Led flashing when contact... challange to make it small

Thread Starter

Piastrengo

Joined Mar 10, 2023
8
Hello,
I have a challange to make a very small circuit.
It needs to make flash 4 LEDs for a fraction of a second, everytime there is contact, from a button (passage of any voltage from 3 to 12v to decide).

For example: when I press the button it has to flash just a fraction of a second, even if I keep pressing on it, and when I depress the button it has to flash again.

There is a way to make this without an IC? but with just simple transistors, I mean with less components as possible for spacing.
 

ElectricSpidey

Joined Dec 2, 2017
2,758
The problem with that circuit is the button press must be shorter than the output time.

Show the circuit with the button coupled to the trigger via a capacitor.
 

Thread Starter

Piastrengo

Joined Mar 10, 2023
8
Ok I calculated it to flash to around 0.1 seconds, but I don't know what's happening if I keep pressed the button/trigger?
And how to make this flash when re open the trigger?
 

Papabravo

Joined Feb 24, 2006
21,159
It sounds like you are describing a double edge triggered one-shot without re-triggering while the output is active to account for the switch bounce. That is, you want a single flash when the button is pressed, and you want another flash when the button is released.
 

ElectricSpidey

Joined Dec 2, 2017
2,758
Here is a 555 version using a SPDT pushbutton.

The area inside the dotted lines are the button.

This circuit is simulated with the NE555 model but should work with a low power 555, maybe needing some adjustments.

You will need a low power 555 if you plan to go down as low as 3 volts.

Keep in mind using variable voltages IE: 3 to 12 will result in varying LED brightness.

I can show how to wire the button if you want to try this circuit. (but, maybe not tonight, it's simple tho)

1678494903780.png
 
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WBahn

Joined Mar 31, 2012
29,978
There is a way to make this without an IC? but with just simple transistors, I mean with less components as possible for spacing.
You seem to be making a self-defeating situation.

If you want as few components as possible for spacing, then you WANT to use an IC -- why not use a small 6-pin microcontroller, such as the PIC10 or ATTINY. Even at DigiKey-onesies prices, you are looking at about 50 to 75 cents for the MCU. Add your LEDs, your switch, current limiting resistor(s), a bypass capacitor, and not much else. You do have to scale the learning curve (which wouldn't be much at all for this) and get a suitable programmer.

Why do you want to do this without an IC?

You don't seem to object to using a 555 (which is an IC) along with a bunch of external components.
 

Papabravo

Joined Feb 24, 2006
21,159
Shouldn't be. Q1 flashes when the button is pressed. Q3 when released.
Sure you configured the circuit correctly in the simulator
I confirmed the operation on the breadboard.
Pretty sure. This is what I see.
1678500584550.png
If OUT1 is mostly low and I(D2) is constant at 12mA it means the LED is on most of the time. If I made a mistake, I'm hoping you can point it out, because I'd like to undestand what is going on.
 

sghioto

Joined Dec 31, 2017
5,380
When the button is pressed the LED is only on as long as it takes C1 to charge.
C2 is charged at this time and when the button is released discharges through R4 and R5 turning on M2.
M3 shorts the gate of M2 while the button is pressed.
I think the problem you're seeing is because of the duty cycle of Vin.
Circuit is dependent on how long the button is held and the flash duration required.Try using smaller value caps.
 
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