Simple LED circuit with delay off

Thread Starter

readmatt

Joined May 9, 2016
3
I have:

1 x Ultra bright LED
1 x Push button switch (momentary)
1 x 3V supply (2xAA)

I'd like: when the button is pushed once, the LED stays on for ~60sec (delay off).

I've looked at 555 timers, but it seems I'll need more voltage.

Any help greatly appreciated.

Thanks, Matt
 

Sensacell

Joined Jun 19, 2012
2,544
What color is the LED? (forward voltage?)

You will probably need a boost converter to run the LED from 3V, a white / blue LED Vf is just about 3 Volts.
 

TheButtonThief

Joined Feb 26, 2011
237
Are your AA's definitely 1.5V each? Alkaline batteries are normally 1.5V but rechargeable AA's are almost always 1.2V

Have you considered an RC circuit?
 

Thread Starter

readmatt

Joined May 9, 2016
3
Hi guys. Thanks for the responses so far.
Yes, an ultra bright white LED and two alkaline AAs
---- but I could go to a 9V PP3 batt (just thought of this!)

What I'd like to make is a auto-on/off bicycle light, using a mercury tilt switch(es) as the activator, hence the delay off.

Can you elab on the 'RC circuit'

Let me know your thoughts.

Cheers, M
 

Bernard

Joined Aug 7, 2008
5,368
Still confused; you get on bike, sloshing Hg turns on light & keeps light on until sloshing stops, light off after 60 sec. ?? Try C555, CMOS version of 555 connected as one shot, Hg SW bypasses timing cap. Output drives logic level N ch. FET; Phillips Lumiled, LXH8-PW-30, smd, 150 lumen, 2.8 V @ 700 mA, warm white, 120 deg. ??
 

crutschow

Joined Mar 14, 2008
24,130
I would use three AA batteries to give you 4.5V, (much longer life then a 9V battery)
And then, since the delay time is not critical, use a simple RC circuit at the MOSFET gate to give the desired delay to off.
Below is an LTspice simulation of such a circuit.
The N-MOSFET can be just about any logic-level type rated to fully turn on at a Vgs of 3.3V or less [the voltage where the ON resistance is specified in the data sheet (not the the Vgs(th) voltage)].
The value of R1 may have to be tweaked to get the desired delay.
The value of R2 determines the LED current.

upload_2016-5-10_19-15-56.png
 

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