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
Quote:
Originally Posted by Occasionally6
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.)
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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
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.
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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.