# Single flash for an LED

#### Phoebe607

Joined Oct 25, 2021
4
Hi I’m looking for a very simple circuit to take a long input switch down to a quick flash for an LED

#### Ian0

Joined Aug 7, 2020
3,768
Put a capacitor in series with the LED‘s current limiting resistor.
C = t/R where R is the LED’s current limiting resistor, and t is the length of the flash.
(Also put a diode in inverse parallel with the LEDs so that the capacitor can discharge)

#### Phoebe607

Joined Oct 25, 2021
4
Thanks but I need the power to stay on the LED if another shot is fired straight after the first
Put a capacitor in series with the LED‘s current limiting resistor.
C = t/R where R is the LED’s current limiting resistor, and t is the length of the flash.
(Also put a diode in inverse parallel with the LEDs so that the capacitor can discharge)
[/QUO

#### Phoebe607

Joined Oct 25, 2021
4
I’ve tried a 555 timer and a capacitor and resistor circuit with various values and can get it to work on the first switch close but if I try to do three quick hits then the power for the LED isn’t strong enough. Any suggestions would be most welcome. It has to trigger on the switch closing but only fore the LED for a very short bright flash and I only have a very very small space to put any circuits. Many thanks for any help

#### Yaakov

Joined Jan 27, 2019
3,755
I am having a lot of trouble working out just what you want. I get the idea you want an LED to flash on the press of a momentary switch, and I think you want it to ignore the switch until it is released and pressed again.

The part that is unclear is the behavior you want when the switch is pressed repeatedly in a short time, and just what is wrong with the current approaches in getting that. Could you right a little narrative for each case to clarify:

1) Switch pressed and released with a long pause.
2) Switch pressed and released, then pressed again in a short interval.

If there is a difference between the behavior if the interval is shorter than the duration of the flash, what is it?

#### Ian0

Joined Aug 7, 2020
3,768
Either capacitively couple the trigger to a 555, or use a retriggerable mono stable such as a 74HC123.
The standard HC123 is a dual in a 14-pin package but there is a tiny 74LVC1G123 version.

#### Phoebe607

Joined Oct 25, 2021
4
I am having a lot of trouble working out just what you want. I get the idea you want an LED to flash on the press of a momentary switch, and I think you want it to ignore the switch until it is released and pressed again.

The part that is unclear is the behavior you want when the switch is pressed repeatedly in a short time, and just what is wrong with the current approaches in getting that. Could you right a little narrative for each case to clarify:

1) Switch pressed and released with a long pause.
2) Switch pressed and released, then pressed again in a short interval.

If there is a difference between the behavior if the interval is shorter than the duration of the flash, what is it?
Ok so to clarify it’s supposed to simulate gunfire so the switch that’s closed is on the trigger so the person firing it can hold the trigger down for as long or as short a time as he/she sees fit. But the muzzle flash LED has to be just that a bright flash. But the gun can be fired three times in quick succession but the LED must still only be a bright flash each time. The interval between the switch being closedand opened and closed again can never be shorter than the LED flash as it’s a millisecond flash

#### crutschow

Joined Mar 14, 2008
28,208
How much current does the LED take?

#### ElectricSpidey

Joined Dec 2, 2017
1,972
These value right here will give you about 3 shots a second...is that enough?

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#### MisterBill2

Joined Jan 23, 2018
9,854
The simple way is to use a double-throw switch with both a normally closed contact and a normally open contact. Connect the capacitor between the common terminal and the supply common side, and it will charge up while the button is not pressed, Connect the LED between the normally open contact and the power negative, along with the capacitor. When the button is pressed the capacitor discharges through the LED, delivering a flash. As soon as the button is released the capacitor recharges and is ready for the next flash. The recharge is not instant but it is fast, limited by the voltage source internal resistance.