Hi to all!

I made monostable timer by scheme from https://forum.allaboutcircuits.com/threads/555-monostable-self-triggering.64555/

The difference is the time, I need 2.5 hours. Therefore my RC has the values of 1M8 and 4700 uF. Everything works ok though the R and C values are much larger than the datasheet permits.

The problem is that after the expiration period in my app I have the need to turn it off again, then switch on for another period if there is a need for it. The capacitor remains charged for 10 to 15 seconds and I have to wait for the circuit to be switched on again. How to make a faster discharge after switching off?

My scheme is attached.

 

Place a diode 1N4148 in parallel with R3, Cathode to pin 8, and a 1K resistor across the supply pins 1,8, this should discharge it in 5 seconds .

Ideally you're better using a CD4060 timer very accurate upto 24Hrs.

http://www.gadgetronicx.com/how-to-build-24-hours-timer-circuit-cd4060/

 

Place a diode 1N4148 in parallel with R3, Cathode to pin 8, and a 1K resistor across the supply pins 1,8, this should discharge it in 5 seconds .

Ideally you're better using a CD4060 timer very accurate upto 24Hrs.

http://www.gadgetronicx.com/how-to-build-24-hours-timer-circuit-cd4060/

@Dodgydave, thank you, seems like good solution. Javit ću rezultat za 2.5 sata

Anyway, my area are microcontrollers and I'm sorry now that I've chosen 555 for this project. PCBs have been finished so now not acceptable modification to some microcontroller or 4060.

 

hi Zex,
Is the 555 a NE55 type or TLC555.?
The TLC being CMOS enables the use of lower value timing R/C's.
E

 

@Dodgydave, thank you, seems like good solution. I will post the result for 2.5 hours

Anyway, my area are microcontrollers and I'm sorry now that I've chosen 555 for this project. PCBs have been finished so now not acceptable modification to some microcontroller or 4060.

 

Javit ću rezultat za 2.5 sata = I will post the result for 2.5 hours

 

hi Zex,
Is the 555 a NE55 type or TLC555.?
The TLC being CMOS enables the use of lower value timing R/C's.
E

Yes, it is TLC555 but equations are similar I think. Or I am in wrong?

 

hi,
Post your calculations, so that we can compare results, for the two 555 types
E

 

Hi,
In this project tolerance is big, +- 20 minutes so i use simple equation T = 1.1 x R x C

 

hi Zex,
The main problem with using very high value capacitors is the wide capacitance tolerance of upto +/-40%.
Look at the tolerance rating of the 4700uF that you are using and check the datasheet tolerance.

The other main point is the high internal leakage of high value electrolytic's.

The combination of tolerance and leakage can give timing values very different to the equations used.

E
EDIT:
https://www.electronics-tutorials.ws/capacitor/cap_3.html

 

Yes, the initial setting of my project is wrong.
In my storage I have enough ATTiny's, must remake this. Thanks for help.

Zex

 

hi Zex,
I would also use a mini-MCU, makes the timing more stable.
For future reference look at the DS3231 RTC datasheet, very stable with battery back up.
E

 

I was using DS3231 but for this purpose ATTiny with internal clock will be enough.
After reset just initialize 16-bit timer, put some output high, count to 2.5 hours and put this output low.
Simple as that.
I'm angry at myself for the time and money that I lost because I was not thinking
Speed is slower...

 

hi Z,
There is an old saying ' more haste, less speed'.
Let us know how your upgraded Timer project goes.
E