Electrolytic capacitors & playing with 556 chip

I wired up a 556 (two 555 timers on one chip) as follows:

Timer A: astable circuit with equal mark & space width, R = 267 ?, C = 1000 µF (electrolytic with 25 V rating), output: 320 ? in series with an LED, also tied to the reset pin on Timer B

Timer B: same astable timing arrangement, R = 100 k ?, C = 10 nF (non-electrolytic), output: piezo speaker in series with a resistor

I powered this concoction with a 9 V battery, and got more or less the expected result: the speaker alternates between silence & a high-pitched noise, and the LED lights up when the speaker is on. The first few cycles of timer A are a bit irregular (it stays on for a few seconds before first switching off), but then it evens out.

When I supply the whole circuit with 3 V (two AAAs) instead, however, (and reduce the resistance in series with the LED accordingly), the LED stays on all & the speaker whistles all the time.

My questions:

Is tying the output of timer A to the reset pin of timer B the most correct way to switch B on and off with A?

One of the books of 555 circuits says that electrolytic capacitors only work properly as capacitors above 10% of their rated voltage. Is this why timer A doesn't work on a 3 V supply?

Even with the 9 V supply, the LED doesn't go completely off when the speaker does --- it's just noticeably dimmer than when the speaker is on. Why?

--
When a man tells you that he got rich through hard work, ask him
whose?                                           --- Don Marquis
Reply to
Adam Funk
Loading thread data ...

R = 267 is asking for a lot of current from the chip. You may want to try using a 2.67k resistor and a 100uF cap, or even a 26.7k and a 10uF.

Also, an astable multivibrator with a 555 uses two resistors and one cap to set the frequency and duty cycle -- what's the other resistor value?

Have you bypassed the control inputs on both timers?

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
 Click to see the full signature
Reply to
Tim Wescott

"Adam Funk"

** Where does it say a 555 will work at 3 volts??

IIRC, the range is 5 to 15 volts.

.... Phil

Reply to
Phil Allison

Yes, and that was the obvious thing I noticed about the post.

I've never actually tried the 555 at a lower voltage.

Perhaps the CMOS 555 will work at lower voltages.

Michael

Reply to
Michael Black

Dang. I missed that point.

I checked on DigiKey, and there's a butt-load of timers with '555' in their names, many that operate down below 3V. One (the ZSCT1555 by Zetex) even claims to work down to 0.9V.

But, as Phil will point out amid tiresomely repetitive swear-words if I don't mention it, they're not NE555 chips, which, of course, is what the OP must have meant even if he didn't say, because that's what Phil assumed.

And the SE/NE555 is only specified down to 4.5V

--
Tim Wescott
Control system and signal processing consulting
 Click to see the full signature
Reply to
Tim Wescott

which 556? they are not all the same.

[works off 9v]

Some 556s need a 4.5v supply to function correctly.

it seems perfectly acceptable there are several other ways to get a similar result.

I've never heard of that, I've used a 250V capacitor on a 9V circuit before with no obvious problems.

possibly pin 7 is not connected battery negative

--
?? 100% natural

--- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net
Reply to
Jasen Betts

the CMOS versions will usually run from a lower voltage, fairly weak in the output stage though

eg:

formatting link

--
?? 100% natural

--- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net
Reply to
Jasen Betts

Now I'm not sure where that number came from. I just checked what I'd left on the breadboard and it's 10 k?. (The book also recommends using R between 1 k? and 1 M?.)

The circuit I was using (on both timers, but with different R & C values) was taken from the book _IC 555 Projects_ as follows:

Vcc to the Vcc pin

Vcc connected to reset on timer A

output connected to the top of R

bottom of R connected to trigger, threshold, and top of C

bottom of C and ground pin connected to ground

output of timer A connected to reset on timer B

discharge pin unconnected on both timers

with the description "equal mark space ratio" and a total period of T = 1.4 × R × C.

--
Bob just used 'canonical' in the canonical way.  [Guy Steele]
Reply to
Adam Funk

Thanks to all of you. I checked the data sheet for what I have (TI NE556) and the recommended operating range is 4.5 to 16 V.

--
In the 1970s, people began receiving utility bills for
-£999,999,996.32 and it became harder to sustain the 
 Click to see the full signature
Reply to
Adam Funk

You mention in another response that you're using an NE555, which has (I'm pretty sure) a TTL-style totem-pole output, which, in turn, does not pull the output all the way to VCC.

That circuit will work DANDY on a CMOS version of the 555, because you can pretty much count on the output going nearly to the rails.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
 Click to see the full signature
Reply to
Tim Wescott

Right, the discharge pin is not connected at all in this circuit.

Thanks.

--
When a man tells you that he got rich through hard work, ask him
whose?                                           --- Don Marquis
Reply to
Adam Funk

pin 7 is the groun pin of a 556. -

--
?? 100% natural

--- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net
Reply to
Jasen Betts

I assumed Jasen was refering to the equivalent 555 pin numbers.

But in the circuit I was using, 556 pin 7 was connected to the battery negative (and 14 "Vcc" was connected to the positive).

--
Mathematiker sind wie Franzosen: Was man ihnen auch sagt, übersetzen
sie in ihre eigene Sprache, so daß unverzüglich etwas völlig anderes
 Click to see the full signature
Reply to
Adam Funk

Thanks for that tip! If I get around to doing anything serious, I might buy some of those.

--
I heard that Hans Christian Andersen lifted the title for "The Little
Mermaid" off a Red Lobster Menu.                         [Bucky Katt]
Reply to
Adam Funk

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.