555 timer current consumption

[David]

I'm considering building something after years of inactivity.  I joined a Christian rock band as bassist last year.  We haven't gigged yet, but when we do, I will probably be using anywhere from four to six basses during a gig.  I know ABY switches exist and are easy to find.  I have never seen a ABCDEFY switch.  That's OK, I think I found a way around that.

I found an old Forrest Mims electronics book I almost forgot I had.  This book had a drawing of a circuit using a 555 timer that generated a "clean .1 second pulse".  I thought, "Gee, just what I need for the above.  And Tayda has them really cheap."
Yeah, but then I read that 555s use a lot of current.  All the time.  Even when they aren't doing anything.  My proposed toy will be powered by a wall wart, so I'll have power to spare, but I have this...  THING... about egregious power consumption.

So is this an urban legend, or should I maybe look into the 7555?  I know there are a multitude of ways to do switching, but for some reason, doing it this way appeals to me.

[FSFX]

The basic 555 timer takes about 3mA to 4mA of supply current at 5 volts with no load, so a simple oscillator doesn't need much power.
It will take 10mA to 15mA running at 15 volts.
It is only when you drive a low resistance load with it that you will need more power which will apply to the CMOS version as well.

[David]

The basic 555 timer takes about 3mA to 4mA of supply current at 5 volts with no load, so a simple oscillator doesn't need much power.
It will take 10mA to 15mA running at 15 volts.
It is only when you drive a low resistance load with it that you will need more power which will apply to the CMOS version as well.

Ten to fifteen?  For some reason, I thought it would be a whole lot more that that.  If that's the case, I can deal with it because my "toy" is only going to use four of them and the wallwart I plan to use will supply at least an amp at 9VDC.

Excellent! There's a "con" I can scratch off the list.

[GibsonGM]

Even just a couple hundred mA would do it - maybe down the road after confirming how much current your monster requires, you would get a slightly smaller power supply - then you can use the beefy 1A for other projects       

If it's a generic wall wart, you might want to measure the DC voltage it's putting out before you connect it to your 'stuff'.  Unregulated 'warts will deliver their rated voltage (9V) at the RATED current (when actually drawing 1A).  This means they 'sag' to the right voltage.   You COULD see much higher voltage from it than you expect as you're not drawing anywhere NEAR the rated current.    MAX voltage for the 555 is 15v so it could be worth it to see if you have higher than that unloaded.    I personally haven't fried anything this way, but it is possible - maybe I worry too much.  It boggled my mind when I'd read "18V" on my meter from a 9V supply!       Wall warts rated for lower output current tend to have a lower unloaded voltage and will sag to the right level with less loading (of course) - hope that makes sense.

[Rob Strand]

So is this an urban legend, or should I maybe look into the 7555?  I know there are a multitude of ways to do switching, but for some reason, doing it this way appeals to me.

For the basic bipolar NE555 the currents FSFX gave look OK to me.

Most of those CMOS variants, like the ICL7555 and TLC555 devices, only draw 100's of *micro* amps.

When you get to low currents don't overlook the fact the external R1 timing resistor (see datasheet) is pulled to ground  when the output is low.  So if you use a low R1 value it will cause the *circuit as a whole* to pull unnecessarily high current, perhaps even more than the chip.

FWIW, the standard NE555 has three 5k resistors in series across the supply rails; inside the chip.  The CMOS parts use higher valued resistors in those locations in order to reduce the power consumption.   (If you use connect pin 5 to other circuits this difference can make the circuit behave differently with an NE555 and a CMOS 555.)