How to light up LED with a 1.5V battery? but minimum is 1.8v to light up LED

I wish to build a 1.5V batter power indicator, can someone give me some guideline ?

tengyy said:

I wish to build a 1.5V batter power indicator, can someone give me some guideline ?

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Here are two circuits

Buy a solar garden light and disconnect its solar panel or cover it with opaque tape. Most come with a single Ni-Cad battery cell.
Most still light an LED when the battery voltage drops to 1.0V.

Audioguru said:

Buy a solar garden light and disconnect its solar panel or cover it with opaque tape. Most come with a single Ni-Cad battery cell.
Most still light an LED when the battery voltage drops to 1.0V.

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I dont understand your meaning, in simple a rechargeable 1.5V battery can light up the LED ?

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thank you for your provided circuit, i have follow the 1.5V LED flasher.
May i ask how to identified power of the rechargeable battery? only the brightness of led ?

Hi,

There are capacitive charge pump as voltage doublers.
Like ICL7660.
Don't have the min supply voltage in mind..

Klaus

**broken link removed**

Older GaAs deep Red LEDS require less voltage 1.4 as the colour wavelength of the LED determines the diode threshold voltage. (IR is around 1.2, Blue or White is 2.6V (dim) and 3V bright and higher from series resistance.

If you use low Vce sat transistors from Diodes Inc(Zetex) for T3 then you can get close to doubling the battery voltage to pulse the LED voltage thru C2.

The ESR of the 5mm LED is around 15 Ohms at 20mA and increases for lower current. This ESR * C2 will determine the Pulse ON time, while R3C1 time to reach 0.6V determines the OFF time interval.

tengyy said:

I dont understand your meaning, in simple a rechargeable 1.5V battery can light up the LED ?
May i ask how to identified power of the rechargeable battery? only the brightness of led ?

Click to expand...

A cheap solar garden light has a 3.2V white LED that is lighted from a voltage stepup circuit powered from a Ni-Cad rechargeable AA battery cell. The cell is 1.4V when charged and the LED is lighted until the voltage drops to about 1V. The led gets dimmer as the battery voltage runs down.

The voltage stepup circuit is similar to the first circuit shown in post #2.

is that possible to change 6.9V to 1.5V for switching between the green and red LED ? and how to do it? is that just adjust higher the resistance?

The circuit you posted is designed for a Vcc of from about 6V to about 10V. It will not work with Vcc as low as 1.5V.

The left transistor turns on when its base is +0.7V. The 6.2V zener diode conducts when it has a voltage of 6.2V across it. Then the left transistor turns on and lights the green LED when Vcc is 0.7V + 6.2V= 6.9V or more. Use the voltage spec for the green LED (about 2.2V for an old dim one), the actual Vcc voltage and Ohms Law to calculate the current in the LED.
The right transistor base voltage will be very low when the left transistor turns on then it will not turn on so the red LED will not light.

When Vcc is less than 6.9V then the zener diode and the left transistor do not conduct so the green LED does not light.
I think the red LED will NEVER light since the green LED needs a resistor parallel with it so that the base of the right transistor has base current for it to turn on.

The value of the 47k resistors is much too high for the transistors to saturate. It is a hopeless circuit.

You do not limit the voltage to an LED because it limits its own voltage. Instead you must limit its current with a series resistor.