Astable 555 timer circuit for long duration.

[Airickf]

Hi,
I'm trying to build a 555 timer circuit that has an output low time of 5 minutes and an output high time of 250 milliseconds. I cant seem to find a good example online. Can someone help me?

thanks!,
-Eric

 

[Airickf]

Oh, also, want to start output high...

 

[rjenkinsgb]

Those times are extreme for a simple analog timer such as a 555, it would require a current ratio of 2400:1 between the capacitor charging and discharging.

You could try this, see what happens?

Connect trigger and threshold pins together and add a 10000uF capacitor from them to 0V.

Connect a 33 Ohm resistor in series with a diode from output pin to the capacitor; anode to output pin.

Connect an 82K Ohm resistor in series with another diode from output to the capacitor; anode to capacitor end.

The capacitor is initially discharged, the output is high, the capacitor charges via the 33 Ohm resistor until it reaches the threshold level.
The output then switches low and the cap discharges via the 82K until it's back to the trigger level and the cycle repeats.

The first pulse will be somewhat longer as the cap is totally discharged rather than in the later working voltage range.

You need the usual capacitor across the overall supply, a fairly large one.

You could minimise the difference in the first pulse time by using two timing capacitors arranged as a divider;
Something like 6800uF from the trigger and threshold to 0V and 3300uF from them to V+, so they set an initial voltage on those pins.

If you do that, add a small cap from pin 5 to power instead of the usual pin 5 to ground cap; that will momentarily increase the input thresholds to help ensure triggering on power on.

You will probably have to play around with the resistor values to get accurate times, and the big caps need to be low leakage to not mess things up...

Edit - just realised that's to much current for the 555 output.
Double the resistor values and half the capacitor values, to keep it within safe limits.

 

[Wp100]

If you need long periods of accurate timing, then the 555 is always going to be a challenge.

Probably cheaper and much more accurate to use a little micro like a £4 Arduino Nano clone board, which needs no separate programmer.
The free software/IDE even comes with simple timing example to do what you want.

Depending on what Load you are driving you may need an extra simple output transistor

 

[Nigel Goodwin]

If you need long periods of accurate timing, then the 555 is always going to be a challenge.

Probably cheaper and much more accurate to use a little micro like a £4 Arduino Nano clone board, which needs no separate programmer.
The free software/IDE even comes with simple timing example to do what you want.

Depending on what Load you are driving you may need an extra simple output transistor

I'd go for an 8 pin PIC (not the silly 6 pin one ) or AVR, although you then need a programmer of course - but it's then the same size as a 555, and will VASTLY outperform it, with no added components (just a little code).

 

[eTech]

Hi,
I'm trying to build a 555 timer circuit that has an output low time of 5 minutes and an output high time of 250 milliseconds. I cant seem to find a good example online. Can someone help me?

thanks!,
-Eric

how accurate does the times need to be?
A CD4060 may be a better timer device than a 555.

 

[danadak]

This will manage the problem, and be very accurate in timing -

Pulse Sequence Generator / Smart Timer

Often users, developers, hobbyists need timed and qualified event generators, many resorting to 555 timers and the like. The venerable 555 has had a long run but its limited in accuracy and capability. This approach uses block language to create...

www.electro-tech-online.com

Easy with a $3 Nano board -

Can even do it with ATTINY85, but programming code into it a tad more involved.

Question do you want the two delays separate trigger input and output pins or one
trigger that creates both delays ? Both outputs high and pulsed low ? Triggers need
to be digital or analog, like a V threshold ?



Regards, Dana.

 

[Airickf]

Thanks Guys! Thanks rjenkinsgb, I'll try the circuit you described.

In the beginning, I did use an Arduino (sparkfun pro micro) but I didn't like the power consumption since this project is going to be run on AA batteries.

The 8 pin pic is an idea. I do have some 12F675s that I got free when Microchip was giving samples... hmn.... not sure if they still do... and I still have a programmer I got a while back... (PICkit 2)

The CD4060 seems cool. I'll have to read more up on it. As far as timing, doesn't have to be critical. More of a circuit that checks a status every ~5 min.

Dana, one trigger for both delays and digital, or could be TTL.

Thanks again guys! Going to research now..

-Eric

 

[Nigel Goodwin]

Thanks Guys! Thanks rjenkinsgb, I'll try the circuit you described.

In the beginning, I did use an Arduino (sparkfun pro micro) but I didn't like the power consumption since this project is going to be run on AA batteries.

The 8 pin pic is an idea. I do have some 12F675s that I got free when Microchip was giving samples... hmn.... not sure if they still do... and I still have a programmer I got a while back... (PICkit 2)

The 12F675 would be fine, and as you've got a PK2 for it would be easy to do - personally I moved from that to the 12F1840, and since then to the 16F18313 for my 8 pin needs. Consumption wise you should be able to make it nice and low, and VERY, VERY low if you can add a 32KHz crystal to it for TMR1 and run the PIC in sleep mode and wake once a second. But even just running the PIC at a slow clock rate will drop consumption low enough for decent life on AA's.

 

[danadak]

Question, is trigger from mechanical switch or button ? Or its a "clean"
digital signal ?

When one trigger occurs do both outpouts have to simultaneously go to "0" ?


Regards, Dana.

 

[rjenkinsgb]

A 4060 is a counter with a built in oscillator, and is excellent for many long time delay tasks.
It does not fit well with your need for 2400:1 ratio though, as not all the counter bits are brought out to pins.

Using the 555 running at around 4Hz feeding the clock input of a 4020 12 stage counter should work though, if a 2048:1 time ratio is ok?
Just add a diode from each output 1 to 11 (cathodes to the IC), all to a common pull-up resistor, eg. a 4k7

That will only allow the pullup to go high when the count is exactly 2047 decimal; 11111111111 binary.
One in 2048 clock cycles, so once every 512 seconds at 4Hz input.

If whatever you are triggering is sensitive to short glitches, you may need to add a small cap, eg. 1nF, from the final output (diode-resistor junction) to ground, as the 4020 is a ripple counter and the outputs do not change synchronously; it is possible for it to give spurious values for fractions of a microsecond as the count increment propagates through all the stages.

edit - Just remembered the "high to start" requirement.
Use a pulldown and reverse the diodes, then invert the output. That will give the short pulse when the counter is at zero, all bits low, instead of all bits high.

Add a capacitor and resistor trigger to the reset pin to guarantee it starts at zero when power is applied.

 

[AnalogKid]

How precise do the high and low times have to be? For example, if the high time is 290 ms, and the low time is 5 minutes, that fits perfectly within a CD4060's output codes.

ak

 

[simonbramble]

what about some of these parts:
https://www.analog.com/en/parametricsearch/11410#/

Failing that, it has been a while since I used a 555 (I replace that function with a PIC), but can you not feed the output of one monostable into the input of another one and multiply up the delays? Not sure if this will work.

The Timerblox above will work (but might be a bit small for prototype work)

- Simon

 

[gophert]

Below is an example of the schematic needed for discrete logic chips.
A PIC microcontroller will require a circuitboard or a protoboard and jumper wires to program and some hours to download and setup/figure out.
An Arduino (clone) will cost a few dollars and require a USB cable (if the dilvs a few lines of code and about 15 minutes to download and figure out.

 

[gophert]

Or here is a circuit set up to trigger pin D7 (output) and D13 (onboard LED)