i'm hoping someone can help me in my quest for a super simple 5 second LED control circuit.

NE555 version.
EDIT: Added diode D1

Transistor version with fast cutoff after appx 5 seconds.

 

NE555 version

As in post #5, add a 10 K resistor from Vcc to GND to discharge C1.

ak

 

C1 will discharge through the 555 but a diode across R1 will speed up the discharge.

Another version using a cmos 555.
LED current limited to 10ma.

 

A rising edge triggered timed pulse with an automated reset of the output, NOT USING A 555 timer, with two in a single package, can be done with a CD4027 DUAL JK flipflop wired as a flopshot. That allows an adjustable delay using lower capacitor values for the timing portion, and having the input strictly edge triggered. One resistor from the "Q" output to the associated R input, and a suitable capacitor from that "R" input to common. The "J" input held high and the "K" input, sand "set" tied low. The button to +5 volts to the trigger input with 10K ohms to common. The "Q" output also drives the gate of the mosfet as shown in post #1.
The benefit is a much lower standby current , and the bigger benefit is it does not use a 555 timer.

 

The CD4027 Reset input is not conditioned with a Schmitt trigger circuit, so it might not like a looooong rise or fall time..

ak

 

The benefit is a much lower standby current , and the bigger benefit is it does not use a 555 timer.

Why not a 555? Can't get much simpler then this with zero standby current.

 

THAT circuit requires that the button be held the entire time. I have included the flop-shot circuit in a standard product and it works well. The reset is not so very demanding on a fast rise time.

 

THAT circuit requires that the button be held the entire time.

That's what the TS requested in post #1.

 

For rapid turn-off you can use a schmitt trigger made out of popular LM393 comparator.
The R1 & C1 sets the 5sec timing (change C1 for precise tuning), the R2 & C2 forms a low pass filter to debounce the button transitions.
The schottky diode like Bat46 quickly discharges the C1 after button is released to be ready for the next button press.

 

Remember the TS requested a "super simple " circuit.

 

THAT circuit requires that the button be held the entire time.

i'd like to be able to press and hold a tactile switch, which will keep Q1 On for 5 seconds, even if the button is held on for longer.

ak

 

Transistor version with fast cutoff after appx 5 seconds.
View attachment 352707

I quite like the idea of this one.
Am i correct in thinking that as the Gate voltage of M1 starts to fall, the voltage at the Drain will rise as M1 is not 'fully' On?
once the drain voltage gets to >0.6V, Q1 turns on and pulls the gate of M1 to GND, therefore turning off M1 immediately?

 

I quite like the idea of this one.
Am i correct in thinking that as.......

Exactly

 

Here's the LTspice sim of sghioto's bottom circuit in Post #21:

 

The time-out for the circuit with a MOSFET can vary significantly due to the wide value that its Vgs(th) can have from unit to unit (0.8V to 3V).

Below is the sim of a simple circuit using the low-cost TLV431 voltage reference configured as a comparator, which has a precise Ref switch point of 1.24V.

 

The CD4027 circuit is simple, and you get two complete systems in one IC. One resister and one capacitor per system. And it can work from 4 volts to 15 volts.

 

Below is the sim of a simple circuit using the low-cost TLV431 voltage reference configured as a comparator, which has a precise Ref switch point of 1.24V.

If you look at all of these circuits from the point of view of constants and variables. the constants are the circuit elements that are in all designs: a switch, the LED, its resistor, one timing R and one timing C. Everything else - other R's, other C's, transistors, IC's, etc. - are variables. The #35 circuit has only one such part, and its is only 3 pins.

I think that's the winner.

(I'm not counting the discharge resistor because it does not serve an explicit requirement.)

ak

One of these years I'll remember the 431 *before* Carl uses it.

 

The CD4027 circuit is simple, and you get two complete systems in one IC. One resister and one capacitor per system. And it can work from 4 volts to 15 volts.

Plus a transistor to switch the LED current, unless you only need a mA or two of current.

 

Of course that is true. BUT we we

Plus a transistor to switch the LED current, unless you only need a mA or two of current.

re "given" that transistor in post#1.
Besides all of that, the "flopshot" circuit is cool.

 

Here's the LTspice sim of another minimal KISS circuit that gives a sharp turn-off with only one active device:

It uses a single-chip CMOS Hex Schmitt-trigger with 5 of the outputs paralleled to get sufficient drive current for the LED.

Since the Schmitt trigger point is largely proportional to the supply voltage, the delay time is fairly insensitive to any voltage change.