Quick question on 16F628A

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Hi Eric,

I'm using a 40106 inverting buffer, in fact the exact one I use in the old working circuit. Swapped all ICs from old to new to remove chance of bad ones.

Thats the thing that makes me think board problems/miswiring, but I can't find anything.

Al
 

hi Al,
Do you have a 74LS14, 7414 or 74LS04 , 7404 you could try in place of the 40106.?

EDIT:
Are you powering the 40106 from 5V, OK.?
 
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bigal_scorpio said:
Hi Eric, still the same I'm affraid.

Al
Click to expand...

ok,
Try this,,, unplug your 74LS04, leave it out, power up, then connect the ULN inputs one at a time [ jumper wire] to 0V, This should switch off that Digit LED in turn

Try each of the 4 digits in turn,

If this doesnt work I may have to make a house call..
 
Still stay on Eric! Don't think it can be anything else but the TIPs causing this, but since the colon flashes I just don't see how.

With the colons common anode from the minutes I just can't see how that is flashing without at least one digit doing the same.

Well baffled! You will always be welcome for a house call mate. Will tea and bakewell tart do you?

Al
 

hi Al,
That must mean the output of the ULN is close to 0v.! [ you have not connected the common diode pin of the ULN to 0v by any chance.??]

You have my address if all else fails.

E.
 
Hang on, just noticed the colon flashes even when the 40106/7404 is removed! So it can't be the inverter that is making it flash, it must flash by the PIC switching the other 2803.

Still puzzled though.

Al
 
Hi Eric,

All smiles here! Its working now.

I found a miniscule trace of copper between the bottom two pins of the 2803! The Gnd and common diode connections.

Can't believe that caused all that trouble. But boy am I glad its over.

Sorry to be a pain yet again Eric, I bet I keep you busy just helping me out mate.

Thanks again Al
 
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ericgibbs said:
hi Al,
That must mean the output of the ULN is close to 0v.! [ you have not connected the common diode pin of the ULN to 0v by any chance.??]

You have my address if all else fails.

E.
Click to expand...

Hi Eric,

Just seen this post, mine has moved on to next page with my last post so I hadn't seen this one.

We both found the cause of the problem at about the same time eh! Difference is you found it without ever seeing the board and did it purely by deduction! All hail Eric the Sherlock Holmes of electronics.

Anyway I am so glad just to get it going and thankful as usual for all the help.

Kindest regards Al

PS Eric, the tea and buns are always here for you mate, in fact sunday dinner if you like anytime. Al
 
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Doh! Darlingtons?

Hi to all,

I have my circuit built and working now but have one small problem.

My new display has strings of 3 50-50 leds that draw about 85mA per string (per segment) and my circuit does show the time ok but very flickery and with odd sements partially lighting.

The flickering is not just the sort you notice out of the corner of your eye it is very apparent and way too much to be used as the clock it was intended to be.

When I put my original display onto the new circuit to test it then all is well, no flickering or digits partially lighting as on the new display.

All I can think of that is different in the new circuit is the TIP125 darlingtons that I replaced the original 2N3906s with. Are darlingtons slower than standard transistors?

I am wondering if I should be using P Channel FETs instead of the darlingtons, does anyone out there have experience of this? I don't want to rejig the circuit only to find no difference.

Thanks for looking Al
 
hi Al.
As its been pointed out the bigger the multiplexed display area the wider the view angle to the eye, so it will 'appear' to flicker more.

You could try this small change.

Reduce the 10K's in the TIP bases to 2k2 and increase the 1K's to 4k7, also place a 220pF or a 470pF in parallel with the 4k7's. This will improve the turn off speed of the TIP's
 
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Hi Eric,

I will try those mods and let you know if its any different.

I don't want you to be dissappointed with the display if you call for that tea and buns mate!

Al
 
Hi Eric,

Tried the resistor changes and the caps, did the resistors first and tried in stages, but no effect on the ghosting.

I also tried a 20Mh xtal in and apart from the clock being 5 x too fast the flicker is gone.

Don't know if I will ever get all the good bits working at once? hehe

Al
 
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