Powered by Invision Power Board


Forum Rules Forum Rules (Please read before posting)
  Reply to this topicStart new topicStart Poll

> My Stopwatch Design (reality Check?)
g33kb0ard3r
Posted: February 05, 2012 09:47 pm
Reply to this postQuote Post


Newbie
*

Group: Members+
Posts: 3
Member No.: 16,993
Joined: July 30, 2008




So I drew this design up a while back, and I want to run it by you guys before I order some parts. I'm pretty sure it will work, but I just wanted to make sure.

Schematic (linked because it's pretty big)

The ICs:
7 Segment display
Counter
7 Segment Decoder
Quad AND Gate
NTE977 (says it's not recommended for new designs? What would be?)

Please tell me if I'm way off base. My goal with this was basically to do it without a microcontroller.

Sorry if this is the wrong area to post this in.
PMEmail Poster
Top
Halborr
Posted: February 05, 2012 09:50 pm
Reply to this postQuote Post


Jr. Member
***

Group: Trusted Members
Posts: 64
Member No.: 22,540
Joined: February 14, 2009




Sorry, g33kb0ard3r and this account are the same person. I'm not entirely sure why I have two accounts, but I was too late when I realized it to switch accounts.

(My bad)

This post has been edited by Halborr on February 05, 2012 09:51 pm
PMEmail Poster
Top
Sch3mat1c
Posted: February 06, 2012 03:00 am
Reply to this postQuote Post


Forum Addict ++
Group Icon

Group: Moderators
Posts: 19,610
Member No.: 73
Joined: July 24, 2002




NTE977 = 78L05

Generally avoid NTE/ECG parts, they are expensive replacement/substitute parts.

At a glance, the only thing missing from your schematic is a pull-down on your diode-wired-OR reset logic, and a diode from each AND gate (otherwise the reset is hard driving two AND outputs).

You'll want to use all 74HC logic; there should be a 74HC4511 decoder chip out there.

555 isn't stellar for accuracy, but you aren't exactly setting any records with only three digits so that won't be any problem. (If you wanted to make one literally good enough for official records, all you really need is a quartz crystal oscillator of 1MHz or so, and a few more counters / clock dividers to bring it down to as many decimal points as you wanted.)

I'm not sure if the clock-reset connection will work. That tends to result in "runt pulses", where the counter's value increments, the new value appears on the output, the AND gate sees its active combination and resets the counter, which goes to zero, terminating the pulse. Pulse widths can be 30ns or less, which might not make it to the next counter. You can, however, use a "synchronous reset" type, which I believe is somewhere else in the 74x91-92-93 (x=null,1,2,3) family. You might give 74HC162 a shot -- as a bonus, it's a decade counter, so it already includes the reset logic to generate decimal numbers for you. You could use a '163 for the top counter if you still want counts up to 15:59 (though what the 4511 decodes anything above 9 as, depends on the datasheet).

Tim


--------------------
Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
PMEmail PosterUsers Website
Top
tekwiz
Posted: February 06, 2012 09:57 pm
Reply to this postQuote Post


Forum Addict ++
Group Icon

Group: Moderators
Posts: 28,711
Member No.: 5,746
Joined: July 24, 2006




QUOTE (Halborr @ February 05, 2012 12:50 pm)
Sorry, g33kb0ard3r and this account are the same person. I'm not entirely sure why I have two accounts, but I was too late when I realized it to switch accounts.

(My bad)

No worries. We aren't strict about one account per member...we go by what is posted on any particular account. wink.gif


--------------------
Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
PMEmail Poster
Top
Halborr
Posted: February 07, 2012 04:46 am
Reply to this postQuote Post


Jr. Member
***

Group: Trusted Members
Posts: 64
Member No.: 22,540
Joined: February 14, 2009




Alright, I think found the correct 5V regulator and 74HC4511.

When you're talking about keeping the reset from driving the AND gates high, would it be beneficial to put the diodes before the CK pins on the counters (as in both the AND output and the CK were behind the diode)? You lost me a little bit there talking about the clock reset.
PMEmail Poster
Top
atom
Posted: February 07, 2012 05:32 pm
Reply to this postQuote Post


Forum Addict ++
*******

Group: Trusted Members
Posts: 2,121
Member No.: 21,032
Joined: December 02, 2008




Regulating 9 volts down to 5 volts is a bit inefficient for a battery-powered project, and is kinda pointless anyway. You might as well just use 3 AA or AAA batteries in series to get 4.5 volts, then ditch the regulator. I would think the ICs would run off this just fine.

Btw, the battery symbol in your schematic is upside-down (not that it really matters...).
PMEmail PosterUsers Website
Top
Halborr
Posted: February 07, 2012 05:43 pm
Reply to this postQuote Post


Jr. Member
***

Group: Trusted Members
Posts: 64
Member No.: 22,540
Joined: February 14, 2009




QUOTE (atom)
Regulating 9 volts down to 5 volts is a bit inefficient for a battery-powered project, and is kinda pointless anyway. You might as well just use 3 AA or AAA batteries in series to get 4.5 volts, then ditch the regulator. I would think the ICs would run off this just fine.

Btw, the battery symbol in your schematic is upside-down (not that it really matters...).


I suppose you're right. I don't know why I did it that way, maybe I figured the 555 needed more than 5v for some reason. I'll ditch that.

This post has been edited by Halborr on February 07, 2012 05:43 pm
PMEmail Poster
Top
Sch3mat1c
Posted: February 07, 2012 09:19 pm
Reply to this postQuote Post


Forum Addict ++
Group Icon

Group: Moderators
Posts: 19,610
Member No.: 73
Joined: July 24, 2002




HC logic will function perfectly from 6V (the absolute maximum) down to about 2V (where it still works, but is quite slow and feeble). The plain vanilla LM/NE555 won't do much less than 5V, but CMOS versions (7555, LMC555, etc.) will operate on much the same level as HC logic, and the output is directly compatible (whereas the LM555's output is marginal to drive relatively low voltage CMOS logic).

Tim


--------------------
Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
PMEmail PosterUsers Website
Top
Halborr
Posted: February 07, 2012 10:55 pm
Reply to this postQuote Post


Jr. Member
***

Group: Trusted Members
Posts: 64
Member No.: 22,540
Joined: February 14, 2009




The only problem with going with a 7555 or LMC555 is they're all surface mount. What are the odds of the NE555 working at 4.5V, and would there be a way to make the output more suited for HC logic?
PMEmail Poster
Top
Sch3mat1c
Posted: February 08, 2012 01:03 am
Reply to this postQuote Post


Forum Addict ++
Group Icon

Group: Moderators
Posts: 19,610
Member No.: 73
Joined: July 24, 2002






--------------------
Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
PMEmail PosterUsers Website
Top
tekwiz
Posted: February 08, 2012 11:08 pm
Reply to this postQuote Post


Forum Addict ++
Group Icon

Group: Moderators
Posts: 28,711
Member No.: 5,746
Joined: July 24, 2006




QUOTE (Halborr @ February 07, 2012 01:55 pm)
The only problem with going with a 7555 or LMC555 is they're all surface mount. What are the odds of the NE555 working at 4.5V, and would there be a way to make the output more suited for HC logic?

The NE555 should function at 4.5V. However, an easy way to ensure proper operation is to use 4 cells, for 6V. Place a silicon diode in the positive line from the battery & this will lower the maximum voltage to 5.3V, so it's well within the safe area for HC logic. wink.gif


--------------------
Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
PMEmail Poster
Top
Halborr
Posted: February 09, 2012 01:54 am
Reply to this postQuote Post


Jr. Member
***

Group: Trusted Members
Posts: 64
Member No.: 22,540
Joined: February 14, 2009




Sch3matic - Nevermind, I could have sworn they didn't have any through hole versions.

tekwiz - I had no idea that was a thing.

I'll probably go with the 7555 then to make sure everything's compatible.

PMEmail Poster
Top
Halborr
Posted: February 20, 2012 09:18 pm
Reply to this postQuote Post


Jr. Member
***

Group: Trusted Members
Posts: 64
Member No.: 22,540
Joined: February 14, 2009




Update for potential internet search travelers -

This is the final schematic. I breadboarded it up and after some trouble that included setting the display drivers up correctly (pins 3 and 4 have to be high, pin 5 low) and using a trim pot correctly for the 21.4k resistor, I got it working.

Thanks for the help guys, I'm going to start working on making this into a final product.

This post has been edited by Halborr on February 21, 2012 04:14 am
PMEmail Poster
Top
Halborr
Posted: March 06, 2012 07:16 am
Reply to this postQuote Post


Jr. Member
***

Group: Trusted Members
Posts: 64
Member No.: 22,540
Joined: February 14, 2009




So I built it, and it works as expected...

Except for some really weird behavior. The 10's digit will only display 1 and 0, it just cycles when incremented. It actually counts fine, and the system will count all the way to the maximum value. The tens will just only display 0 or 1. This leads me to believe it is 'after' the counter, either in the display driver or the led display. I switched display drivers around and it didn't change anything, so the chip is fine. Any idea what could be going on here? I'll try and upload a video of the problem if I can get a hold of a video camera. I've gone over the circuit a few times and I haven't found anything obviously wrong yet, but I'll probably go over the suspected areas a couple more times.
PMEmail Poster
Top
Sch3mat1c
Posted: March 06, 2012 03:18 pm
Reply to this postQuote Post


Forum Addict ++
Group Icon

Group: Moderators
Posts: 19,610
Member No.: 73
Joined: July 24, 2002




Well you didn't fix any of the diode gate business, which is sure to malfunction.


--------------------
Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
PMEmail PosterUsers Website
Top
Halborr
Posted: March 06, 2012 05:04 pm
Reply to this postQuote Post


Jr. Member
***

Group: Trusted Members
Posts: 64
Member No.: 22,540
Joined: February 14, 2009




I thought I fixed that because I breadboarded the circuit and it functioned perfectly. Are you talking about the pull down on the OR reset logic? What do you mean by "pull down"?

EDIT: Here's the video.

EDIT #2: Just as a crazy idea, the 10's place (as seen in the video) is the only place being routed by wires instead of entirely by solder traces. I used solid wire because that's what I had laying around. The wires are also very close together and parallel (again, as seen in the video). I couldn't be getting interference on these wires, could I?

This post has been edited by Halborr on March 06, 2012 06:37 pm
PMEmail Poster
Top
Gorgon
Posted: March 07, 2012 09:56 am
Reply to this postQuote Post


Forum Addict ++
*******

Group: Trusted Members
Posts: 3,009
Member No.: 3,861
Joined: December 18, 2005




Your schematics is of little value since you have not included pin numbers and what you've done with all the control inputs to the chips. One thing that obviously are wrong is the naming of the counter outputs vs the decoder inputs.

OK, I suddenly saw the fly shit like pin numbers, so cancel that. (Normally pin numbers are put on the outside of the packages.)

You also need pull down resistors on all the RESET inputs that are controlled by diodes. 10k should be ok. The reset inputs are in fact floating behind the diodes, so over time you'll get a static charging of the signal input, that can't be turned off.

From the video you seem to have a bad connection on the C (and D) lines between the counters and the decoder on the 2nd digit. Since the advancing code to the 3rd digit appear to work after 6 pulses, I would think that you've got a missing connection on C.


TOK wink.gif


--------------------
Light travels faster than sound. That's why some people appear bright until they speak!
PMEmail Poster
Top
Halborr
Posted: March 08, 2012 01:32 am
Reply to this postQuote Post


Jr. Member
***

Group: Trusted Members
Posts: 64
Member No.: 22,540
Joined: February 14, 2009




doh.gif

Thanks for the help, I found two traces that were connected that shouldn't have been. I really need to get in the habit of probing with a multimeter faster. I was looking for connections that weren't made, instead of extra connections.

I also added the pull-down resistors.

I'll put up some pictures sometime when I can make a case. I'm thinking a wooden box with a plexiglass top.
PMEmail Poster
Top
0 User(s) are reading this topic (0 Guests and 0 Anonymous Users)
0 Members:

Topic Options Reply to this topicStart new topicStart Poll

 


:: support us ::




ElectronicsSkin by DutchDork & The-Force