The availability and relatively low prices of ultra-bright light-emitting diodes (LEDs) allow them to be used in various amateur devices. Beginning radio amateurs who are using LEDs in their designs for the first time often wonder how to connect an LED to a battery? After reading this material, the reader will learn how to light an LED from almost any battery, what LED connection diagrams can be used in this or that case, how to calculate the circuit elements.
What batteries can the LED be connected to?
In principle, you can simply light the LED using any battery. Designed by radio amateurs and professionals electronic circuits allow us to successfully cope with this task. Another thing is how long the circuit will operate continuously with a specific LED (LEDs) and a specific battery or batteries.
To estimate this time, you should know that one of the main characteristics of any batteries, be it chemical element or battery, is the capacity. Battery capacity – C is expressed in ampere-hours. For example, the capacity of common AAA AA batteries, depending on the type and manufacturer, can range from 0.5 to 2.5 ampere-hours. In turn, light-emitting diodes are characterized by an operating current that can be tens and hundreds of milliamps. Thus, you can approximately calculate how long the battery will last using the formula:
T= (C*U baht)/(U work led *I work led)
In this formula, the numerator is the work that the battery can do, and the denominator is the power consumed by the light-emitting diode. The formula does not take into account the efficiency of the specific circuit and the fact that it is extremely problematic to fully use the entire battery capacity.
When designing battery-powered devices, they usually try to ensure that their current consumption does not exceed 10–30% of the battery capacity. Guided by this consideration and the above formula, you can estimate how many batteries of a given capacity are needed to power a particular LED.
How to connect from a AA 1.5V AA battery
Unfortunately, it doesn't exist simple way Power the LED from one AA battery. The fact is that the operating voltage of light-emitting diodes usually exceeds 1.5 V. For this value lies in the range of 3.2 - 3.4V. Therefore, to power the LED from one battery, you will need to assemble a voltage converter. Below is a diagram of a simple voltage converter with two transistors that can be used to power 1 – 2 super-bright LEDs with an operating current of 20 milliamps.
This converter is a blocking oscillator assembled on transistor VT2, transformer T1 and resistor R1. The blocking generator produces voltage pulses that are several times higher than the voltage of the power source. Diode VD1 rectifies these pulses. Inductor L1, capacitors C2 and C3 are elements of the anti-aliasing filter.
Transistor VT1, resistor R2 and zener diode VD2 are elements of a voltage stabilizer. When the voltage across capacitor C2 exceeds 3.3 V, the zener diode opens and a voltage drop is created across resistor R2. At the same time, the first transistor will open and lock VT2, the blocking generator will stop working. This ensures stabilization of the converter output voltage at 3.3 V.
It is better to use Schottky diodes as VD1, which have a low voltage drop in the open state.
Transformer T1 can be wound on a ferrite ring of grade 2000NN. The diameter of the ring can be 7 – 15 mm. Rings from converters can be used as a core energy saving light bulbs, filter coils of computer power supplies, etc. The windings are made of enameled wire with a diameter of 0.3 mm, 25 turns each.
This scheme can be painlessly simplified by eliminating stabilization elements. In principle, the circuit can do without a choke and one of the capacitors C2 or C3. Even a novice radio amateur can assemble a simplified circuit with his own hands.
The circuit is also good because it will operate continuously until the power supply voltage drops to 0.8 V.
How to connect 3V batteries
You can connect a super-bright LED to a 3V battery without using any additional parts. Since the operating voltage of the LED is slightly higher than 3 V, the LED will not shine at full strength. Sometimes it can even be useful. For example, using an LED with a switch and a 3 V disk battery (popularly called a tablet), used in computer motherboards, you can make a small flashlight keychain. This miniature flashlight can be useful in different situations.
From such a battery - 3 Volt tablets you can power an LED
Using a pair of 1.5 V batteries and a purchased or homemade converter to power one or more LEDs, you can make a more serious design. The diagram of one of these converters (boosters) is shown in the figure.
The booster based on the LM3410 chip and several attachments has the following characteristics:
- input voltage 2.7 – 5.5 V.
- maximum output current up to 2.4 A.
- number of connected LEDs from 1 to 5.
- conversion frequency from 0.8 to 1.6 MHz.
The output current of the converter can be adjusted by changing the resistance of the measuring resistor R1. Despite the fact that from the technical documentation it follows that the microcircuit is designed to connect 5 LEDs, in fact you can connect 6 to it. This is due to the fact that the maximum output voltage of the chip is 24 V. The LM3410 also allows LEDs to glow (dimming) . The fourth pin of the chip (DIMM) is used for these purposes. Dimming can be done by changing the input current of this pin.
How to connect 9V Krona batteries
“Krona” has a relatively small capacity and is not very suitable for powering high-power LEDs. The maximum current of such a battery should not exceed 30 - 40 mA. Therefore, it is better to connect 3 light-emitting diodes connected in series with an operating current of 20 mA to it. They, as in the case of connecting to a 3 volt battery, will not shine at full power, but the battery will last longer.
Krona battery power supply circuit
It is difficult to cover in one material all the variety of ways to connect LEDs to batteries with different voltages and capacities. We tried to talk about the most reliable and simple designs. We hope that this material will be useful to both beginners and more experienced radio amateurs.
I don't know about you, but I'm modern world Irrational use of batteries is depressing. We buy a 1.5-volt one for a TV remote control, for example. It works and pleases us with its ability to change channels without leaving the couch. But over time, malfunctions begin, the buttons have to be pressed many times in order to achieve at least some action, the remote control must be held at arm's length... The battery is dead. As always, we change what to do. But if you check the voltage in it, it is unlikely to be at zero. Let's say there is one volt left. And where should I put it? It’s a shame to throw it away, but there’s nowhere to use it; you can’t power anything sensible.
It is in connection with such monstrous waste of energy that I put together a “joule thief” circuit to “afterburn” batteries rejected by other consumers using an LED. It is called so because it is capable of almost completely draining the battery, depriving it of the last joule of energy. And in general, the “flashlight of the Apocalypse”, which runs on any kind of garbage, is a very cool idea.
The most interesting thing about this device is, in fact, the very fact that the LED operates from a low-voltage power source. Typically, an LED needs 2.5 - 4 volts (depending on the color), if the voltage is lower, it simply will not turn on. This circuit works as a boost converter, and its output is exactly the amount of voltage that the LED needs.
The circuit is very simple, with a minimum of details. The capacitor and diode can be eliminated. 
The heart of the device is the transformer. It is wound on a ferrite ring. Rings made from used materials work well motherboard PC. 
We take an enameled copper wire (mine has a diameter of 0.3, or something - a rusty caliper), fold it in half and start winding it around the ring. 
A total of 20 turns are needed. Looking ahead to the future, the second version of the circuit has 26 turns (for variety).
Afterwards we decide on the coils. We get two outputs at the top and two at the bottom. We remove varnish from them using any known method - sandpaper, fire, Aspirin. Using the dialing function in the multimeter, we find the combination of pins “one on top, one on bottom”, when it does not squeak - this will be the junction of the two coils. They connect in antiphase, that is, the end of one is to the beginning of the other. 
I used the transistor KT315G, but it is possible with a different terminal letter. My electronics engineer friend, when I show him my latest homemade product (or someone else’s on the Internet), immediately asks how many KT315 are inside. If there is less than one, the device is useless and soulless; if there is one, but together with other transistors, then everything rests on it; on several KT315, it is good and correct; all functionality is provided by a single transistor of this brand - the highest class.
In the second version of the circuit - KT361D. Accordingly, the polarity of the LED and battery switches on.
The resistor in the base circuit is 1 kOhm.
Warm white LED with a yellow tint. The Chinese crafts that have flooded the market all have a cold white glow and have a bluish tint. I have a 100 ohm resistor soldered under my LED. It limits the current. 
Wow, it works. Very powerful sorcery. 
Miniaturization work. Based on this circuit, I really want to build myself a flashlight that burns batteries. I removed the resistor in front of the LED to make it shine brighter.
From a battery with a voltage of 1.5 volts or lower, it’s simply not realistic. This is due to the fact that most LEDs have a voltage drop exceeding this figure.
How to light an LED from a 1.5 volt battery
A way out of this situation may be to use a simple one transistor and inductance. In essence, it is peculiar. The circuit is a simple blocking generator, powered by a 1.5 volt battery, generating fairly powerful pulses as a result of pumping energy into the inductor. The circuit is simple and can be assembled in literally 10 minutes.
The T1 inductor is made on a ferrite ring with a diameter of 7 millimeters (its dimensions are K7x4x3). The winding contains 21 turns, made of double-folded enameled PEV copper wire with a diameter of 0.35 millimeters.
After winding is completed, the end of one of the wires must be connected to the beginning of the other wire. The result is a tap from the center of the winding. By selecting the resistance, you can achieve better light output.
LEDs have long replaced incandescent light bulbs in almost all areas. This is understandable: LEDs are brighter than lamps, given their energy consumption.
But LEDs also have a number of disadvantages. Of course, we won’t talk about all of them, but we will discuss one. This is a high initial power threshold - it is about 1.8-2.2 volts. Naturally, you can’t power it with just one battery...
To overcome this shortcoming, we will build a simple converter using an absolute minimum of parts.
Thanks to this converter, you can connect an LED (or several LEDs) to one battery and make a small flashlight.
We will need:
- LED
- 2N3904 or BC547 silicon transistor, or any other n-p-n structure.
- Wire.
- Resistor 1 kOhm.
- Ring cores or ferrite cores.
Converter circuit
I will give you two diagrams. One for winding a ring transformer, the other for those who do not have a ring core on hand.
This is the simplest blocking generator, with a free excitation frequency. The idea is as old as time. The device will have a high efficiency.
Winding the inductor
Regardless of whether you use a ring core or a regular ferrite core, wind 10 turns of each winding. Your inductor is ready for this.
Generator check
We assemble according to the diagram and check. The generator should work and does not need adjustment.If suddenly, even though the elements are working properly, the LED does not light up, try changing the ends of one of the windings of the induction transformer.
Now the LED shines very brightly even with a dead battery. The lower limit of the power supply for the entire device is now somewhere around 0.6 volts.
The efficiency of a transformer with a ring core is slightly higher. Not critical of course, but just keep it in mind.
The repair and modernization of the next VEF 202 has been completed. 
He was such a pig. Released in October 1975. My favorite scale design option. 
It is very interesting that from the inside of the front panel the texture of the plastic does not look the same as in my other 202s, and there are a lot of extra holes. Most likely, this was done to reduce material consumption and weight, because the design has a decent safety margin. In the 1976 receiver with the previous version of the scale (see), these holes no longer exist (or not yet, if the additional holes were intended only for this “red” version of the scale). 
And this is a receiver housing with a “red” scale produced in 1977. And also with holes...
In principle, there is nothing special to add to the article about the repair of “202s”. This time I made the sound input not on the tape socket, but directly connected the wires to the contacts on the board. Perhaps it's more convenient. Once again I was convinced of the futility of repairing telescopic antennas without knowledge and equipment. In mine, the topmost link slightly extended beyond the boundaries of the previous one, and at maximum reach the brass fixing petals were visible. Having squeezed a little top part ringing with pliers, I, having essentially achieved nothing, left the antenna alone. It holds - and that’s okay.
As an experiment, this VEF, like my Speedola 232, was installed LED backlight scales. In the original VEF, the backlight switch is monostable and normally open, that is, current flows only when contact group attached external force— the finger presses the button. But I wanted a bistable option: I wanted it - turned it on and left, I wanted it - turned it off. LEDs consume little power, so you can make a night light from the receiver. 
For this you need a corresponding button. This one was taken from a Chinese battery-powered flashlight, but can easily be found in radio stores or other not so required devices. The button had to be modified by gluing a piece of plastic onto the back of it, so that when it sits in the niche opposite the original switch, it rests with its back against the wall (if you do this, you’ll understand for yourself). The button is glued into place with superglue and soda. 
The original spring contacts remain in place, and the red cam, on which the rod of the “external” button presses, in turn now presses the new button, the green one. I had to work a little, adjusting the depth of the switch in the niche - filing its glued back part. If it protrudes too much, then when the body is fully assembled, switching will not occur, and if the seat is too deep, the “external” button will have to be pressed in strongly. The whole difficulty lies in finding the optimal freewheel. 
The LED strip (200 mm) is best glued to this place, on the side under the top chrome strip. My tape is in silicone, there was no other. When installing the chassis, it slightly interferes with the indicator arrow - it bends (on the cable) and rubs against the tape. No solution was found, and I didn’t really want to. There are no such problems with VEF-Spidola 232; there is plenty of space in it. I'll hope for a chance. And such a “brake” even helps a little when accurately tuning to a station.
Some metallized areas of the scale had to be sealed with foil (there is such self-adhesive foil) so that the small dots would not show through (no joke - the receiver is thirty-nine years old this year!). We also had to use pieces of rubber from the bicycle tube to seal the extra holes in the body located in the grille area. 
The tape is connected through a 1 kOhm trimming resistor (blue parallelepiped) for one-time brightness adjustment: so that it doesn’t dazzle too much at night, and so that it looks nice. The backlight can now light up even when the receiver is turned off - the tape is connected in parallel to the power source. 
