[HBQRP] Weird NE555's?

[Sean Pepin]

On 04/14/2011 08:28 PM, Dan Reynolds wrote:
> I cross posted this to 4SQRP also.
>
> I bought some NE555 chips from the watery auction site, they are marked TI
> and some of them are giving me strange results.
>
>   <remainder of quote trimmed for space>

Greetings,
I'm no expert by a long shot, but I'll put forth my advice anyhow. Best 
case I get corrected by a pro. :)
References:
TI's NE555 datasheet - 
http://focus.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=ne555&fileType=pdf
Sentex 555 timer tutorial - 
http://www.sentex.net/~mec1995/gadgets/555/555.html

What you're describing definitely seems odd. Assuming all is well in 
circuit construction you may be right, it might be some bad chips. 
Still, a lot of counterfeit silicon is turning up lately and your 
supplier might not know that what they have to sell is bad. It might be 
worth contacting TI via email and seeing what they have to say. Either 
the parts made by them might be bad, or someone's selling bad parts with 
their name on it, so I'm sure they'd be interested either way!

Here are a few random things to try that came to mind. None should 
really matter, but they're the sort of thing I'd try if I had the 
circuit in front of me.

* Try moving the key
Theory: A key down condition charges C1

* Try a capacitor from pin 5 to ground of about 10nF
Theory: Pin 5 is often CC to ground with a capacitor in other 
schematics. Important maybe?
Thoughts are mixed on the use of a capacitor on pin 5 when it's 
otherwise unused. It's often omitted from designs because it's not 
really necessary, as all it does (I think) is help keep noise out of the 
voltage divider reference. Relevant sections in the reference are the 
pin 5 section of the Sentex writeup and Note A of figure 12 in the 
datasheet. When I first looked into this option it seemed like a 'why 
not try it' idea, but now it seems more like a 'has little or nothing to 
do with it' idea.

* Double check caps and/or reduce C3 to 10μF or lower
Theory: A sweep implies a time constant, or a capacitor discharging 
through a resistance somewhere. Check everything about all the 
capacitors (good grounding, proper connections, etc.) Also, C3 has a lot 
of energy within and makes the load for the 555 hugely capacitive, and 
some strange path in the circuit might be causing it to power the 
oscillator for a short time
Looking over the circuit the only decent size capacitance is C3. This DC 
blocking cap forms a high pass filter with the speaker and the series 
resistor. The higher the value of C3, the lower the cutoff frequency of 
the filter, and as is it's about 13.5Hz. At 10μF the output will be 3dB 
down at about 260Hz, and at 4.7μF it'll be about 560Hz. Either of these 
values should sound just fine, and they will store much less energy than 
the 220μF cap. If there's an oddball defect in the chip or the circuit 
somewhere, this might help.

* Consider redesign where oscillator runs freely and speaker is keyed
Theory: None that makes sense, really
The Sentex website above has a key oscillator of slightly different 
design. In theirs the oscillator is free running, and only the output to 
the speaker is keyed. If you have the components to breadboard this 
circuit I'd give it a go and see if you get the same problem.

This is a really odd problem, I must say. I hope this helps some, or 
perhaps triggers a thought that makes sense of things.

73

Sean AC0VD