Electrolytic capacitors & playing with 556 chip

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?

"Adam Funk"

IIRC, the range is 5 to 15 volts.

.... Phil

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

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

which 556? they are not all the same.

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

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

eg:

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.

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.

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.

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

Thanks.

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).

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