Voltage sensitive 556?

Hi Guys

I’ve made a considerable number of 556 based circuits, usually only the resistors/capacitors vary for timing purposes, and use them to drive a number of leds.

I’ve just now soldered up five identical circuits using the values in the attached schematic but the flash rates of the leds, instead of being as calculated (ie 1 sec on 1 sec off for one set and 0.07 on 0.8 off for the other), varies depending on the supply voltage.

Anything from around double the flash rate at 12v down to the calculated values at 6v and below seems to be the output rates I’m getting. Other than a duff batch of 556s (all components are smd by the way), what could cause that effect?


556.tif (31.9 KB)

What are the specs on those 10uF electrolytics?

Too much current flowing into the Q1 and Q2 bases??
42mA @12V
20mA @6V
556 CMOS versions max output current when sourcing: 15mA..

C2, C4 voltage dependency?? Low voltage ceramic caps used and get "conductive" with higher voltages?? Crazy.. :astonished:

Too much current flowing into the transistor's bases??
42mA @12V
20mA @6V
CMOS versions max output current when sourcing: 15mA..

Those transistors are on the Q outputs, so I can't see that having any influence
on the timing. And the LM556D isn't CMOS, you thinking of the ICM7556 perhaps?

Most electrolytics are poor performers, lossy, leaky and voltage sensitive.
For precision you'd avoid them. A factor of two is pretty high though - are they actually
rated for 12V?

And the LM556D isn't CMOS, you thinking of the ICM7556 perhaps?

How do you know he is not using CMOS versions?
555 is commodity - the sellers do not care, for them what is important is "555" on the package..

The caps are rated 16v (http://www.farnell.com/datasheets/1484859.pdf)

Here's the 556 - its not cmos (http://www.ti.com/lit/ds/symlink/ne556.pdf)

I've got several other assembled circuits, all made with the same components - the only difference I can possibly think is that the 556s are from a different supplier.

I've also noticed that at higher supply voltages, up to around 14v, the output actually becomes a double flash?!!

Fake 556 :slight_smile:
Try to increase the 270ohm resistors to 1k or 2k2, curious what happens..

I hope they're not fake 556s, otherwise Farnell's will have some questions to answer - I've just bought a whole load of them!!

I'll try the higher base resistor but I'm even more puzzled now. I've just timed the led flashes again, and it actually seems like the rapid flash from ICB is fairly stable. Its the even 1 on 1 off from ICA that's the main problem.

This is getting very twilight zone!

Some sort of short to the CV pins? The internal circuitry of the 555 should be pretty immune to supply voltage...

Don't I feel a fool!! :*

I've just discovered the problem. Its nothing to do with the circuit at all, its my &*%^ soon to be ceremoniously destroyed (dismantled) bench power supply! As soon as I use batteries to provide the higher voltage (I've a fair few 11.1v lipos from my RC hobby that are giving 12.2v) the problem goes away - on all the boards. My guess is that the smoothing caps in the power supply are shot and its giving out a "dirty" supply. Mebe an ac supply would upset a 556!