Simple LED circuit and need a 10 second on timer

Thread Starter

soopytwist

Joined Dec 23, 2016
42
I've got a very small, very basic circuit of a momentary button connected to an LED and an R1 330 resistor. The LED comes on when I hold the button down. What I want to add in is a timer so I press the button once and the LED lights up for 10 seconds then goes off again. What do I need? Need it to be very small to mount on the back of a laptop Hard Drive clock.
 

MisterBill2

Joined Jan 23, 2018
6,997
A third option is an FET of adequate current rating for the LED, with a capacitor between the gate and the source, and the LED in series with the drain. Pushing the button charges the capacitor and the FET stays on until the capacitor charge leaks off. One resistor to limit the charge current and possibly one to discharge the capacitor if it does not leak enough. Smaller than a 555 and no standby current. You already have the resistor to limit the led current, still need that one.
 

SamR

Joined Mar 19, 2019
2,558
Depends... Could be a small perf board, or no-board with components soldered leg to leg and conformal coated, potted, or simply taped to prevent shorts.
 

Reloadron

Joined Jan 15, 2015
5,646
What you want is commonly called a one shot timer. What you fail to mention is if you want a retriggerable or non-retriggerable version. I push a button and a timeout period begins for 10 seconds. Then 5 sec into the cycle I push the button again? What do you want to happen?

You can roll your own or get one off the shelf, build or buy? You also fail to mention any working voltages?

Ron
 

MisterBill2

Joined Jan 23, 2018
6,997
View attachment 216121Drawing not to scale.
SG
This is the circuit that I described, except that I suggested a resistor in series with the pushbutto to limit the charge current spike. But that may not be an issue. And I am not convinced that the timing capacitor needs to be that large. That wil take a bit of experimenting.
To insulate it after it is working I have a cheap trick of brushing on that clear adhesive used for joining plastic pipes. When it gets a bit thick it is still great for bonding and insulating electrical and electronic stuff.
 

sghioto

Joined Dec 31, 2017
1,443
This is the circuit that I described
It is, I was following up on your lead. You going to need a resistor across the gate or it may never shut off. I used a 220pf and the LED is still fully on after several minutes. The TS can change the cap to 1uf and use a 12M resistor with the same results, shouldn't be a problem inside a 1.5cm x 6cm space
SG
 

MisterBill2

Joined Jan 23, 2018
6,997
With the very simple circuit the Timing capacitor leakage, or lack of leakage, will make a lot of difference as to how long it will stay on. That is the downside of really simple circuits.
 

Tonyr1084

Joined Sep 24, 2015
5,039
I press the button once and the LED lights up for 10 seconds then goes off again. What do I need? Need it to be very small to mount on the back of a laptop Hard Drive clock.
Sounds to me like a backlight for a clock. I'm sure 10 seconds is a rough target. It is probably intended to be pushed once in a while when someone wants to see the time in the dark.

As for size, once you determine the capacitor and drain resistor you can in all likelihood solder a surface mount resistor across the leads of the capacitor and save a ton of space. Shorten the leads of the FET and make the whole thing really compact. However, I, too, was concerned about the current capacity of the switch. When the cap is discharged, there's going to be a large inrush of current when the button is pressed. A small resistor will reduce the current but may mean you will have to hold the push button a little longer. There's trade-offs.
 

MisterBill2

Joined Jan 23, 2018
6,997
The inrush to a small capacitor will not be much, and the battery internal resistance will certainly limit it quite a bit. Zero IR battery types only exist in simulators.
 
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