4026 CMOS Decade Counter - Page 1
Forum section: electronics / beginners

author: jobog
author_rank: Regular Contributor
timestamp: April 21, 2015, 09:28:41 pm
content: I've breadboarded a circuit from a "Make: Electronics" book that uses 3, 4026 decade counters and a 3 digit LED display to count from 000 to 999. I'm using a push button switch to increment the count so there is a lot of bounce in the circuit. My problem is that anytime the third digit, the ones counter, lands on 9 the circuit locks up and won't accept any input pulse. As long as that third digit does not stop on 9 the counter works as designed. I've checked over my wiring and can't find anything wrong and I assume if it was not wired correctly it wouldn't work at all. If I connect the 4026's separately they work just fine and switching their positions I get the same result so it's not that one of them has been shorted in some way. I get a pulse from Pin 5, the carry pin, at the appropriate time so it's not that. Is there something wrong in the design of this circuit? All the resistors are 1K.

author: Kjelt
author_rank: Super Contributor
timestamp: April 21, 2015, 09:41:28 pm
content: Only thing i notice is the lack of 100nF decoupling capacitors for each ic.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: April 21, 2015, 09:54:00 pm
content: I can't figure out why they have pin 5 of IC3, which I take it is a carry output, grounded. I don't see how that would cause the symptoms you see but it can't be good.EDIT: Are you using resistors on the segments to limit current? Don Lancaster says it's OK but nothing in the datasheet suggests to me it's OK? Worth a try anyway.

author: jobog
author_rank: Regular Contributor
timestamp: April 22, 2015, 12:08:30 am
content: Quote from: Paul Moir on April 21, 2015, 09:54:00 pm

author: Paul Moir
author_rank: Frequent Contributor
timestamp: April 22, 2015, 03:32:07 am
content: If the decoupling capacitors don't do it, post a photo of your breadboard for a second (well, probably a few hundred) sets of eyes to double check.Inputs should be grounded or tied to Vcc, while outputs should be left floating. Grounding an output is an obvious mistake.

author: Kjelt
author_rank: Super Contributor
timestamp: April 22, 2015, 05:28:31 am
content: Quote from: Paul Moir on April 22, 2015, 03:32:07 amI agree, pin 5 but also pins 4 should be left floating or tied with a high resistor (10k or so) to vcc or gnd.

author: jobog
author_rank: Regular Contributor
timestamp: April 22, 2015, 09:41:58 pm
content: First of all let me thank everyone that has responded.Adding decoupling capacitors did not do anything to solve the problem. Removing the ground from all the outputs, pins 4,14, and 5 on IC3, did not fix the counting problem but did drop the current draw of the circuit from around 90 mA to 20mA.What fixed the problem was finding a bad connector on my breadboard. From the circuit switch S1 is connected to pin 2 of IC1 which is the clock disable. Holding this switch closed puts 9V on pin 2 and keeps the circuit from accepting impulses on pin 1. When the switch is open pin 2 is connected to ground through the 1K resistor and the counting can continue. I noticed that holding down S1 did not stop the counting and a meter on pin 2 showed the 9V was not there. Touching a jumper from Vcc to pin 2 stopped the counting and locked the circuit since the 1K resistor was also not connecting to pin 2. Touching a jumper from ground to pin 2 would allow the counting to resume. Pin 2 of IC1 was inserted properly in the breadboard but was not making contact. Pushing on pin 2 makes everything work properly. Hurrah!!!!My breadboard is a cheap piece of Chinese junk and I've noticed that after putting the lead of a 1/2 watt resistor into a hole the contacts in that hole stay open and a smaller lead is very loose in that hole. What breadboards are people using and what is a good one that wouldn't have that problem?

author: Paul Moir
author_rank: Frequent Contributor
timestamp: April 23, 2015, 03:23:43 am
content: Great troubleshooting! That's an interesting and nasty fault that got created.I for one have not found a decent breadboard in 30 years as a hobbyist. I suspect they're universally bad, but I haven't tried a 3M one yet due to suspicions and sticker shock. Perhaps there are some that are really horrible that I haven't used yet though. And having damage caused by using a component with a largish lead is quite common. 1/2W resistor is a little surprising but if it were a 1 or 2W I would say it was normal.I can tell you they wear out so think of them as disposable. Also intermittents are a fact of life on them. I try to only build a small section of a circuit on them and then rip it up when I get it working properly. By limiting the number of connections on the breadboard, I reduce the likelihood of an intermittent. Not exactly rocket science, but it helps! Also, another thing I find helps is Bud Industries BC-32625 wire kit. Available from Mouser, Digikey, etc, etc. Next time you order check it out. They're nice stranded wires with crimped solid ends.My preferred through hole prototyping method is to use point-to-point rat nest type wiring on perfboard using wire-wrap type wire and something heavier (usually telephone quad) for the power busses. At least you have something reliable when you're done.