Question: I read this book called Practical Electronics for Invetors,and it says that one of the ways to increase magnetic field strenght of solenoid is to increase the current or increase the number of turns.

That got me thinking,why do induction heaters and induction coupled plasma use small number of turns and high current? Why not increase the number of turns and then decrease the current so there is less problem with coil overheating?

I played with online solenoid magnetic field strenght calculator,and as I kept increasing number of turns it kept giving stronger magnetic field without increasing current.So logicaly that extra energy must come by voltage right? I mean energy doesnt just magicaly appear out of nowhere,and since the current stays the same,it must mean that solenoid with more turns needs higher voltage,is that correct?

Can I avoid heat problems by using solenoid that have large number of turns so I can power it with high voltage low current instead of low voltage high current that conventional solenoid with small number of turns would require?

edit : I want to say big thanks for all of you who came here to help me,I didnt expect so much help so fast,this ElectronicsEngineering section seems friendliest of all stackexchange sections I tried,I love you all <3

Answer: 
Especially considering size and weight, I'd use a PIC 10F200 microcontroller. It has a oscillator built in, and needs no other components other than a bypass cap to drive a LED. It can also run from a reasonably wide voltage range, including 3.7 V.

The internal oscillator is good to a few percent. The instruction cycle rate is 1 MHz driven by that oscillator. At 1-5 kHz, you have a minimum of 200 cycles per blink period, or 100 cycles per blink phase. That's plenty to do a little counting to decide when to toggle the LED state.

The output pins can source or sink a few mA, and you can gang three of them together to get more current capability to drive the LED directly with a resistor in series to set the current.
Here is a quick and dirty design using a ZSCT1555 CMOS timer IC as a VCO and PWM to change not only the duty cycle of color from Yellow to Red with battery voltage but also the frequency up to a 2:1 range for an input voltage range of 3.6 to 3.3V .

I chose ~2kHz high to 4kHz when low Vbat and simulated it here with [**Falstad**](http://ZSCT1555), but you may want a slower rate. The output driver toggles between 2 of the 4 existing LEDs changed to Red/Yellow and duty cycle of Red increases as Vbat drops.

There are an infinite number of variations of this including Red Only when below a certain threshold. The 2V zener is actually another LED.

I used a 10Hz triangle wave to simulate Vbat from 3.6 to 3V but it works beyond this range.

[![enter image description here](https://i.stack.imgur.com/ChhNI.jpg)](https://i.stack.imgur.com/ChhNI.jpg)