Need to create 1 second pulse every 30 minutes

Thread Starter

tomerbr

Joined Oct 16, 2017
25
Hello,
I need to create, without an MCU, a pulse that will last approximately 1 second every 35 minutes or so.
This is done to rest a charger.

I tried to do this with 555 in an astable multivibrator mode as follows:

R1 = 300M Ohm
R2 = 200K Ohm
C1 = 10uF

Before I did that I checked to see if the circuit timing work as expected by changing the resistors value to something that will produce more frequent changes.
So I tried:
R1=1K
R2=2M
C=10uF
Which will give approximately 14 seconds on time and 14 seconds off time.

I measured the timing and so that in the first cycle the time if much longer than 14 seconds.
I tried this with various values for the resistors (ranging in time perspective from 10 minutes to 1 minutes) and the results were always off my calculations.

In the 10 minutes variant, (10 minutes on time, 1 minutes off time) it took me nearly an hour to change the state.

Can someone help me understand what am I doing wrong, or is there a better way to achieve this pulse every 30 minutes?

P.S.
I know that I should use a diode in parallel to R2 and change the values accordingly to get the "on time" pulse to be shorter or use an inverter in the output.

Please see the circuit below

1611167356737.png
 

wayneh

Joined Sep 9, 2010
17,153
Hello,
I need to create, without an MCU, a pulse that will last approximately 1 second every 35 minutes or so.
This is done to rest a charger.

I tried to do this with 555 in an astable multivibrator mode as follows:

R1 = 300M Ohm
R2 = 200K Ohm
C1 = 10uF

Before I did that I checked to see if the circuit timing work as expected by changing the resistors value to something that will produce more frequent changes.
So I tried:
R1=1K
R2=2M
C=10uF
Which will give approximately 14 seconds on time and 14 seconds off time.

I measured the timing and so that in the first cycle the time if much longer than 14 seconds.
I tried this with various values for the resistors (ranging in time perspective from 10 minutes to 1 minutes) and the results were always off my calculations.

In the 10 minutes variant, (10 minutes on time, 1 minutes off time) it took me nearly an hour to change the state.

Can someone help me understand what am I doing wrong, or is there a better way to achieve this pulse every 30 minutes?

P.S.
I know that I should use a diode in parallel to R2 and change the values accordingly to get the "on time" pulse to be shorter or use an inverter in the output.

Please see the circuit below
Couple questions. What sort of precision do you need to the timing? In other words, is 30±1 minute ok? 30±5?

Which 555 are you using? Specs vary.
 

ericgibbs

Joined Jan 29, 2010
13,280
hi Tom,
Most high value electrolytic capacitors have a high self leakage current, so with a high value resistor the capacitor may never charge up.

E
 

BobTPH

Joined Jun 5, 2013
3,510
The 555 is not good for 30 second periods. You need a chip like @ericgibbs proposed in post #3.

That chip runs at a higher frequency, then counts pulses to get longer periods.

Bob
 

Papabravo

Joined Feb 24, 2006
16,523
What is more important?
  1. Getting the result you are looking for
  2. Not using a processor
You decide.
Possibly interesting from a pedagogical point of view, but probably not worth the effort in practical terms.
 

crutschow

Joined Mar 14, 2008
27,477
As noted, a 555 timer is not really appropriate for such long time intervals.

Here's my favorite timer for long time intervals using a CD4060 oscillator-counter.
It requires no large capacitors or resistors, and it's easy to adjust the long time-out period by measuring the period of the Q4 output LED (a period of about 65s if you use the Q9 (pin 13) output for a 35min period).

You don't need the relay and its driver transistor so you can eliminate those.
For the repeating mode you also don't need D1.

You can trigger a 555 one-shot to give the one second pulse at each 35 minute time-out.
 
Last edited:

wayneh

Joined Sep 9, 2010
17,153
I'm not so sure a 555 can't do 30±5 minutes. You need a big (and expensive) capacitor like 5,000µF and then R1 can be a more reasonable value, eg. 500kΩ. Is that enough current to overcome any leakage? R2 = 300Ω to give you about one second.

There are more elegant solutions but we don't know the constraints.
 

AnalogKid

Joined Aug 1, 2013
9,297
I agree with most of the posts - 30 minutes is a looooooong time for a simple R-C (resistor-capacitor) timer circuit. There are programmable lamp timers on ebay that will do this, but where's the fun in that?

The CD4060 is a very common part for this type of application. It is an oscillator plus a 14-bit divider, so from a single count output to its maximum value is over 16,000-to-1. 1 second in 35 minutes is only 2,100-to-1, so it fits well within the 4060 range. Depending on what type of circuit or device the output of the timer is driving, the number of additional passive components can be small.

1. What is the power source for the timer circuit? Voltage, current capability, etc.

2. What does the output of the timerr circuit connect to? Does it drive a small relay to simulate pressing a button, or would an open-collector transistor output be ok, or something else?

3. Does the output delay period need to be adjustable?

4. A tricky one - when the circuit is powered up, does it need to make an output pulse immediately? OR, after power up, can it sit there for 35 minutes before making its first output pulse?

5. Would it be convenient to have a small LED blinking every few seconds to lwt you know the circuit is running, or would that be a distraction in the environment?

ak
 

AnalogKid

Joined Aug 1, 2013
9,297
I was reinforcing your position. BUT, in running the numbers, there might be an issue. 1 second in 35 minutes is almost exactly 2048:1. The issue is that the outputs of a 4060 span 1024:1. This can be fixed by increasing the output pulse to 2 seconds.

An alternative is using a CD4040 for the counter, and a CD4093 as the oscillator and decode gates.

ak
 

AnalogKid

Joined Aug 1, 2013
9,297
Still trying for a 1-chip, non-uC solution. A differentiator pulse former on the output of the 4060, going to a 2N7000, hits the numbers. But for a 1 s pulse the trailing edge at the drain will be pretty slow. And, the first output will be in 15 minutes, and then every 30 minutes after that. grrr

ak
 
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