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Topic: Tilt Switch and 555 Timer Astable Circuit for LED (Read 4748 times) previous topic - next topic

jarus

#30
Mar 23, 2018, 08:22 pm Last Edit: Mar 23, 2018, 08:24 pm by jarus
If it is 10x a month you don't need to change batteries.
Calculation can be ok but components not, capacitors not always have the value as is it say on it, increase R1.
Leave  MOSFET as is, just connect everything and observe what is happends.
When you make a drawing dont use description  - Pin1, just 1, thats what is in practice, ehen you describe something in text, use Pin1
Thanks for explaining the convention on schematics - I fixed the labels and also added a ground to pin 1 since I forgot to do that in my last schematic.

I built the circuit using 1 LED and here's what I observed when I used the second MOFSET:
-Without the MOFSET, the LED blink frequency would increase with time
-With the MOFSET, the blink frequency appeared to stay constant
-The brightness of the LED decreased with time for both cases
-There overall brightness of the LED was less when using the second MOFSET compared to not using it   

So what I have learned is that there is a voltage drop across a MOSFET and that drop is equal to Vg(th).

I am going to look for lower Vg(th) MOSFETs and lower voltage 555s.

Wawa

What I am learning is to use N-Channel if between load and ground and P-Channel if between positive and load. So the N-channel could have worked if drain was connected to ground? Correct?

For decoupling of the 555, does that mean a capacitor between drain and ground? It's C3 in the diagram but I don't know to calculate it's capacitance.

How do you recommend switching/powering the LED? Do I still need a MOFSET as Ted recommended? And I am guessing it should be a P-channel if so?

Think I should look into MOSFETS with a lower Vg(th) (currently 2V) and a 555 IC with lower operating voltage (maybe LCM555 or TLC555)? I'd like to power this thing with the minimum number of AA batteries. Right now I will need 2V (Vgth) + 4.5V (555)
Correct.

A capacitor across the supply of the 555 (pin8 and pin1).
100uF with at least a rating of max battery voltage will do.

The LED can be switched with an n-channel mosfet, regardless how you're switching the power to the 555.
But all the diagrams seem to be wrong there as well.
Source to NE555 ground, gate to NE555 output (with or without 1k resistor),
and LED (with suitable current limiting resistor) between NE555 supply(+) and drain.

Battery voltage needed depends mostly on Vf of the LED(s).
White power LEDs have a Vf of ~3.3volt, red ~2.4volt.
Add at least a volt for the LED CL resistor and mosfet(s) losses.
A Cmos 555 (ICM7555 etc.) works reliably from 2volt, a common NE555/LM555 needs at least 4.5volt.
A mosfet with a Vgs(th) of 2volt needs at least ~3.3volt.
Up to you do do the rest of the maths.
Leo..

ted


Wawa

Now compare that one to the more common 2N7000.
Not much difference aye.

Note that both have a fairly high Rds(on) of 1.8ohm typical at 4.5volt/75mA.
That might be a problem with low voltage/high current applications.
Leo..

ted

:
-Without the MOFSET, the LED blink frequency would increase with time = overloaded 555, frequency is changing because LED resistance  is decreasing

-With the MOFSET, the blink frequency appeared to stay constant = MOSFET is controlled by voltage , does not take a current from 555


-The brightness of the LED decreased with time for both cases = look at the link, you need low voltage  MOSFET


-There overall brightness of the LED was less when using the second MOFSET compared to not using it =
LED is taking power (current) from 555 not directly from battery.  

So what I have learned is that there is a voltage drop across a MOSFET and that drop is equal to Vg(th) = MOSFET is not completely open, low voltage MOSFET is need..

I am going to look for lower Vg(th) MOSFETs and lower voltage 555s. = first yes, second no

ted

Watch the video, this guy achieved what you need @ 12v =increase your voltage or look for  another MOSFET

https://www.elcircuit.com/2017/07/mosfet-timer-circuit-simple-and-easy-to.html

the problem is in  first MOSFET, short   D and  S of  it,  and make working LED.

jarus

#36
Mar 23, 2018, 11:24 pm Last Edit: Mar 23, 2018, 11:57 pm by jarus
Correct.

A capacitor across the supply of the 555 (pin8 and pin1).
100uF with at least a rating of max battery voltage will do.

The LED can be switched with an n-channel mosfet, regardless how you're switching the power to the 555.
But all the diagrams seem to be wrong there as well.
Source to NE555 ground, gate to NE555 output (with or without 1k resistor),
and LED (with suitable current limiting resistor) between NE555 supply(+) and drain.

Battery voltage needed depends mostly on Vf of the LED(s).
White power LEDs have a Vf of ~3.3volt, red ~2.4volt.
Add at least a volt for the LED CL resistor and mosfet(s) losses.
A Cmos 555 (ICM7555 etc.) works reliably from 2volt, a common NE555/LM555 needs at least 4.5volt.
A mosfet with a Vgs(th) of 2volt needs at least ~3.3volt.
Up to you do do the rest of the maths.
Leo..
The polarity of the N-channel and P-channel MOSFETs do not make sense to me. Put simply, I am confused by the diode symbol arrows and following current flow. I tried to follow your instructions, can you please point out my error? Sorry I am a beginner and have been Googling.  Thanks

jarus

Watch the video, this guy achieved what you need @ 12v =increase your voltage or look for  another MOSFET

https://www.elcircuit.com/2017/07/mosfet-timer-circuit-simple-and-easy-to.html

the problem is in  first MOSFET, short   D and  S of  it,  and make working LED.
I found this video really hard to follow, but thanks for all the other great resources! I think I am finally beginning to grasp this stuff! 

Wawa

Just Google "fet symbols" (images) to confuse you more. There are many.
Just go by Source/Drain/Gate.

The diagram in post#36 seems correct (assuming you have the timing of the 555 right), but I think it's better to move the LED supply (R5) from the 555 supply to the battery.
Then you have more LED power (LED current through one fet only), and you can use a smaller p-channel fet between battery and 555.
Leo..

ted

TLE7272-2

Here is your solution - voltage regulator with EN pin, attach to it R1C1 and you done.

https://www.google.ca/search?q=TLE7272-2&rlz=1C1AFAB_enCA475&source=lnms&tbm=isch&sa=X&ved=0ahUKEwif7uSu1IPaAhUG6YMKHTsNAVQQ_AUIDCgD&biw=1440&bih=787#imgrc=Vp2FaHNMYtjDYM:

Watch video again min; 8:00 to 9:07. One min. timer with 3 sec dimming effect.

JohnRob

Hi,

I just saw this thread and have not followed it from the beginning.   However a note if you are still planning on using the 555 timer.  Look for a 555C which is a CMOS version of the 555 timer and much more battery friendly.

Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

Wawa

Already mentioned a Cmos 555 in post#31.
Current draw of the 555 is irrelevant here, since the 555 is switched off with a mosfet most of the time.
Leo..

ted

Why guys you think that problem can be in NE555 ?

jarus

Just Google "fet symbols" (images) to confuse you more. There are many.
Just go by Source/Drain/Gate.

The diagram in post#36 seems correct (assuming you have the timing of the 555 right), but I think it's better to move the LED supply (R5) from the 555 supply to the battery.
Then you have more LED power (LED current through one fet only), and you can use a smaller p-channel fet between battery and 555.
Leo..
Haha yeah the transistor symbols are confusing!

I agree with the idea of moving the LED power supply to the battery. I am hoping to find a better MOFSET with a lower Vg(th) bc these things are eating up my supply voltage and I don't want too many batteries.

Why did you recommend a P-channel MOSFET to power the 555 rather than a N-channel MOSFET placed between ground and the load? I'm just curious if there was a practical difference. I'd like to build this with cheap and readily available parts.

Thanks for your help!

 
TLE7272-2

Here is your solution - voltage regulator with EN pin, attach to it R1C1 and you done.

https://www.google.ca/search?q=TLE7272-2&rlz=1C1AFAB_enCA475&source=lnms&tbm=isch&sa=X&ved=0ahUKEwif7uSu1IPaAhUG6YMKHTsNAVQQ_AUIDCgD&biw=1440&bih=787#imgrc=Vp2FaHNMYtjDYM:

Watch video again min; 8:00 to 9:07. One min. timer with 3 sec dimming effect.
What problem does this voltage regulator solve?

Hi,

I just saw this thread and have not followed it from the beginning.   However a note if you are still planning on using the 555 timer.  Look for a 555C which is a CMOS version of the 555 timer and much more battery friendly.


Already mentioned a Cmos 555 in post#31.
Current draw of the 555 is irrelevant here, since the 555 is switched off with a mosfet most of the time.
Leo..
Thanks, yes I looked these up. From a current standpoint, yes it's irrelevant. but since it's battery powered I am looking for a 555 with a lower operating voltage since these MOFSETs are chewing up some voltage and I am trying to keep this as small and compact as possible.

ted

Voltage regulator pin Enable is equal gate of the MOSFET and is working at low voltage.
Using P channel MOSFET it is the same as N channel, just different connection. Look at post # 32, it is using popular  BS270

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