suggestions for 3 digit 7-segment display counter.

[stj]

Re: suggestions for 3 digit 7-segment display counter.

keyboards usually are matrix-based when you buy them.
usually 4x3

[mariushm]

Re: suggestions for 3 digit 7-segment display counter.

The most modern mechanical keyboards don't multiplex buttons, or they only multiplex keys that are very far away or with different functions... like A and PgUp for example, combinations that aren't commonly pressed.

You can test the keyboard by simply keeping both SHIFT pressed at same time and typing "the quick brown fox jumps over the lazy dog" or some phrase that has all the letters"

If some keys are multiplexed you won't see them show up on screen. It varies from controller to controller.

[stj]

Re: suggestions for 3 digit 7-segment display counter.

we are talking about numeric keypads (9-0) not full keyboards!

[mariushm]

Re: suggestions for 3 digit 7-segment display counter.

Ah yes, the cheap keypads (4x3 or 4x4 are just boring simple matrix)

Here's an example for 2 cheap models of keypads: https://cdn.badcaps-static.com/pdfs/...58e95cc733.pdf

Some models are available only with 7-8 pins and you have to do some fun stuff with the microcontroller to detect what button is pressed, others have on pin for each button and a common pin (for voltage or ground or whatever).

[Spork Schivago]

Re: suggestions for 3 digit 7-segment display counter.

We were trying to use multiplexing where basically we'd flash the 7-seg displays at a high rate, and using persistence of vision to trick people into thinking all the digits are lit at the same time. This trick reduces the number of pins needed such that a single microcontroller with no external decoders needed to display a number - have it all done in software.

Using a 40 pin ARM processor, there are enough pins likely to drive all three digits directly and not need to multiplex... That's sort of what the old 7107 ADC does...

Wow, cool. Did you guys use an ARM processor? If so, which one?

Nowadays, we have better microcontrollers and smaller microcontrollers with more pins than what we had a decade or two ago. Is it common practice now to drive the 7-segment displays directly and not use any decoders or converters (ie, the 7107)?

[stj]

Re: suggestions for 3 digit 7-segment display counter.

no,you still multiplex them because they are often wired that way internally, go look at the 2digit or 4digit pinouts.

[Spork Schivago]

Re: suggestions for 3 digit 7-segment display counter.

Keypads are easy, and can be done in multiple ways.

The easiest would be to connect every button to an input pin of your microcontroller but that uses a lot of pins.

Another method would be to use the built in ADC (analogue to DC converter) to convert an analogue voltage to a value.

Most PIC micros have 10bit ADC and some have built in voltage references or can use input voltage as maximum voltage. So for example, you can power the PIC with 5v and you can configure the voltage reference to 4.096v and then the ADC will return a number between 0 and 1023, basically 4 mV per step.
If you use the 5v input voltage as "reference", you just divide 5v by 1024 and multiply with the value the ADC gives you, to get the voltage on the ADC pin.

So now you can simply use resistors to create voltage dividers and whenever you push a button, you create a voltage divider which sends a voltage between 0v and 5v on that ADC pin and your microcontroller measures it.




See TIP 7 in the first PDF I linked to :



Whenever you press a button, there's always two resistors on the "top" side in series, and the fixed resistor on the bottom side. So the voltage the ADC pin sees is always (adapting the formula above)

Vout = Vin x [ Rbottom / (Rbottom + Rtop1 + Rtop2) ]

So just by using different resistors for each column and each line on that 4x4 matrix of buttons, you can make it so every time you press a button, you'll get a different voltage on the ADC pin (for example 16 buttons, 16 different levels, so you aim for 1024 / 16 = 64 steps or 64 x 4mV = 256 mv but you want some safety margin on both sides, like around 20mV ...

First button should output something between 0v and 256mV, with safety margins anything between 20mV and 230mV ... let's say aim for 0.15v
Second button should output something between 256mV and 512mV, with safety margins anything between 280mV and 490mV, let's say aim for 0.35v

.. and so on..

The problem with this scheme is obviously that you'd need a lot of 1% tolerance resistors and different values, which increases the cost of this.

You can do a sort of hybrid of this by splitting the matrix in 2 (2x4) and use 2 adc pins and scan half the keyboard first and then scan the other half

Or you can separate the columns at the top and power only one column at the top using a pin for each column (so for 4x4 matrix you'd have 4 output pins and one input pin used) and you'd need 4 consecutive adc conversions to determine if a button is pressed. You save money on resistors and you only need a few different resistors, but it takes more time to scan all the keys.

Note that these schemes save pins but obviously when you press two buttons at the same time, you get garbage, can't detect two or more buttons at same time.

If you want that, cheap options would be to use port extenders or shift registers that can work the other way around, meaning they have 8 or more input pins and you can read the pin states by using two pins on your micro (clock and data) ... this way, each button is connected to an input pin and your microcontroller reads the 8-16 pin states every time into a variable, so multiple buttons pressed at same time is no problem.

Ex 74HC597D https://www.digikey.com/product-deta...2822-ND/763089 (8 parallel in, 2 serial out)

You can also do hybrid style like use one output pin to send voltage to 2x4 buttons and read the 8 bits from the shift register, then turn off pin and use another output pin to power the other 2x4 buttons which are also connected to the same shift register... read the 8 bits again and you have the state of all 16 buttons using one shift register and 4 mcu pins.

Slightly more expensive (but cheaper than microcontrollers) are port expanders: https://www.digikey.com/short/3dcb2j
You get 8/16/16+ i/o ports that you can read state or set state through i2c or spi (2 pins on your micro) but they may also give you some benefits that your micro may not have (if it's on the cheaper side), like interrupt on change (so instead of constantly reading button states in a loop, just let the port expander IC interrupt the cpu through i2c and basically say hey a button state has changed)

Newer PIC micros have interrupt on change on specific ports or pins, so if you connect 8 buttons to 8 io pins on a port and then when someone presses a button, you automatically get an interrupt and make it so a function in your code is executed at that moment, instead of constantly checking if buttons are pressed.

That resistor / ADC thing is ingenious! Who ever came up with that was really using their brain. I don't mind spending some money and I am definitely all about going a little more complicated route to gain experience. I like the idea of being able to press two buttons at once. I love the idea of using interrupts. I haven't used them before but I've read up on them.

I'm thinking of using the shift register and using the interrupts. I haven't played with SPI or I2C too much, so this would be a great time to learn about them. You guys have been more than helpful and have given me some great ideas.

A quick question about resistors and tolerances though. My understanding was the tolerance meant that the resistor could be off by whatever percentage the tolerance was when it was made. For example, a 100k resistor with a 5% tolerance would be 100k +/- 5%. Here's my question though. If I have a 100k resistor with a 5% tolerance, if my DMM shows the resistance to be 100k, that resistor is 100k, right? That tolerance doesn't mean that the value of the resistance can change + or - 5% over time, right? It'll always read 100k? If so, then the 1% resistors (if I went that route) might not be needed. I'd just have to sort through my fairly large collection of resistors to find ones that measure really close to what they're supposed to measure....

Thanks!

[stj]

Re: suggestions for 3 digit 7-segment display counter.

tolerance can mean how much it can drift at different temperatures.

btw, if you look at the tiny arm pcb i linked,
http://www.ebay.co.uk/itm/291693366485

this may be good!!
http://electronut.in/stm32-start/

[Spork Schivago]

Re: suggestions for 3 digit 7-segment display counter.

Ah yes, the cheap keypads (4x3 or 4x4 are just boring simple matrix)

Here's an example for 2 cheap models of keypads: https://cdn.badcaps-static.com/pdfs/...58e95cc733.pdf

Some models are available only with 7-8 pins and you have to do some fun stuff with the microcontroller to detect what button is pressed, others have on pin for each button and a common pin (for voltage or ground or whatever).

I plan on using a 12-button one, like the one in your link (0 - 9, #, and *). Maybe the # I'll use as a clear key and the * as an enter key, or something similar? My Make: Electronics Component Kit 2 came with one ( https://www.amazon.com/Make-Electron.../dp/B00H8G1224 ). I have both kits, 1 and 2. Bought them when Radio Shack was going out of business. I got a digital copy of the book that goes with it and it's an okay book. A little different than a normal book, in the sense that you make the project first and then learn how it works. It's not the most in depth book either, but a good starter book. I've used the book and badcaps (when I needed clarification) to learn some stuff.

First thing was how to burn out an LED. It didn't say we were burning out the LED until after you burn it out. If I knew we were going to burn out the LED, I wouldn't have done it! But eh, I learned what voltage divider circuits where and how to calculate the resistor value needed to get the voltage I desired.

[Spork Schivago]

Re: suggestions for 3 digit 7-segment display counter.

tolerance can mean how much it can drift at different temperatures.

btw, if you look at the tiny arm pcb i linked,
http://www.ebay.co.uk/itm/291693366485

this may be good!!
http://electronut.in/stm32-start/

Thank you for explaining the tolerance. So I guess 1% would be needed then.

I don't mind using an ARM processor, but I do not want to use a development board. I want to etch the board myself and do all the soldering, etc, at least for this project.

[stj]

Re: suggestions for 3 digit 7-segment display counter.

the dev board is for developing

[Spork Schivago]

Re: suggestions for 3 digit 7-segment display counter.

Yeah, but I prefer to etch my own boards. We got a laser now, and I want to try the transparency / laser printer / UV exposure thing. I think it'll be fun.

I was looking at some PICs and I think the PIC16F886 will work, especially if I use I2C or SPI for the keypad. It's fairly cheap and I have a generic PicKit3 (not made by MicroChip). I have everything I need to program the chip. I might even have a 28-pin socket around here somewhere that I might be able to put it in. I think Mariushm recommended the PIC16F886 as well. Kinda neat my searching came to the same PIC he recommended.

That PCBWeb Designer software is very lacking. I want to find something better. The main issues I've found is the component library isn't very large. It has the PIC16F886 in it, but the component isn't fully defined and it wants me to edit the part. A lot of components seem to not be fully defined and I have to edit them to add a footprint and all that jazz.

Another issue, it just seems to not have a lot of features. It just seems really limited. The copyright on the About page shows 2014. So maybe it's been abandoned or something. Guess it's time to go back to searching for a good free (or fairly cheap) PCB layout / schematic editor.

I want one where the PCB layout is linked to the schematic editor, like Eagle does. So if the schematic shows a resistor connected to an LED, in the PCB Layout editor, the two will be connected with a trace and I just have to move it around on the board.

[stj]

Re: suggestions for 3 digit 7-segment display counter.

your pretty much limited to eagle & kicad if you want a big upto date library and dont want to spend a shitload of cash!

[Spork Schivago]

Re: suggestions for 3 digit 7-segment display counter.

Do you know Kicad? How hard was it to learn? I didn't like how I had to add the footprint to every part I added.

[stj]

Re: suggestions for 3 digit 7-segment display counter.

i use eagle, i need to get around to trying kicad - i have it installed.

[mariushm]

Re: suggestions for 3 digit 7-segment display counter.

A quick question about resistors and tolerances though. My understanding was the tolerance meant that the resistor could be off by whatever percentage the tolerance was when it was made. For example, a 100k resistor with a 5% tolerance would be 100k +/- 5%. Here's my question though. If I have a 100k resistor with a 5% tolerance, if my DMM shows the resistance to be 100k, that resistor is 100k, right? That tolerance doesn't mean that the value of the resistance can change + or - 5% over time, right? It'll always read 100k? If so, then the 1% resistors (if I went that route) might not be needed. I'd just have to sort through my fairly large collection of resistors to find ones that measure really close to what they're supposed to measure....

Thanks!

Some clarifications...

The 1% tolerance or the 5% tolerance means the value of the resistor will be within -1% .. +1% or within -5% .. +5%

In real world however, what usually happens is that manufacturers try to make all resistors as precise as possible and then may or may not use laser trimming to make the resistors even more precise.

Also these days the manufacturing process today is good enough that most resistors that are made are already precise enough, so there's generally little interest to bin the resistors aggressively.


In the case of +/- 1% resistors, you'll probably find that if you take 1000 resistors out of a box, most will be around below +/- 0.3-0.7%
Maybe if you take 1000 and measure the resistance of each resistor, you may find out that there's no resistor within +/- 0.2% , it could be possible that the manufacturer took those out to sell them as +/- 0.5% tolerance

In the case of +/-5% resistors, basically it's kinda of whatever is less precise than 1% ... you'll get anything between +/- 1% to 5% but from experience the majority of 5% resistors I used are usually within 1.5%...2.5%

Another thing you have to keep in mind is temperature fluctuations... it's some value advertised in ppm (parts per million) ... 5% resistors' value often varies more with temperature change, the resistance drifts more towards the edges of that 5% tolerance or may even exceed it.

Here's a good video about resistor values and tolerance, you can see how resistor values are distributed in a box of resistors : https://www.youtube.com/watch?v=1WAhTdWErrU


Some notes about multimeters :

What you measure depends multimeter's accuracy , depends on the number of counts the multimeter has and depends on whether the multimeter has a RELative mode or not.
The probes you use have their own resistance which is small but it's there (usually less than 0.2 ohm) so usually it doesn't matter when you try to measure a 100 kOhm resistor (the tiny resistance won't change the result too much).
However, for proper measurements you would normally short out the probes of the meter by touching them together and then pressing the REL button on the meter when the display is stabilized.

Then, the value on the multimeter will depend on the meter's count number and it's accuracy.

For example, you may have a 4000 count multimeter rated for a (+/- 1% +2) accuracy ... that's +/-1% + 2 digits.
A 4000 count multimeter would typically auto-range like this :

0.000 ohm .. 400.0 ohm
400.1 ohm .. 3.999 kohm
4.000 kohm .. 39.99 kOhm
40.00 kOhm .. 399.9 kOhm
[..]
So if you measure a 100 kOhm resistor, the multimeter may read 100.06 kOhm or 100060 ohm but you only see 100.0 kOhm on screen (and you may think the resistor is perfect at 100 kOhm.

Also remember the accuracy which could be +/- (1% + 2 digits)

1% of 100'000 ohm is 1000 ohm, so the multimeter may output anything between 999 kohm and 101 kohm PLUS 2 digits and technically it does its job right (in reality, the meters are usually calibrated to measure better than extremes of 1%.

I have a Uni-T UT61E which is 22000 count and has +/- 0.5% +10digit accuracy on the 100 kOhm range.

So on screen, a 100 kohm would show up as 100.00 kohm (the 22000 count allows for an extra decimal) and the better accuracy would also reduce the error of the multimeter... this means my multimeter may show on screen a value closer to the actual value of the resistor.


Another important note

For reading voltages using ADC pins on micros, you want the resistors that form the voltage divider to be relatively small, let's say below 10kohm ... with high values there's too small current flow for the adc to properly sample data in reasonable amount of time.
So I'd suggest using something like 1000-4700 ohm as the bottom resistor and then maybe use 100 / 150 / 220 / 330 / 470 ohm etc resistors at the top, values from E24 (easy to buy) and whatever is 1% and that would give you those voltage steps when doing the math.
The microcontroller documentation (datasheets) has some notes about this.

[Spork Schivago]

Re: suggestions for 3 digit 7-segment display counter.

i use eagle, i need to get around to trying kicad - i have it installed.

I went for KiCad again. I found a lot of how-to videos on their website and I guess I'll just have to get used to it. The layers on the PCB confuse me a bit still. Like F.brd and B.brd or whatever they are. It's just going to take some time adjusting to something other than Eagle. I like how it's free and there's a Windows and Linux version.

The first quick-start video I watched showed a guy creating a new part, completing a schematic, and a PCB layout. I can just rewatch it a few times if I get stuck.

[Spork Schivago]

Re: suggestions for 3 digit 7-segment display counter.

Some clarifications...

The 1% tolerance or the 5% tolerance means the value of the resistor will be within -1% .. +1% or within -5% .. +5%

In real world however, what usually happens is that manufacturers try to make all resistors as precise as possible and then may or may not use laser trimming to make the resistors even more precise.

Also these days the manufacturing process today is good enough that most resistors that are made are already precise enough, so there's generally little interest to bin the resistors aggressively.


In the case of +/- 1% resistors, you'll probably find that if you take 1000 resistors out of a box, most will be around below +/- 0.3-0.7%
Maybe if you take 1000 and measure the resistance of each resistor, you may find out that there's no resistor within +/- 0.2% , it could be possible that the manufacturer took those out to sell them as +/- 0.5% tolerance

In the case of +/-5% resistors, basically it's kinda of whatever is less precise than 1% ... you'll get anything between +/- 1% to 5% but from experience the majority of 5% resistors I used are usually within 1.5%...2.5%

Another thing you have to keep in mind is temperature fluctuations... it's some value advertised in ppm (parts per million) ... 5% resistors' value often varies more with temperature change, the resistance drifts more towards the edges of that 5% tolerance or may even exceed it.

Here's a good video about resistor values and tolerance, you can see how resistor values are distributed in a box of resistors : https://www.youtube.com/watch?v=1WAhTdWErrU


Some notes about multimeters :

What you measure depends multimeter's accuracy , depends on the number of counts the multimeter has and depends on whether the multimeter has a RELative mode or not.
The probes you use have their own resistance which is small but it's there (usually less than 0.2 ohm) so usually it doesn't matter when you try to measure a 100 kOhm resistor (the tiny resistance won't change the result too much).
However, for proper measurements you would normally short out the probes of the meter by touching them together and then pressing the REL button on the meter when the display is stabilized.

Then, the value on the multimeter will depend on the meter's count number and it's accuracy.

For example, you may have a 4000 count multimeter rated for a (+/- 1% +2) accuracy ... that's +/-1% + 2 digits.
A 4000 count multimeter would typically auto-range like this :

0.000 ohm .. 400.0 ohm
400.1 ohm .. 3.999 kohm
4.000 kohm .. 39.99 kOhm
40.00 kOhm .. 399.9 kOhm
[..]
So if you measure a 100 kOhm resistor, the multimeter may read 100.06 kOhm or 100060 ohm but you only see 100.0 kOhm on screen (and you may think the resistor is perfect at 100 kOhm.

Also remember the accuracy which could be +/- (1% + 2 digits)

1% of 100'000 ohm is 1000 ohm, so the multimeter may output anything between 999 kohm and 101 kohm PLUS 2 digits and technically it does its job right (in reality, the meters are usually calibrated to measure better than extremes of 1%.

I have a Uni-T UT61E which is 22000 count and has +/- 0.5% +10digit accuracy on the 100 kOhm range.

So on screen, a 100 kohm would show up as 100.00 kohm (the 22000 count allows for an extra decimal) and the better accuracy would also reduce the error of the multimeter... this means my multimeter may show on screen a value closer to the actual value of the resistor.


Another important note

For reading voltages using ADC pins on micros, you want the resistors that form the voltage divider to be relatively small, let's say below 10kohm ... with high values there's too small current flow for the adc to properly sample data in reasonable amount of time.
So I'd suggest using something like 1000-4700 ohm as the bottom resistor and then maybe use 100 / 150 / 220 / 330 / 470 ohm etc resistors at the top, values from E24 (easy to buy) and whatever is 1% and that would give you those voltage steps when doing the math.
The microcontroller documentation (datasheets) has some notes about this.

Okay, good to go. I didn't take into account the accuracy of my DMM. It's old and probably needs to be calibrated as well. It does have the REL button and I use that regularly. Didn't ever think about the probes having resistance, I always just thought my DMM wasn't properly calibrated! Thanks for pointing that out.

I want to look into using SPI or I2C for the keypad. So, with that in mind, the 1% resistors would only be if I planned on using the ADC of the microcontroller, and not SPI or I2C, right? I'll go through and re-read everything probably tomorrow. It's been a busy day and my mind's all garbled now!

I'm leaning towards I2C right now. I'd still need to debounce the keys, wouldn't I? I know how to do it via software, but I never found a good hardware way to debounce buttons. I've seen a couple ideas out there, but none of them seemed to do what I wanted. I just wanted the buttons debounced every time I hit them. There was a hardware solution which used transistors (I think), but I couldn't get it work the way I wanted. I'd hit the button, and it'd register as a hit, but hitting it a second time never registered again. If I remember though, this was the way it was designed to work. There were two buttons, if that matters.

I don't really like the just wait a few hundred milliseconds and hope they can't push really quick software solution.

[mariushm]

Re: suggestions for 3 digit 7-segment display counter.

- . I think it would help NOT quoting my huge messages every time you reply. -

I recommend trying out DipTrace ... may be easier to understand than KiCad and it's free for simple circuit boards (2 layers , 500 pin count or something like that)
There's good tutorials (easy to follow and learn) inside DipTrace and on their website.


As for debouncing, for just 1-2 buttons, I generally find it enough to just use a resistor and a capacitor .. the resistor would limit the current and charge speed and the capacitor basically blocks the erroneous hits..

5v ... [ resistor ] ---[button ] ---[ capacitor to ground ] -- [ input pin ]

For example let's say 1kohm for resistor , 1uF ceramic for capacitor.

When you press the button, the capacitor slowly charges to 5v. If you have some random 1s and 0s and 1s, the capacitor simply filters those 0s and it slowly charges.
When the capacitor charges up to around 0.7v or 2v (threshold depends from micro to micro) you get the HIGH / 1 on that input pin.
When the button is released, the internal resistance on that pin of the micro (and whatever protection circuitry is there) will slowly discharge the capacitor back to nothing, so very quickly you'll have LOW/0 on the pin
It's basically just a few us delay between pressing/releasing button and getting the changed state.

In software, some very simple debouncing technique would be to just read the pin state multiple times and change the internal state when enough 1s or enough 0s are read.
For example, have a byte variable (0..255) .. every 1 read adds to the variable, every 0 decreases the variable by 1 .. but make sure you don't go below 0 or above 255. When you have a number higher than let's say 10 you know maybe 10 out of last 15 reads are 1s so the button is probably pressed. When the number goes down to 0, then the button is probably not pressed.

A simpler version could be made which uses more memory in theory (probably works best with 2 or 4 bytes for each input) but saves cpu cycles because you don't have to check if number is higher than 255 or lower than 0 when adding or substracting
For example, you have an unsigned int (16 bit variable) that's 0 initially .. every time you read the button state you either shift to left and add 1 if the button is 1, or you shift the number to the right if the value is 0.
When you have 8 consecutive bits on the right (basically you do something like number & 0xFF == true or something like that), you're fairly confident that the button is pressed.
Shifts and additions are fast (1-2 cycles) while if then else would use more cycles.

Other solutions ... read about schmitt triggers and how could be used to debounce ... there's even special ICs to do debouncing (but usually expensive and actually meant to be used for high frequency switches not manual push buttons)

Here's a good paper (includes my simple RC filter solution to hardware debouncing on page : https://cdn.badcaps-static.com/pdfs/...6fc31016ff.pdf

Here's another good page: http://www.labbookpages.co.uk/electronics/debounce.html

I probably could have saved 15 minutes of my life and just paste these links but i didn't think of googling it before answering.

[stj]

Re: suggestions for 3 digit 7-segment display counter.

i cant use something with pincount limits