555 timer questions.

[goodpickles]

Can anyone see why this circuit does not work in real life? The output pin is low all the time. It is supposed to have the output low for about 0.5 second , then high for about 5 minutes.

For some funny reason it seems to work sometimes when the timing capacitor goes to the + instead of -. can anyone explain this?

Thanks for taking the time to help me!

 

[jrz126]

I think you need to attach your trigger and thresh. inbetween the cap and resistor c1 & r2
I found this prog awhile ago, its great for 555's

 

[goodpickles]

Ok I changed that...it still does not work. Anyother ideas?

 

[jrz126]

maybe the IC is bad? other than that, I dont know

 

[k7elp60]

A possibility is the leakage current for the 100uF capacitor connected between pins 2&6 and ground.
Five minutes timing for a 555 is near impossible to achieve, mainly because of the leakage current of high value electrolytic capacitors.
If you have a high impedance digital multimeter(most modern ones are about 10Meg), connect it across the capacitor and see if it is charging.
The voltage across the capacitor should vary from 1/3 to 2/3 of the Vcc voltage.

 

[Roff]

As k7elp60 said, it's probably leakage, which prevents the cap from being able to charge to the pin 6 threshold. If so, the multimeter will only make the problem worse unless you measure from VCC (instead of GND) to the cap. You can pretty much get guaranteed oscillation, albeit at higher than the calculated frequency, by connecting the + end of the cap to VCC and the - end to pins 2 and 6. The leakage now aids the 4Meg resistor instead of fighting it. I doubt that you will be happy with the results, though. See schematic below.
The output will be low when you turn on power, instead of being initally high as it is with the standard cap connection.

 

[goodpickles]

Ok what would be the best way to get a 5 min. timing cycle? On the data sheet it says time from nanosecondes to hours....

 

[Roff]

Ok what would be the best way to get a 5 min. timing cycle? On the data sheet it says time from nanosecondes to hours....

You can buy ceramic chip caps up to 100uF (@6.3v) from **broken link removed**, which I assume are low leakage. For the astable mode,

(Ra+2*Rb)*C=1.44*T, where T=300 sec.
(Ra+2*Rb)*C=432
If C=100uF=1e-4,
Ra+2*Rb=4.32Meg, not unreasonable.
You could also use 10uF and 43.2Meg. This combo might be more available and/or less expensive. I know that other mfrs make 10uF ceramic caps (e.g., Kemet).
You should be using a CMOS 555 for this application, which I think you are.

 

[Nigel Goodwin]

Ok what would be the best way to get a 5 min. timing cycle? On the data sheet it says time from nanosecondes to hours....

The datasheet is highly optimistic 8)

Easiest way (you all knew I was going to say this), would be to use an 8 pin PIC - this will give more accurate delays from microseconds to as long as you want!. It will also have a lower component count than a 555 circuit.

 

[lavenatti]

You could try a smaller cap for CT just to check the operation of the IC and the wiring. This could help confirm the leakage current problem with the cap. Maybe getting rid of R1 and just connecting pin 4 to Vcc would help also.

 

[audioguru]

How about using a 4060 oscillator/divider? You could use reasonable-value 2 resistors and 1 capacitor to get accurate timing for any time up to days. It can't drive a heavy load like a big relay but adding a transistor will do that.

 

[Sebi]

Always 555.... Nobody thought about 4541? Very stable, no need big cap, and easy setup for hours in the short time range.

 

[audioguru]

Thanks Sebi,
I didn't know about that one. It is much better than a 4060 and is smaller too.