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555 frequency problem

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jerryd

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Electro Tech forum,

I have a 555 timer circuit built up on a Radio Shack bread board.

It's running in astable mode putting out a square wave that is
high for .5 sec and low for .5 sec and driving a couple of leds.
It works fine with a 9 volt battery and a LM317T voltage regulator.

Recently I replaced the battery and regulator with a 110 to
9 volt transformer(actually puts out about 10vac), a BR805D/TP2
full wave bridge and a 7805T 5 volt regulator with a 220uf
capacitors on the input and output.

This also works fine, until I plug in an SN74HC14 device anywhere
else on the board and power it up. Then the output of the 555
increases about 20% in frequency. Plugging in other types of ic's
like 7447, 7490 ect. don't cause this problem.

There is a .1uf bypass cap on the 7414 from power to ground and
the 5 volt supply doesn't appear to have any glitches on it.

Am I doing something wrong or is there something I don't know
about a 7414?

Thanks for any reply,
Jerryd
 
Post your circuit showing values of the timing resistors, capacitors and what the 555 is driving.

Do do have a bypass cap between the 555 Vdd and Vss pins?
Do you have a bypass on the VCO input?
What is connected to the 7414's outputs? If it is driving a short to ground, it would pull down the 5V line and thereby effect the 555 frequency.
 
MikeMI,

Thanks for the questions.

I do have a bypass on the 555 and a 0.01uf cap on pin 5.

The timing components are 22k and 2.2K resitors and a
44uf(2 22uf in parallel) cap. I'm driving 2 leds on the
output pin, one to vcc through a 330 ohm resistor and one to
ground also through a 330 ohm resistor. The leds blink at
a .5 sec rate.

The input and output pins of the 7414 are floating.

Any suggestions?

jerryd
 
With the inputs of the 7414 floating, you have probably created a mini transmitter. Try grounding them, although being a TTL thing, they should really float high when hanging open. Do you own an oscilloscope?
 
Last edited:
MikeML,

I grounded all the inputs and it seemed to help some.

I don't have a scope so I use my sound card and some audio
editing software. With a voltage divider it works OK.

There may be a hint here. I get a spike on the rising and
falling edges on the output pin when the battery is powering
the 555. But when I use the diode bridge power supply I get
no signal even though the led's still blink. And again I only
have this problem when I'm using the diode bridge power supply.

Can it be a current problem?

jerryd
 
Poor regulation of the supply voltage?
 
MikeML,
It's a "right out of the book" circuit.

I don't think the sound card scope approach gives a
good representation of the power stability. I think
it will always try to block or zero out all dc voltage.
How else could I check it?

jerryd
 
There is a .1uf bypass cap on the 7414 from power to ground and
the 5 volt supply doesn't appear to have any glitches on it.

Yes I have noticed that happens to me too when you put a low-value cap on the power rails.
best way to fix that is to have the two circuits on their own power supply.

-Ben
 
The output of an ordinary 555 causes a supply current spike of 400mA when the output switches.
The datasheet for the LM555 says that it needs two supply bypass capacitors to filter the spike.

Intersil's datasheet for their ICM7555 shows the current spike from an ordinary 555 and no spike from their Cmos 555.
 
Elector Tech forum,

Following audioguru's suggestion I replaced the LM555CN with
a TLC555 cmos version and the problems seem to be cured.
I made sure to use the recommended bypass caps.

I'll probably be back with more questions.

Thanks,
jerryd
 
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