Voltage sensing circuit to trigger blinking led

C01000110010011110101100

Joined Jan 19, 2020
10
I need a circuit to flash an LED. The battery is 3.6v 1A, this powers a fairly dim 2.5v micro LED (probably a very low current), I don't think this alone could power a flashing LED circuit, so I want to piggyback off of this LED to trigger the circuit to flash a 3.6v LED.
So I'll have a 3.6v supply and a 2.5v trigger, the size of the circuit needs to be no larger than 25mm x 60mm and 6mm high.
I have attached a diagram to better explain what I need. Can anyone help with this? Many thanks in advance, Chris.

Tonyr1084

Joined Sep 24, 2015
7,991
@C01000110010011110101100 Welcome to AAC.

Thank you for your attempt at being very thorough and descriptive of your needs. However, there is still some clarity needed. It would also be helpful to know a little about your skill level. First, do you understand how to light an LED? Then, this piggybacked LED, what is the voltage source for it? How many volts? What type of LED do you want to flash? What color? Yes, color makes a difference as each different type of LED has a typical forward voltage (Vf) that needs to be considered when calculating the amount of current you can safely use to light that LED. There's a lot of other things as well. But lets start with two basics - what is your skill level and what is the power source for your particular LED. OK, a third thing - what type or color LED are we considering?

ericgibbs

Joined Jan 29, 2010
19,091
hi C,
Welcome to AAC.
Have you considered a MOSFET with a low Vg threshold that could switch ON with a +2.5V signal.?
E

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Audioguru again

Joined Oct 21, 2019
6,782
Your schematic shows two 3.6V batteries. A positive one and a negative one. But I think you have only one battery that produces +3.6V and 0V.

A battery has a range of voltages from new or fully charged to dead. If yours is a Li-PO or Lithium-Ion rechargeable then its voltage is 4.2V when fully charged then it drops to 3.2V as it is used and when it should be disconnected. 3.7V is half charged or half dead. Maybe you have three Ni-CAD or Ni-MH cells? Their voltage also drops as they are used.

A transistor will work fine with your battery.

Is the dim 2.5V LED a flashing LED? It can turn on a transistor that can blink your bright LED if the battery voltage is high enough for it.

Forget about using a Mosfet with a "threshold voltage" when it is almost turned off. Its datasheet shows that it needs 4.5V to 10V to be partially turned on.

ericgibbs

Joined Jan 29, 2010
19,091
Its datasheet shows that it needs 4.5V to 10V to be partially turned on.
Where do you get this data from.???

Why should he forget about using a low Vgs MOSFET when he is driving an LED with about 20mA, with a 2.5V voltage source ????

Audioguru again

Joined Oct 21, 2019
6,782
Hi Eric. He said he has a dim little 2.5V LED which I think is a blinking LED and "piggybacked" he wants it to blink a much brighter LED from his 1A battery. A 350mA or 700mA LED?

The datasheet text for the tiny Mosfet shows that with a Vgs of 4.5V its on-resistance is 6 ohms max which causes a voltage drop of 2.1V when the current is 350mA. He ain't got that much extra voltage.
Also, the tiny Mosfet will smoke and burn with 0.35A squared x 6 ohms= 735mW x 350 degrees C per W= 257 + 30= 287 degrees C. Hey, the solder will melt and it will fall out!

I always design a circuit for "worst case" so that every circuit I make works, not just some of them or none of them.
Your graph is for a "typical" device that is not sold, you get what they have, not what you want. I doubt that he wants to buy many and test them all to maybe find a "typical or better" one.

ericgibbs

Joined Jan 29, 2010
19,091
hi agu,
Lets see what he intends the drive current to be for this additional LED, rather than guessing the current, we can then take from there.
Have you considered the Base current required to drive a BJT by 'piggy backing' from an undetermined 2.5V source, to sink a Collector at 350mA or 700mA thru an LED.?

E

Audioguru again

Joined Oct 21, 2019
6,782
His dim little blinking LED might use only 3.5mA then two common-emitter transistors (not Darlington) can amplify it to drive 350mA.

ericgibbs

Joined Jan 29, 2010
19,091
You originally said this:
A transistor will work fine with your battery.
Is the dim 2.5V LED a flashing LED? It can turn on a transistor that can blink your bright LED if the battery voltage is high enough for it.
Now.
His dim little blinking LED might use only 3.5mA then two common-emitter transistors (not Darlington) can amplify it to drive 350mA.
Again You and I are guessing about the actual LED current required. lets wait and see what the TS actually requires.

C01000110010011110101100

Joined Jan 19, 2020
10
THANK YOU ALL VERY MUCH FOR REPLYING. I have a very very basic understanding of electronics, I learned a little bit back in the 80's. I understand LED's have a forward voltage and require a resistor to limit the current, a little about caps and resistors, but not much else. I should be able to follow a fairly simple schematic, but a breadboard would be easier.

I'm sure there is an IC somewhere to act as a switch to send the main voltage through using a sensing pin, I just can't find one...

To try and make things a little clearer, The set up is the insides of an illuminating PC keyboard. The main PCB is tiny, about 50x20mm, it has 3 onboard steady micro LED's, Num, Caps and power, these are quite dim, so I often don't notice when caps lock is left on. The battery is a Li-ion 3.6v 1000ma flat cell type (yes, 4.2 full, charged from 5v USB), this powers about 20 other micro LED's for the keys. I am a 2 fingered, look directly at the keys I'm striking typist and gETTING cAPS bACK tO fRONT dRIVES mE mad!

I have got a meter reading of 2.5v across the caps lock LED solder tags, I was thinking of using the +v side to trigger the main 3.6v - 4.2v, this in turn to power a circuit to make an appropriate bright white LED flash, speed controlled via a mini pot, although, the blinking speed isn't important. My project is to fit a noticeable caps lock LED placed near to the actual key.

Hope I've made my project a bit clearer.

Audioguru again

Joined Oct 21, 2019
6,782
A white LED is fairly bright at only 20mA to 30mA.
I considered using a 555 timer to blink the lED but its minimum supply his more that your battery produces and a cmos 555 has an output current that is too low.
Then I thought of using a 74HC logic gate IC that produces plenty of current and works until your battery is dead.