# Info for changing voltage

#### spike1947

Joined Feb 4, 2016
496
Hi

I am attaching a drawing of what seems a simple circuit ! , what calculations would I have to do to have this same circuit run on a 2.4v supply .

it may seem simple to others ,but not me ! .

cheers

spike

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#### Alec_t

Joined Sep 17, 2013
14,409
Consider:
What LED will you use? A white LED needs about 3V to give a reasonable light output. A red LED needs about 2V.
A 2.4V supply can't charge a 9V battery unless a DC-DC boost converter is used.
A 1N4001 diode drops about 0.65V; that's a big chunk of the 2.4V you're starting with. Schottky diodes drop less voltage.
Reistor values would need to be reduced.

#### #12

Joined Nov 30, 2010
18,224
You have chosen a random circuit which has nothing to do with 2.4 volts, and it never will.
Are you trying to charge a 2.4 volt battery?
Better to say what you need. Somebody here might know how to do it.

#### spike1947

Joined Feb 4, 2016
496
Hi

Thanks for all your answers , and I am sorry for being to blunt in my enquiry , what I am wanting to do is use 2 x 1.2v AAA rechargable battery's and a solar panel that produces 4.36v , it could be a little higher than that , I am at work and taking this from memory , the resister to the LED I off course can work out , my LED is 1.8 to 2.2 v , I really wanted to know regarding the resister size from the solar panel to the transistor base connector , I do not know how to work that one out .

cheers

spike

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#### AnalogKid

Joined Aug 1, 2013
11,201
Start with the desired LED current. Divide that by 20 or more. An ancient rule of thumb says 10, but, like many other things from the Eisenhower administration, it does not apply to contemporary devices and applications. Remember, every uA than goes through the base does not go through the LED. The tradeoff is that too low a base current prevents transistor saturation, which decreases LED current and brightness.

Rbase = Vbase / Ibase = (Vbat-0.6-Vpanel(dark)) / (Iled/20+)

ak

#### spike1947

Joined Feb 4, 2016
496
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#### wayneh

Joined Sep 9, 2010
17,504
The tradeoff is that too low a base current prevents transistor saturation, which decreases LED current and brightness.
I'd be tempted to use that fact to eliminate the 100Ω resistor. It would require a little experimentation to get the right base resistance to dial in the brightness you want. But when the supply voltage is only 2.4V max anyway, it seems like it should be OK.

#### WBahn

Joined Mar 31, 2012
30,298
Hi

Thanks for all your answers , and I am sorry for being to blunt in my enquiry , what I am wanting to do is use 2 x 1.2v AAA rechargable battery's and a solar panel that produces 4.36v , it could be a little higher than that , I am at work and taking this from memory , the resister to the LED I off course can work out , my LED is 1.8 to 2.2 v , I really wanted to know regarding the resister size from the solar panel to the transistor base connector , I do not know how to work that one out .

cheers

spike
I'm still unsure of what the goal of this circuit is. Under what conditions is the LED supposed to be on?

Notice that your base current is a reverse current in the solar cell. What is the effective resistance of the solar cell when it is being overdriven? Does this damage the solar cell?

But let's start with a statement of the goal -- you have a black box to which you connect an LED, a solar cell, and a rechargeable battery. What is the purpose of the box? What do you want it to do?

#### HitEmTrue

Joined Jan 25, 2016
32
But let's start with a statement of the goal -- you have a black box to which you connect an LED, a solar cell, and a rechargeable battery. What is the purpose of the box? What do you want it to do?

But my money's on this: use solar power to charge the battery.

#### AnalogKid

Joined Aug 1, 2013
11,201
Solar power charges battery and inhibits LED. Darkness turns LED on with reverse current through the solar cell.

ak

#### spike1947

Joined Feb 4, 2016
496
Solar power charges battery and inhibits LED. Darkness turns LED on with reverse current through the solar cell.

ak
Thanks
That makes me understand the circuit a lot better now .

cheers

spike

Start with the desired LED current. Divide that by 20 or more. An ancient rule of thumb says 10, but, like many other things from the Eisenhower administration, it does not apply to contemporary devices and applications. Remember, every uA than goes through the base does not go through the LED. The tradeoff is that too low a base current prevents transistor saturation, which decreases LED current and brightness.

Rbase = Vbase / Ibase = (Vbat-0.6-Vpanel(dark)) / (Iled/20+)

ak

Cheers thanks for that info , again now I understand a bit more in this amazing world of electronics .

spike

I'm still unsure of what the goal of this circuit is. Under what conditions is the LED supposed to be on?

Notice that your base current is a reverse current in the solar cell. What is the effective resistance of the solar cell when it is being overdriven? Does this damage the solar cell?

But let's start with a statement of the goal -- you have a black box to which you connect an LED, a solar cell, and a rechargeable battery. What is the purpose of the box? What do you want it to do?

As mentioned it says what it is in the drawing ! .

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#### spike1947

Joined Feb 4, 2016
496
It says in the schematic what it is ! , I want the flickering LED 1.8v to 2.2v to turn on when it gets dark and turn off and charge the batteries 2 x 1.2v NiMH , when it gets light .

spike

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#### ian field

Joined Oct 27, 2012
6,536
Hi
I have mentioned the battery voltage as 2x1.2v rechargeable , I don't have battery's as yet , prob NiMH .

Cheers

Spike
Most commercial solar garden lights use a single 1.2V cell and usually a single transistor blocking oscillator (Joule thief) to boost the voltage for the LED. Occasionally I've seen 2 cell types and other varieties of oscillator. Most recent types use a custom chip that integrates the sunlight sensing with the oscillator, usually the only external component is a small inductor.

#### wayneh

Joined Sep 9, 2010
17,504
Yes, if I was doing this project, I'd just hack a commercial light - which are often free at Menards - to replace the LED with a flickering one.

#### spike1947

Joined Feb 4, 2016
496
Most commercial solar garden lights use a single 1.2V cell and usually a single transistor blocking oscillator (Joule thief) to boost the voltage for the LED. Occasionally I've seen 2 cell types and other varieties of oscillator. Most recent types use a custom chip that integrates the sunlight sensing with the oscillator, usually the only external component is a small inductor.
Thanks for all your replies , have tried with a cheap garden solar light , but the flickering led does not work with a joules thief , I think it has something to do with a chip built into the flicker led ! .

Cheers

Spike

#### ian field

Joined Oct 27, 2012
6,536
Thanks for all your replies , have tried with a cheap garden solar light , but the flickering led does not work with a joules thief , I think it has something to do with a chip built into the flicker led ! .

Cheers

Spike
Put a small electrolytic across the LED - maybe about 10 - 47uF.