555 woes (monostable oscillator)

On May 25, 6:21 pm, snipped-for-privacy@earthlink.net wrote: [... about LM555 ...] Google on LM555 and datasheet, you are likely to find better information about the chip.

What voltage (DC), do you see on pin 2, pin 6 and pin 3?

If pin 2 is less than 1/3 Vcc, the output should go high If pin 6 is more than 2/3 Vcc, the output should go low.

Which isn't it doing?

No, the pin 7 is th eopen collector. Pine 3 is push pull.

to my knowledge the 555 does not have an open collector on the output.. Try putting the input of your scope on DC and use the

10:1 setting on the probe.

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HTH

Stanislaw.

I haven't checked your reference, but from the description I assume you want a square wave generator rather than a triggered monostable. If you're not locked into a 555 and can accept a duty cycle that's not exactly (but close to) 50%, then you can make an extremely simple squarewave generator out of a CMOS schmitt inverter, such as the 74HC14. Take one of the gates, R from output to input and C from input to 0V. That's all. Many datasheets provide the equation for calculating R and C for a required period, or maybe someone here could oblige....? If you don't need the other 5 gates, don't forget to tie the unused inputs to one or other rail. Or you can run several of them in parallel as a buffer to increase the drive capacity.

You *do* realize that for a *monostable* pulse generator you must keep clicking the switch to get a square wave? The pulse will be just 250 mSec wide. If you have the astable circuit (which I see is #3), then it should oscillate.

Make sure you have a good bypass capacitor on the 5 VDC supply. It is not shown in the schematic but is critical for reliable operation.

Good luck,

Paul

Getting beat up by a 555...omg... :P

If R1 is turned to 0 ohms then the cap cannot discharge. (Assuming R1 connects between the discharge pin and V+). The astable cct will not oscillate. Does the 555 get hot? Make sure R1 is turned to something the 555 can tolerate. D from BC

Is it a CMOS version (eg. LMC555, TLC555 7555) rather than LM555/NE555? If so, you *must* tie 4 to Vcc. Otherwise, it's just a really good idea.

You're only expecting to get a single pulse when the switch is pressed, right?

These questions really belong in sci.electronics.basics, BTW.

Best regards, Spehro Pefhany

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The 555 has been obsolete for two decades now. Use a PIC instead.

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Yes, the reset pin 4(4,10 for 556) is not shown as connected in the schematic, and is almost always connected directly to pin 8(14 on 556). The equivalent schematic for the LM556 does not show an internal pullup on this pin.

Also, the control voltage pin 5(3,11 on 556) should be bypassed with a capacitor. Makes it more stable if the power supply is noisy. Tell us what voltages you read at each of the pins on your 556. Might as well fix this here rather than moving to SEB.

Paul

Oh come on... Has mixed mode electronics gotten that PICy?

D from BC

Your component values yield an oscillation frequency of about 4Hz, so you will want something like 100ms per division time scale to view the pulse train.

Is there a CMOS 556? Sounds rare...

With the right RC values, a PIC makes an ok oscillator.

I see, and minimum astable frequency of 1.2MHz at 5V too...looks like someone is using a lot of these "hobby" chips.

Not rare. TLC556; about 30,000 in stock at Digikey. Probably others.

Best regards, Spehro Pefhany

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speff@interlog.com             Info for manufacturers: http://www.trexon.com
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555's are for kids.. Now PIC's..that the Arrfff! Arrrfff! muscle machine of choice... :)

However.. I don't care if 555's have a newbieish reputation, sometimes they fit the problem and I include them in designs. D from BC

There's a new rule here in the s.e.* hierarchy. Nobody's allowed to say "Use a Pic" without providing a circuit and a program. ;-)

Cheers! Rich

For example:

Use a PIC! Put 2 resistors on an I/O pin.. Here's the code:

INCF FSR, 0x1 INCF INDF, 0x1 MOVLW 0x22 XORWF INDF, 0 BTFSS STATUS, 0x2 GOTO detdone CALL rndgen GOTO chgmode

Have a nice day! :) D from BC

You don't need code (or even program the PIC*) to make an oscillator. Just use the clock pins and add whatever RC network you like for the frequency on GP4. The PIC12F629 has the RC mode as default, and the CLKOUT is fOsc/4 on GP5. You can eliminate the R and C if you set the clock mode to internal, but then you only have a 4 MHz clock with 1 MHz output.

*(Maybe you have to set the GP5 pin as output)

Of course it won't have the drive capability of the 555 :)

Paul