Max (resistor/capacitor) values for 555 timer monostable circuit ?

Thread Starter

vtc

Joined Apr 3, 2018
34
Hi All,

I am trying to build a circuit to turn lamp on for 10 minutes once any object crosses the light sensor.

I see i can use 555 timer in monostable mode (i have NE555) to do what i need . i found to get 10 min period i should use a large value of R(R~1.2M) and C(C~470u).

Is there a max value for R and C should i consider when i build a monostable circuit ? or can i use the above calculated values without any problem ?

Regards
 

AnalogKid

Joined Aug 1, 2013
8,102
Two problems with large-value R-C timers of any kind is that they are not precise or stable.

Low cost electrolytic capacitors have a tolerance of -20% to +100% (yes, 100; that is not a typo), and better ones are only +/-20%.

The capacitance value wanders around considerable with changes in temperature.

The capacitor leakage current might be equal to or even greater than the charging current through the resistor. This can be a huge source of error, that (of course) varies with changes in ambient temperature.

For some applications, one solution is to use a CD4060, which is an oscillator and 14-stage divider. This can deliver very accurate long timing periods, but the circuit is a bit more complex.

ak
 
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dl324

Joined Mar 30, 2015
8,877
Is there a max value for R and C should i consider when i build a monostable circuit ?
You're probably near the realistic maximums.
or can i use the above calculated values without any problem ?
If this is a one off and you've determined experimentally that it gives the desired interval, then it's fine.

If you wanted to mass produce the circuit, that's different. The tolerance on electrolytic caps is probably +/-10% at best. Worst case, they could be -20%/+80% because they're usually used for filtering applications where more is generally okay.
 
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Thread Starter

vtc

Joined Apr 3, 2018
34
Hi ,

I will order cmos 555 and 4060 counter ,, 30 days at least to deliver :D:(

any time between 5-15 min accepted :)

Regards
 

iimagine

Joined Dec 20, 2010
388
any time between 5-15 min accepted :)
Mean while, play with this circuit, It consumes no idle current!
DeleteMe2.PNG

Edit: Correction it does consumes a tiny amount of current via the 1M bias resistor
 
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AnalogKid

Joined Aug 1, 2013
8,102
During the 10 minute timing period, what do you want the circuit to do when another object crosses the sensor:

1. restart the timing period back to 10 minutes and keep going.

2. ignore the event and finish the original timing period.

Also, what is your power source / battery voltage?

ak
 

Bernard

Joined Aug 7, 2008
5,260
I keep timing R to 1 meg.& C up to 10,000 uF. A 3 hour timer over a period of a week held to +- 2.5 min.
I have looked but have never found a max. current for dump, pin 7. Tried 30,000 uF with no problem, seemed to act like a 300 mA constant current discharge?
 

iimagine

Joined Dec 20, 2010
388
Seriously, you dont need a 555 for this. Its just a waste of power; it does not improve accuracy. The circuit that i posted above met your requirement, give it a try
 
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iimagine

Joined Dec 20, 2010
388
No, any generic bjt and diode would do
If you are using 12V to test the circuit, most mosfet would work
 
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Thread Starter

vtc

Joined Apr 3, 2018
34
i have tried the circuit using 5v and 12v as a power source.

5v: i got about 50 seconds once the button pressed.

12v: i got about 90 seconds once the button pressed .

how to get period between 5 - 15 min using your circuit ?

Regards :)
 

AnalogKid

Joined Aug 1, 2013
8,102
I'm not a "555-for-everything" guy, but ...

Given the softness of a FET's conduction curve and the relatively wide variation in threshold voltage from one part to the next, I think a CMOS 555 (LMC555) offers a couple of improvements over the circuit in post #15.

Separate from that - vtc: what is the object sensor output? relay contacts, open collector transistor, optocoupler, etc?

ak
 

iimagine

Joined Dec 20, 2010
388
I think a CMOS 555 (LMC555) offers a couple of improvements over the circuit in post #15.
I agree, but since accuracy isnt a problem and that the circuit consumes no other current then just charging the cap, i think its a winner for the job
 

Thread Starter

vtc

Joined Apr 3, 2018
34
The object sensor output is open collector transistor .

i used 470 uf with 12v , i got 7 min period :)

sorry guys, i forgot to till you while the object still crossing the sensor the lamp should be on :(
and this cant achieve with iimagine circuit and i dont know if i can do that using 555 timer :rolleyes:


could you advice me to use microcontroller to do what i need? :rolleyes:


Regards
 
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