Electrolytic capacitors & playing with 556 chip

[Adam Funk]

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

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

Timer B:
same astable timing arrangement,
R = 100 k Ω, 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?

 

[Phil Allison]

"Adam Funk"

When I supply the whole circuit with 3 V

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

IIRC, the range is 5 to 15 volts.


..... Phil

 

[Michael Black]

"Adam Funk"



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

IIRC, the range is 5 to 15 volts.

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

 

[Jasen Betts]

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

which 556? they are not all the same.

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

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

[works off 9v]

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.

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

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?

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

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?

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

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?

possibly pin 7 is not connected battery negative

 

[Jasen Betts]

"Adam Funk"



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

IIRC, the range is 5 to 15 volts.

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

eg: http://www.ti.com/product/tlc556

 

[Phil Allison]

"Jasen Betts" <[email protected]>


** **** OFF to HELL !!

you bullshitting, putrid pile of Kiwi sheep dung !!!

 

[Adam Funk]

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.

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

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?

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.

 

[Adam Funk]

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.

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.

 

[Adam Funk]

possibly pin 7 is not connected battery negative

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

Thanks.

 

[Jasen Betts]

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

pin 7 is the groun pin of a 556.
-

 

[Adam Funk]

pin 7 is the groun pin of a 556.

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).

 

[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.

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