4020 divided outputs

[cyborg 7]

Hello all,
designing a 5 minute timer (as accurate as possible without using Xtal) but when i divide the clock frequency using 4020, the output at pin 3 increases in time, first 5 minutes the out put is fairly accurate but after 6 thru 45 min the output is increased ,

another words i should get accurate 5 min. blocks, but at 45th minute I get 47 min and 37 seconds. I tried resetting the 4020 with One shot pulse at the end of 5th min. that did not work, any any thing i tried still increases the time at pin 3 of 4020 as time reaches to 45 min range. I appreciate the propogation times of the chip but is there anything i could do to RESET the 4020 to as it was first start up ?
Please see the circuit attached to have a clearer explanation. the circuit is drawn as block diagram to help me to explain.

Thank you for your help before hand

Regards

 

[dorke]

Welcome to EP.

The problem is very likely not related to the 4020.
Most probably it is related to the 555 circuit.

What is the purpose of R5 ?
Please provide all values of resistors and capacitors.

 

[cyborg 7]

Hello and thank you for your reply, R5 is a part of variable frequency circuit using 555. by the way i got the same result when i used a 4060 ic with external RC network, no matter what the result is the same of increasing the time on pin 3 of 40420.. i tried 2 other 4020's to make sure that the IC is OK. so i am just about to give up and try entirely different way ..Thank you for your reply.

 

[dorke]

Well ,your circuit is wrong.
Pins 4,8 of the 555 should be tied directly to VCC (not through R5)!

Please provide all values of resistors and capacitors.

 

[Audioguru]

Are the CD4020B ICs cheap fake Chinese ones from ebay?

You should use a modern Cmos 555 called LMC555, TLC555 or ICM7555 because an ordinary old 555 draws a 400mA current spike each time its output switches that might be killing your battery voltage and your circuit is missing a supply filtering capacitor. The datasheet for a 555 recommends TWO supply filtering capacitors so that other circuits like your cD4020B do not get messed up.

 

[Audioguru]

Sorry for the stereo current spikes.

 

[Harald Kapp]

You may get some insight reading our ressource on long duration timers courtesy of (*steve*).

 

[Colin Mitchell]

Put 4 and 8 on supply.
Use a 12v battery
Put a resistor on the output of 555 say 10k or higher.
That will fix it

 

[(*steve*)]

Are you measuring from the start of timing to the rising edge of the output? And then measuring from the first rising edge to the second?

If so, your second and subsequent times will be twice the first.

In addition, you don't have a circuit to reset the 4020 at power on. It is possible it is starting on a non zero count, reducing the time for the first cycle.