Odd duty cycle timer circuit.

Thread Starter

Razzle308

Joined Mar 17, 2016
49
Hi all.
I need what appears to be an odd duty timer.
I have looked at many both in google and here but most seem to be for high frequency or just too complex for me. :(

I need to build a circuit with a 1 to 3 minutes OFF and .5 to 3 seconds ON with the application of a 12vdc source, cycling continuously. The project is to run on gas tiki torches that I want to flame up every few minutes.

(it needs to be "fail safe" - as in if it fails the connected relay/solenoid is off.)

Any one able to help please?

TIA.
 

Picbuster

Joined Dec 2, 2013
1,047
use a pic make a minute counter reset @ x (1>N) minutes
@ N produce a high pulse use the same mechanism count @ 100mS low @ wanted time.
make a watchdog forced halt on the pulse (wanted time +1) so when duration of pulse >wanted time + one system stop.
This will cost you 30min programming. use a pic 12f675 and a fet to control torch.
 

Thread Starter

Razzle308

Joined Mar 17, 2016
49
Thanks picbuster but that qualifies as "to complex" especially since i have no ability or software to pic program.
Hoping for a schematic with a 555 or so.
 

AnalogKid

Joined Aug 1, 2013
10,986
3 seconds to 3 minutes is a 60:1 duty cycle, and 0.5 sec to 1 minute is a 120:1 duty cycle. Both of these are within the range of an 8-bit counter driven by an oscilator. A CD4060 or CD4521 counter/oscillator and two signal diodes can produce the timing waveform you describe. What is the power source, what is the output, what is the relay/solenoid, and what is the rest of the stuff we need to know?

ak
 
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Dodgydave

Joined Jun 22, 2012
11,284
Use 555 timer or cd4093, or cd4060 chip,

I would use a cd4093 one gate for the timer with two presets and steering diodes one cap, .do you need a drawing.

How are you going to detect flame by heat or light?
 

AnalogKid

Joined Aug 1, 2013
10,986
1. Do either the on time or off time have to be adjustable?

2. If yes, do they have to be independent? For example, 64 seconds off and 1 second off fits nicely in a 7-bit counter (no, not 63 and 1). If the counter is driven by an adjustable oscillator, then turning up the delay to 2 minutes automatically increases the output pulse to 2 seconds. Is this OK?

Of course, non-adjustable is easier. Here is a first pass. Approximate times: on = 1 s; off = 64 s. These can be adjusted by changing R1. For example, increasing R1 to 47K increases the off time to 1 m 23 s, and the on time to 1.3 s.

Note - this is a preliminary schematic and has no output driver, power source, power supply decoupling, on/off switch, etc. because the TS has not given up any details.

ak
Torch-Timer-1-c.gif
 

Attachments

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AnalogKid

Joined Aug 1, 2013
10,986
OR -

I'm not a big fan of running a 555 timer with very high component values, but this application is on the edge of being in CMOS 555 territory.
Calculator: http://www.ohmslawcalculator.com/555-astable-calculator
R1 - 860 K
R2 =15 K
C1 = 100 uF
Thigh = 61 s
Tlow = 1 s

This works if the output stuff is activated by a low signal, such as pulling down on the gate of a p-channel power MOSFET or the base of a PNP power transistor. Note that the times will not be very accurate because large electrolytic capacitors have very poor component value tolerance, and temperature coefficient, and the charging current is a significant fraction of the leakage current. With a little tweaking it probably can do the job, but its not my first choice.

If you need the other output polarity, you can add a 1-transistor inverting stage to the 555 output, or change the 555 circuit to the 2-diode/separate resistor version for a fully-adjustable astable.

ak
 
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dannyf

Joined Sep 13, 2015
2,197
A RC oscillator plus a diode will do it.

However, a mcu is the way to go for accuracy, flexibility plus low part count.
 

GopherT

Joined Nov 23, 2012
8,009
Thanks picbuster but that qualifies as "to complex" especially since i have no ability or software to pic program.
Hoping for a schematic with a 555 or so.
Are you looking for something that appears to be random in the 1 to 3 minute range or is a consistent time between flare-ups ok (with a consistent duration of each flare-up).
 

Thread Starter

Razzle308

Joined Mar 17, 2016
49
Hi.
@Dodgydave. A drawing of what you suggest would be great. See if it is suitable. I have gas igniters that sense the flame.


@AnalogeKid The 555 calculator is great, added that to my favorites.

The high (on) time needs to be fairly finite as the time it takes to create the flame i want is going to be precise. I just don't exactly know yet what the burner will be. This is what is happening if you can open it:

Here is a flow of what the setup will look like.Torch actuation flow chart.jpg
 

#12

Joined Nov 30, 2010
18,224
To adjust the three second part you attack the 27K resistor. Replace it with a 2.2k resistor and add a potentiometer of 50K or 100K, linear taper.
 

Dodgydave

Joined Jun 22, 2012
11,284
Here's my take using a Cd4093, two complimentary variable pulsed outputs of 1-4 seconds and 2-4 mins.( drawn in a hurry soz)

DSC_0013.JPG
Uses two gates, a 556 timer would do the same job as posted earlier.
 

Thread Starter

Razzle308

Joined Mar 17, 2016
49
Thanks everyone for their input - great to get some helpful people.

Is this correct Dodgydave... Love the simplicity :)
Torch timer quad nand.jpg

@ Picbuster...you got me thinking now. I have long wanted to do pics, or audrino. There is another facet of this system (pressure pads to accuate - Tombraider style) which require a modded keypad format. But right now I don't have time to learn. I'll use staged 555's for now. Maybe when i'm finished with the landscaping I can pursue a pic do do it all. :)
 

Thread Starter

Razzle308

Joined Mar 17, 2016
49
Here's my take using a Cd4093, two complimentary variable pulsed outputs of 1-4 seconds and 2-4 mins.( drawn in a hurry soz)

View attachment 102639
Uses two gates, a 556 timer would do the same job as posted earlier.
Thanks so much Dave for the circuit. I have it up and running now and it's perfect.
I'll post a vid soon but it didn't want to run the automotive relay so I need to get a lower milli-amp one i think.
Thanks again for your help :D
 
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