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Electronics => Projects, Designs, and Technical Stuff => Topic started by: xtal_01 on January 20, 2025, 07:53:22 pm
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OK ... this probably could have gone under "beginner" ... as electronics is more of a hobby to me. I understand just enough to be dangerouse.
Problem ... I was given an old board and asked if I could "fix it" ... there is a long back story but I will get into that on anther post.
For now, I want to replace any suspect parts. There are three IC's. All common (quad op amp, opto isolator, and timer).
My issue is the NE555N timer. Is is obsolete.
OK ... in my mind (have used 555's forever), they are all the same.
Then I go to Digikey ... NA555P, SA555P, LM555CN, NE555P, NE555P, SE555P
OK, I know I don't want a cmos chip ... just the run of the mill bipolar.
I have read post after post ... looked at spec sheets for hours now ... totally confused!
SE555P has the widest temp and voltage range ... LM555CN says "direct replacement for SE and NE", but has Threshold priority rather than Trigger priority (what ever that means) ...
Friends keep telling me any will work .. just pick one.
This is a 30 plus year old board .... I can't believe it is critical ... but do I just put the number in a hat and pick one?
The only other part I am having an issue with is a 1uF (100V, 20%) metallized polyester cap. The original was made by ELPAC (seems to be out of business). I can't find any that look the same from Digikey or Mouser. I found some NOS but I don't like the idea of using a 30 year old cap ... that is why I am replacing all the caps. Does it matter what I pick just as long as it is rating? Fore example, digikey has a ECQ-E1105JF / P14633-ND ... Plastic Film Cap, Metallized Polyester, 100V, 1uf, 5% Looks completely different (original was axial) but I would think looks mean nothing.
Any advice would be awesome !!!!!!!!
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but has Threshold priority rather than Trigger priority (what ever that means) ...
What I write it's just a guess.
In probably all typical applications Threshold comparator and Trigger comparator never try to drive flip-flop at the same time so which one win in such case is not important.
But may be in some very specific applications it is important and because of this is the information which one wins.
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Go with the LM555CN, it is stated as a direct replacement for the NE/SE555: https://www.ti.com/lit/ds/symlink/lm555.pdf?HQS=dis-dk-null-digikeymode-dsf-pf-null-wwe&ts=1737405322325&ref_url=https%253A%252F%252Fwww.ti.com%252Fgeneral%252Fdocs%252Fsuppproductinfo.tsp%253FdistId%253D10%2526gotoUrl%253Dhttps%253A%252F%252Fwww.ti.com%252Flit%252Fgpn%252Flm555 (https://www.ti.com/lit/ds/symlink/lm555.pdf?HQS=dis-dk-null-digikeymode-dsf-pf-null-wwe&ts=1737405322325&ref_url=https%253A%252F%252Fwww.ti.com%252Fgeneral%252Fdocs%252Fsuppproductinfo.tsp%253FdistId%253D10%2526gotoUrl%253Dhttps%253A%252F%252Fwww.ti.com%252Flit%252Fgpn%252Flm555)
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A search for "LM555 vs NE555" will turn up info on the differences between the two, e.g.:
https://tubetime.us/index.php/2016/04/02/the-battle-of-fives-the-ne555-vs-the-lm555/
Other pages on the topic seem to concur with this conclusion:
From the above description, we can conclude that the main difference between NE555 and LM555 is that the NE555 output timing depends on temperature and supply voltage, and the trigger pin has a higher priority. However, the LM555 output timing is relatively independent of temperature and supply voltage, and the threshold pin has a higher priority.
The LM555 will probably work just fine. For peace of mind I would figure out what the 555 is doing in the circuit and what voltage it is running at. If it's a conventional application like a monostable or astable vibrator I wouldn't worry about using a substitute.
Oh, and maybe just extract and test the NE555 to see if it still works - maybe you don't have to replace it.
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Thanks for the info. I was leaning toward the LM555 just for that reason ... "direct replacement for SE/NE ...." My only other choose would be the SE555 just because it has a wider temperature and voltage range.
It is a loooong story ... I actually can't find anything wrong with this board. 5 volt regulator works (put 12 v on it) .... all caps check out ... resistors check out ... zener checks out.
I don't even know what this came from but as low man on the job, I was told just replace every part on the board if you need to ... hmmmm
OK so I am replacing all the caps, diodes, IC's and regulator ... plus I will re-flow all the joints "just in case" .... don't know what else I can do to it.
Here was what I got when I went to digikey ...
Thanks so much for the advice!
[attachimg=1]
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You can make your own 555, with a cheap MCU that has A/D and a small adapter board. ;-)
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I just overthink everything ......
In the end, a friend took a quick look at the schematic of the board I drew out ... said it was a simple oscillator circuit ... only doing 24 Hz ... so basically anything will work.
Thanks ....
PS ... I did buy a few extra chips ... about time I learned more about electronics :)
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If you just need a simple oscillator, the MIC1557YM5-TR is a good option. Set it up in Astable mode, which only requires one additional resistor and capacitor. Provides a 50% duty cycle.
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If you just need a simple oscillator, the MIC1557YM5-TR is a good option.
If ya wants to pay about 7 times the price of the 555.
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A standard 555 timer is an 8-pin package component that requires 6 external components. So it takes up a lot more real estate. Looks to me like the LM555 costs $1.00, I just checked Digi-Key. This part is 40 cents cheaper in quantity as well.
The MIC1557YM5-TR solution that I posted only requires 2 external parts and is a smaller chip. The part costs $1.40.
LM555 requires 4 additional external components...making up the $0.40 difference in cost.
Not sure where you are getting 7x cost multiplier from. The two solutions are equivalent in cost, but the MIC1557YM5-TR will take up less board area.
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Not sure where you are getting 7x cost multiplier from
mouser,digikey etc,cheapest 555 15p,cheapest MIC1557YM5-TR £1.34,so closer to 9X,more than enough to pay for the 1 extra cap and resistor.
6 external components
6? 1 cap and 1 resistor will do the job,although the classic circuit stretches it to 2 of each,still not a massive amount of real estate
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The CMOS version is very expensive.
Just buy the cheapest one; I guarantee it won't be CMOS.
Also, for electronics, I recommend Tayda Electronics:
bit.ly/tayda
For the login page: bit.ly/taydalog