wrong input, it will warn me)
  • Fused inputs (again for safety)
  • Preference for a 400mA low amp input, but 200mA is acceptable


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Originally Posted by Occasionally6 View Post
Have you tried playing with a few sketches in the Arduino IDE? That will give you some idea of the size of the sketch. (It's displayed at the bottom of the IDE window after compiling.)
I downloaded the Arduino zip file. At first, it wouldn't work for me, but it did this time, so I will play with it and see what I can get out of it before I start buying the pieces (which I plan to do at the beginning of next month).

I've been trying to understand the use of the transistors. With some research, I've seen that transistor have a million and one uses, so just to confirm, the transistor is not acting as an amplifier, rather as a switch in order to multiplex each display. In the sketch I will set it up so that the base pin (D37 & D38 in my diagram) will be high when that particular display/transistor will be on and it will switch back and forth. How do I set it up so pins D37 and D38 switch high and low back and forth in the sketch?

I did the calculation for the resistor for each segment of the LED. I am assuming a Vs of 5V and Vf of 3V (per specs) and a max current of 10ma (Don't know what the spec is, but I don't want much more than that). So (5V-3V)/.01A=200Ω. I'll try the circuit with a 200Ω resistor and if it is too dim I'll change it accordingly.

Quote:
Originally Posted by Occasionally6 View Post
You will be switching up to 8 x LED's so the maximum current C-E will be higher than the 20mA he is using for one LED. That means you might want a lower resistance than 4.7k on the current into 'B' i.e. through B-E, to maintain the 100 ratio.

If you use 20mA per segment, 8 x 20 = 160mA. Dividing that by an Hfe of 100 gives 1.6mA minimum required into 'B'. Note that while the maximum current limit for the BC337 is 800mA, the continuous current limit is 100mA. Since you're going to be switching the transistors on and off for half the time 160mA is OK.
Isn't the current flowing through each segment flowing through the collector of the transistor, not the base, or is my diagram setup wrong?

Using your calculations, but assuming the current through the collector, not the base, I've tried to work out the resistance for R2. Ic will be 10mA and Ib will be 20mA, with an Hfe of 100 (20mA * 5V). So my resistor has to be able to drop the current to no less than .2mA (20mA/100Hfe). So the calculation is (5V-.7V)/.2mA = 215Ω. I think I went wrong somewhere...
According to this calculator, my resistor should be 4300Ω, which is pretty close to that guys 4.7kΩ resistor. Not sure where my math went wrong, but this seems to be more appropriate. (I assumed .1A, 100Hfe, 5V supply & .7V drop)

Found this: Resistor Color Code Thought it might be useful in the future to look back upon.
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Old 06-27-2013, 02:41 AM   #66 (permalink)
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Originally Posted by jeff88 View Post
I've been trying to understand the use of the transistors. With some research, I've seen that transistor have a million and one uses, so just to confirm, the transistor is not acting as an amplifier, rather as a switch in order to multiplex each display.
Yes, as a switch.

Quote:
In the sketch I will set it up so that the base pin (D37 & D38 in my diagram) will be high when that particular display/transistor will be on and it will switch back and forth. How do I set it up so pins D37 and D38 switch high and low back and forth in the sketch?
The "joy" of the sketch is that there's not a single "right" way to do it. If you go back a few posts I put up a link to a page from a computer magazine (APC) that was using two 7-segment displays to display a "weather station" output. That has a circuit diagram for the display and Arduino and, most relevant, a downloadable zip file with the sketch that includes how the display is switched. Basically, it sets up an array for each numeral and accesses them as required, assigning them to the pins to perform the switching.

The array used is 21 by 7 but some of those are used to display things specific to the weather station. You will need just 10 by 7 i.e. one for each numeral. That does present another means by which you could indicate a negative slope though; set up an eleventh row (11 by 7 array) to flash a "-" in between the numbers.

The Arduino programming is based on C++ and there are additional information sources around for C++ as well as what is described in the context of Arduino. You might like to use that if you want more info. on (say) arrays.

Quote:
Isn't the current flowing through each segment flowing through the collector of the transistor, not the base, or is my diagram setup wrong?
There is current flow (at times) through both C-E (i.e. Ic) and B-E (i.e. Ib). The current flow from collector to emitter, that goes through the LED, is only present when the current flow that goes through the base to emitter is present.

Quote:
Using your calculations, but assuming the current through the collector, not the base, I've tried to work out the resistance for R2. Ic will be 10mA and Ib will be 20mA, with an Hfe of 100 (20mA * 5V). So my resistor has to be able to drop the current to no less than .2mA (20mA/100Hfe). So the calculation is (5V-.7V)/.2mA = 215Ω. I think I went wrong somewhere...
A bit. With (your) 10mA per LED segment and all the segments on a digit lit (i.e. with an '8' and dp displayed; the worst case), Ic is: (7+1) x 10mA = 80mA.

With an Hfe of 100, Ib is then: 80mA/100 = 0.8mA.

From the pin that is switching the digit, (5V - 0.7V)/0.8mA = 5375 Ohm = 5.375k Ohm. Use the next resistor value that is larger than that.

If you are still stuck, if you go back to the APC weather station link above, and assuming that the display that is being used for that (I haven't looked for the specs. for it) has the same Vf (3.0V) as the one you will be using, you can simply use those resistor values.

Last edited by Occasionally6; 06-27-2013 at 02:51 AM..
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Old 06-30-2013, 03:16 AM   #67 (permalink)
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Quote:
Originally Posted by Occasionally6 View Post
The "joy" of the sketch is that there's not a single "right" way to do it. If you go back a few posts I put up a link to a page from a computer magazine (APC) that was using two 7-segment displays to display a "weather station" output. That has a circuit diagram for the display and Arduino and, most relevant, a downloadable zip file with the sketch that includes how the display is switched. Basically, it sets up an array for each numeral and accesses them as required, assigning them to the pins to perform the switching.
Found it. Wow! How much of the void update () part to multiplex the display can I just copy and paste? Or do I need to add/subtract any of it?

Quote:
Originally Posted by Occasionally6 View Post
The array used is 21 by 7 but some of those are used to display things specific to the weather station. You will need just 10 by 7 i.e. one for each numeral. That does present another means by which you could indicate a negative slope though; set up an eleventh row (11 by 7 array) to flash a "-" in between the numbers.
I get the part about not using the weather station lines, but not sure what you mean by flashing the negative. Would it just flash a negative really fast before the number displays, like old fashion movie theater adverts?

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Originally Posted by Occasionally6 View Post
A bit. With (your) 10mA per LED segment and all the segments on a digit lit (i.e. with an '8' and dp displayed; the worst case), Ic is: (7+1) x 10mA = 80mA.

With an Hfe of 100, Ib is then: 80mA/100 = 0.8mA.

From the pin that is switching the digit, (5V - 0.7V)/0.8mA = 5375 Ohm = 5.375k Ohm. Use the next resistor value that is larger than that.
So I used your equation with 20mA, but substituted it for 10mA and got the 5.3k Ohm answer, but somehow missed it (probably sleepy and/or exhausted and/or distracted )

Amazon is going to love me pretty soon, I've got a DMM, all this Arduino stuff, the car needs new struts and now I might be doing an Arduino buzzer project for my work! Woohoo!
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Old 07-02-2013, 02:49 AM   #68 (permalink)
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Found it. Wow! How much of the void update () part to multiplex the display can I just copy and paste? Or do I need to add/subtract any of it?
I think you can pretty much copy that part straight over. The difference is going to be in getting the data in i.e. swapping the DHT11 sections for however the accel. and gyro. board inputs the data into the Arduino (datasheet for the board?), and in the void loop() where the display is done.

It might be useful for you to look at the included functions directly. You can do that by finding the relevant text in the Arduino library files.

What happens when an include is called is the compiler (pre-compiler) writes that part of the sketch in from the library as if you had written it explicitly. It can be a bit easier to follow with that copied into the sketch.

Quote:
I get the part about not using the weather station lines, but not sure what you mean by flashing the negative. Would it just flash a negative really fast before the number displays, like old fashion movie theater adverts?
If you look at the void loop() in the weather station sketch, the array lines for the degree, C, r, h etc. are used in between the numerals. eg.

digit1 = 11; // show 'degrees C' code
digit2 = 12;

You could do it the same way. (The line for the '-' is already in the weather station array at row 18 anyway.)
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Old 07-03-2013, 05:32 PM   #69 (permalink)
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Old 07-29-2013, 05:16 PM   #70 (permalink)
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I bought one of these and it's not accurate enough to easily be able to tell if the road grade is just a tad positive or negative. It's a good item, but definitely not worth it's weight in a hypermilers car. I hardly ever glance at it. "Seat of the pants intuition" will give more useful information. For anyone curious, accelerating and braking are noticeable using a gravity "level gauge." However if your just coasting down the the road anyway it's good enough. I give it a don't buy:not worth it's weight.

You really need a digital leveling gauge with number readouts that go into the tenths of a degree to be useful.

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Originally Posted by Nevyn View Post

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