555 Problem - New

Discussion in 'Electronic Basics' started by Jim Dye, Nov 18, 2005.

1. Jim DyeGuest

As described previously, I have my first 555 project, a LED flasher that
looks like

http://ourworld.compuserve.com/homepages/Bill_Bowden/555.htm (The one
with
the 9V battery shown)

I have found numerous examples of the same circuit around the net...but
this one looks the clearest.

I have C as 100uF, R1 as 1K and R2 as 22K. Using the calculator, that
should have the LED flashing on and off for about 1.5 seconds each state.

The LED is set to draw 10mA on 9V

I got some solid core wire and was able to get things running....sort of.

Using that diagram above, I have one LED/resistor connected from pin 3 to
(+) and the other from pin 3 to (-).

When I hit the juice, the two lights alternate on and off as expected at
about 1.5 second intervals. However, after a few minutes the lights stop
flashing and the LED connected to (+) remains solid on.

If I remove the LED to (-). The other LED blinks for a while then goes
steady on. If I put the (-) one back and remove the one connected to (+)
it too blinks for a while then goes to solid on.

Any ideas?

2. Bill BowdenGuest

If I remove the LED to (-). The other LED blinks for a while
I would check your battery voltage to make sure it's not dead or below
6 volts. Also you are wasting power using a 1 K resistor for R1. When
pin 7 goes low to discharge the capacitor, it supplies 9 milliamps into
R1, so you have about a constant 4.5 mA loss in R1. I would use 10K for
R1 and 100K for R2 and maybe 22uF for C. That will give you same times
and save 5 milliamps.

There is also another problem that the 555 output will not move all the
way to +9. It will only get within 2 volts, or about 7 volts on a 9
volt battery. This means when the 555 output moves high to 7 volts, you
still have 2 volts across the LED and resistor connected to +9 which
may turn it on. Also, you need a larger resistor for the LED connected
to +9. For 15 mA current, the resistor will be 470 ohms, not 330.

There is a CMOS version of the 555 (I forget the number) that moves all
the way from ground to +V on the output. The dual LED circuit may work
better with that part and both resistors can be 470 ohms.

Another idea is to use 2 LEDs in series connected to +9 with a 330 ohm
resistor. That way the two LEDs cannot light with only 2 volts applied.

Another idea is to use ultra bright LEDs that are bright at a few
milliamps so the little 9 volt battery lasts longer. Raise the LED
resistor values about 5 times and the battery will last much longer.

-Bill

3. Flavius VespasianusGuest

So, as a general rule is less capacitance/more resistance better here?

What might be the down side of going to

10K 1M 2.2uF

??????
I have some of those TLC555s. Might try them.

I have also been fooling around with some 556 (dual 555s). However I get
some funky results.

I was using 1.2K and 12K to get a one second flash and running along side
of the configuration mentioned previously. The 556 did not have the
problem of the flash stopping. However, there was some "residual
current". Every other flash of the 1 second LED gets followed by a very
noticable dim period before final cut off. The same happens on 1.5 on/1.5
off circuit but it is less noticable.

As a general rule would one be better off with one 556 or two 555s?

4. Bill BowdenGuest

So, as a general rule is less capacitance/more resistance better here?
I like to stay in the range of 10K to 100K. If you go higher than 100K,
you have insulationn problems and the resistance drops with any
moisture on the circuit board. If you go lower than 10K, you waste too
much power.
I imagine a 556 would be better in most cases. But, there could be some
feedback problems between the 2 circuits that might be cured by using
two 555s with separate decoupling networks to the power supply. When
you have several circuits on the same PC board that may talk to each
other, it's sometimes helpful to run a small resistor from +V to each
circuit and also a filter cap from the other side of the resistor to
ground. That prevents any noise from one circuit getting into the power
supply line of another circuit. Just choose the resistor so it drops a
small percentage of the supply voltage. Maybe give up 1 volt on a 12
volt supply and let the circuits run on 11 volts.

-Bill

5. Jim DyeGuest

Is that both resistors in the range 10K to 100K or just R1.

Would you see a problem with R1 == 10K, R2 == 1M, C = 2.2uf

6. Jim DyeGuest

Any idea what might cause this oddity....

I tested

R1 = 10K, R2 = 220K, C = 10uF as suggested (1.5S on, 1.5S off)
and
R1 = 120K, R2 = 12K, C = 10UF (1S flash)

on a 555 and all worked fine.

However, I tried it on a 556 (dual 555) and first configuration looks as
expected. However, the second looks like

________^______^^_______^______^^

In other words OFF, FLASH, OFF, FLASH-FLASH, OFF, FLASH, OFF, FLASH-FLASH

I switched the pairs of resistors and got the same result on both sides.

7. Bill BowdenGuest

Any idea what might cause this oddity....

Usually, the problems are associated with the power supply, or inputs
not terminated.

Try using a 1uF cap across the power terminals of the 556 and terminate
all unused inputs to ground or +v. Pins 6,8,2,12 can be grounded if not
used. Pins 3 and 11 can remain open unless you want to apply an
external voltage to change the trigger and threshold voltages. Pins 4
and 10 are resets and should be connected to +v if not used for the
reset function.

-Bill

8. Jim DyeGuest

Do you meen connect the input (+) and input (-) with a 1uF capacitor?

9. Bill BowdenGuest

Do you meen connect the input (+) and input (-) with a 1uF capacitor?

Yes, connect the capacitor from pin 14 to 7. Plus side goes to pin 14.

-Bill