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Defective Solderless Breadboard Causes Confusion

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WA4WZP
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Iron
Re: You can't use a 555 for 1 hour delays
WA4WZP   9/26/2014 12:54:02 AM
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The 555 timer circuit was not an ideal solution, but this circuit was for a consumer electronics product, so needed to be cheap, cheap, cheap! After solving the breadboard problem, I 'solved' the other 'problem' of not being able to reliably use the 555 to generate lengthly and stable time duration mark and space pulses. An article concerning this is almost complete.    

WA4WZP
User Rank
Iron
Re: Erratic solderless breadboards
WA4WZP   9/26/2014 12:43:16 AM
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Yep Bob...been there done that! The contact strip I pried out, and measured for resistance, had not had component lead or wire 'pushed in.'

WA4WZP
User Rank
Iron
Re: Invisible Coatings?
WA4WZP   9/26/2014 12:40:37 AM
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Could be an 'invisible coating.' I had two of the breadboards, ordered from the same supplier at the same time. I measured the resistance of the metal contact strip in board #2 and it showed the same erratic resistance reading. I still have both the breadboards and one of these days (famous last words) I will try to solve the mystery.

jrh_engineer
User Rank
Iron
Re: Invisible Coatings?
jrh_engineer   9/25/2014 7:46:19 PM
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This sounds like leaching of plasticizer from the plastic bag. With a lot of attention being paid to the fact that bags are not bio-degradeable, we often infer that they are immutable. However, when considering electrical contacts I always assume that the bag is likely to have leached at least one unknown chemical.

The contacts didn't need sanding - they needed a clean with an appropriate solvent. Of course, knowing what solvent is appropriate when the contaminant is unknown presents some difficulty ...

BobKoblish
User Rank
Iron
Erratic solderless breadboards
BobKoblish   9/8/2014 3:43:47 PM
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It is easy to ruin a contact on a solderless breadboard by inserting a component lead that's too large. Use a jumper wire that's too large in diameter, or a resistor lead that's too big, and the contact is ruined. AWG24 wire, certainly no heaver than AWG22. And 1/4 watt resistors or smaller, not 1/2 watt or larger -- the larger lead diameter will kill your breadboard contacts.

 

BobKoblish
User Rank
Iron
long 555 timeouts
BobKoblish   9/8/2014 3:23:19 PM
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When long time intervals are required from a 555 timer, the resistor and capacitor values are high. High capacitance values typically require the use of electrolytic capacitors. When electrolytics sit uncharged for a long time, the oxide layer can tend to un-form, causing the capacitor's DC leakage current to increase. This can happen if the capacitor has sat for a long time, either in a warehouse prior to assembly, if the equipment is rarely used, or if the timing function is infrequently used. The capacitor's increased leakage current can change the timeout of the 555 circuit, or in extreme cases cause the timer not to trip at all. Been there, seen that.

When the capacitor is connected in the usual way with its negative terminal grounded and its positive terminal to the timer's Discharge and Threshold pins, it stays discharged until the timer is triggered. It can be useful instead to connect the positive side of the capacitor to the positive supply rail, and the negative side of the capacitor to the 555 chip's Threshold and Discharge pins. This way, the capacitor is sitting fully charged (and, thus, fully formed) until the timer is triggered.

But other commenters are right; there are better ways to generate long time intervals these days, either with a shorter RC time constant and an external counter, one of the new 555 variants with an internal counter, or an output from the microcontroller that seems to be at the heart of most products anymore.

BobKoblish
User Rank
Iron
Re: You can't use a 555 for 1 hour delays
BobKoblish   9/8/2014 12:27:51 AM
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When long time intervals are required from a 555 timer, the resistor and capacitor values are high. High capacitance values typically require the use of electrolytic capacitors. When electrolytics sit uncharged for a long time, the oxide layer can tend to un-form, causing the capacitor's DC leakage current to increase. This can happen if the capacitor has sat for a long time, either in a warehouse prior to assembly, if the equipment is rarely used, or if the timing function is infrequently used. The capacitor's increased leakage current can change the timeout of the 555 circuit, or in extreme cases cause the timer not to trip at all. Been there, seen that.

When the capacitor is connected in the usual way with its negative terminal grounded and its positive terminal to the timer's Discharge and Threshold pins, it stays discharged until the timer is triggered. It can be useful instead to connect the positive side of the capacitor to the positive supply rail, and the negative side of the capacitor to the 555 chip's Threshold and Discharge pins. This way, the capacitor is sitting fully charged (and fully formed) until the timer is triggered.

But other commenters are right; there are better ways to generate long time intervals these days, either with a shorter RC time constant and an external counter, one of the new 555 variants with an internal counter, or an output from the microcontroller that seems to be at the heart of most products anymore.

William K.
User Rank
Platinum
Re: You can't use a 555 for 1 hour delays
William K.   9/6/2014 5:01:17 PM
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TOM is correct, the 555 chip can be made to provide longer time delays, but if you use the very best low leakage parts you still won't get better than within 5% of the target time repeatability. I had to fix some assemblies that used them for longer delays and the first step, after veriying that the board functioned, was to do a complete cleaning with a moisture displacing flux remover. Next came an application of acrylic "ignition system waterproofing" spray. That would usually bring the assembly within the 5% tolerance band, at least enough to be acceptable for the application. A far better system is a cheap crystal oscillator/divider chip such as the CD4060, followed by a CD4040 if a longer delay was needed. The reason for selecting those chips instead of a more exotic one is that they are still multiply sourced and available from many suppliers. And they were cheaper than the high value low leakage capacitor needed to use the 555 circuit for a similar delay.

William K.
User Rank
Platinum
Re: Excellent Advice
William K.   9/6/2014 4:52:41 PM
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Among the engineerrs and techs that I have worked with the solderless prototype blocks have a universal reputation for providing poor connectivity. So if a circuit did not function as expected the breadboard was always the FIRST suspect. This saved lots of time.

Wayne Eleazer
User Rank
Iron
Invisible Coatings?
Wayne Eleazer   9/5/2014 10:56:20 AM
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Some years ago I was working on a riding lawnmower.  The engine had only a weak spark.  I bought a new set of points for the magneto and installed them but then it had no spark at all.

Examination of the magneto revealed no problems.  The new contacts looked beautiful, clean and shiny - and the instructions that came with the points said "Do not sand these points!  They are fine as is and sanding will only damage them!"

But finally, desperate, I lightly sanded the points.  They were no longer quite as shiny - but then I got a nice spark.

The points had been in a sealed package and looked very nice, but I can only assume that at some point, perhaps in the packaging process, some kind of invisible coating had been applied that acted as an insulator.  And the instructions had specifically said not to sand the points!

I wonder how often this occurs, the packaging people perhaps doing something that makes perfect sense to them but proves to be disasterous to the component's use? 

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