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Topic: Tilt Switch and 555 Timer Astable Circuit for LED (Read 4919 times) previous topic - next topic

Wawa

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.
No difference in using a p-channel fet high-side or an n-channel fet low side.
Assuming this circuit is stand-alone (not connected ot anything else).
Leo..

jarus

Voltage regulator pin Enabled is equal gate of the MOSFET and is working at low voltage.
Sounds interesting but I don't understand how it works. So this regulator can act like a switch?

Using P channel MOSFET it is the same as N channel, just different connection.
Thanks that's what I was thinking! The circuit we originally talked about with the N-Channel to power the 555 worked but I set it up wrong. Drain should not have been attached to Vss+

No difference in using a p-channel fet high-side or an n-channel fet low side.
Assuming this circuit is stand-alone (not connected ot anything else).
Leo..
Ah okay that makes sense. Thanks!

I'm also wondering what's the use case for a MOSFET vs NPN transistor for switching power. Does one work better for this case?

ted

NPN is using current, MOSFET voltage to operate, it has very high resistance.
Enable pin it put regulator in sleep mode when current consumption is very low, 1-5 uA

jarus

NPN is using current, MOSFET voltage to operate, it has very high resistance.
Enable pin it put regulator in sleep mode when current consumption is very low, 1-5 uA
Ah I see, thanks.

So how does the TLE7272-2 work?

ted

When pin EN is connected to 0V regulator is off = sleep, when is 5V regultor is on

ted

I think for beginner is too much info in short time, take your time read again the answers my and other guys,and you will have a perfect toy.

Wawa

I'm also wondering what's the use case for a MOSFET vs NPN transistor for switching power. Does one work better for this case?
The mosfet that switches the LED uses no gate current from the 555.
An NPN transistor there would need ~30mA base current from the 555 for the ~300mA LED current it has to switch.
The mosfet power switch draws no gate current from the RC time circuit, so that capacitor can be small.
A normal transistor would influence that RC circuit too much.


If you decide to use a Cmos 555, then you can try the RC circuit on the reset pin to enable/disable the 555.
You would then only need a mosfet for the LED (if you can live with the ~60uA idle draw of the 555).
Leo..

jarus

When pin EN is connected to 0V regulator is off = sleep, when is 5V regultor is on
Brilliant! I'd have to play with it to see how it works. Looks like only 4 pins, Input, Reset, Enable, and Output, and ground.

Looks like the current consumption is ~20uA....not bad.

The mosfet that switches the LED uses no gate current from the 555.
An NPN transistor there would need ~30mA base current from the 555 for the ~300mA LED current it has to switch.
The mosfet power switch draws no gate current from the RC time circuit, so that capacitor can be small.
A normal transistor would influence that RC circuit too much.


If you decide to use a Cmos 555, then you can try the RC circuit on the reset pin to enable/disable the 555.
You would then only need a mosfet for the LED (if you can live with the ~60uA idle draw of the 555).
Leo..
Ah I see, thanks. MOSFETS are a beautiful invention.

I'll check with my power consumption calculations to see how the 60uA draw would effect the battery life.

ted


jarus

Average battery = 1000mAh
Assume 350mA current Consumption During LED flashing

4hz for 30 seconds means on time is 15 Seconds

Current Consumption during 15 Seconds is  350/60/60*15 = 1.45mAH

Assuming the LED turns on 10 times a month this works out to 14.5 maH per month

Over 5 Years this would be 870maH - battery leakage can be negated due to claimed shelf life.

Battery is rated for operating temps -40F to 140F so it is suitable for outdoor use.

Energizer L91 fits this requirement (Capacity 3500mah). It also meets max discharge requirements >350mA.

I  would need 3 in series to meet voltage requirements - 4.5V (depends if I use MOFSET, 555, CMOS 555, or gated voltage regulator.

Over 5 years the CMOS 555 would consume ~2.5mAh and the TLE7272-2 would consume ~.9maH. So either one could be a potential solution.  =)

Wawa

Correction to post#45.
If you use an n-channel fet power switch low-side, then you can't power the LED/mosfet straight from the battery, as you could do with a p-channel fet that only powers the 555.
Leo..

jarus

Correction to post#45.
If you use an n-channel fet power switch low-side, then you can't power the LED/mosfet straight from the battery, as you could do with a p-channel fet that only powers the 555.
Leo..
I'm curious why this would not work? What's wrong in the diagram I drew?

ted


Wawa

the led is all the time on ?
Correct.
All the pins of the 555 are "high" (connected to batt+) when the battery switch is off.
So also a "high" on the gate of the LED fet.
Leo..

jarus

Correct.
All the pins of the 555 are "high" (connected to batt+) when the battery switch is off.
So also a "high" on the gate of the LED fet.
Leo..
Oh I see, I thought the 555 only had a 'high' output when it was powered on (connected to V+ and ground). Now I realize Trigger would always be active if the MOSFET was not on the high side, causing output to be high.

Thanks for clearing that up for me

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