Practical LED Use
Part of Lighting
How to wire, protect, and deploy LEDs from salvaged sources to run on battery power.
Why This Matters
LEDs are the single most useful light source available after civilization disruption. They are everywhere — in flashlights, household bulbs, car assemblies, string lights, and indicator panels on every piece of electronic equipment. A salvaged LED strip connected to a 12V car battery gives you more useful light than a dozen candles while drawing power measured in milliamps rather than burning combustibles. Understanding how to wire them correctly, protect them from damage, and get maximum life from your supply is a core survival skill.
The critical barrier to practical LED use is that LEDs are current-controlled devices, not voltage-controlled ones. Apply too much current and they fail in seconds. Apply too little and they give dim, inefficient light. Get the current right and a quality LED will outlast you.
Every LED wiring problem reduces to the same solution: a current-limiting resistor calculated from three known values — supply voltage, LED forward voltage, and desired current. This article shows you how to do that calculation, how to scale it to multiple LEDs, and how to scavenge the most useful LED assemblies from the pre-collapse world.
The Resistor Calculation
Every LED needs a series resistor unless you are using a constant-current driver circuit. The formula is:
R = (V_supply - V_forward) / I_forward
Where:
V_supply = your battery or power supply voltage
V_forward = LED forward voltage (from datasheet or table below)
I_forward = desired LED current in amps
Typical forward voltages by color:
| Color | V_forward |
|---|---|
| Red | 1.8–2.2V |
| Yellow | 2.0–2.4V |
| Green | 2.0–3.0V |
| Blue | 3.0–3.5V |
| White | 3.0–3.5V |
| Warm white | 2.8–3.2V |
Standard indicator LEDs run at 20 mA. Power LEDs (1W, 3W, 5W) run at 350 mA, 700 mA, or 1,000 mA depending on rating.
Example — white LED on 12V supply, 20 mA:
R = (12 - 3.2) / 0.020 = 440 ohms
Nearest standard: 470 ohms (use this)
Power in resistor: (12 - 3.2) × 0.020 = 0.176W → use ¼W resistor
Example — red LED on 5V supply, 20 mA:
R = (5 - 2.0) / 0.020 = 150 ohms
Power: 3.0 × 0.020 = 0.06W → ¼W resistor
If you do not have the exact resistor value, always use a higher value rather than lower. Higher resistance reduces current and dims the LED slightly. Lower resistance increases current and risks burning it out.
Wiring Multiple LEDs
Series Strings
Connecting LEDs in series is the most efficient wiring approach. In series, the same current flows through all LEDs, and voltages add.
(+) ─── R ─── LED₁ ─── LED₂ ─── LED₃ ─── (-)
Calculate the resistor as if the LED bank were a single LED with the combined forward voltage:
R = (V_supply - V_f_total) / I_forward
Three white LEDs on 12V at 20 mA:
R = (12 - 9.6) / 0.020 = 120 ohms
Power: 2.4 × 0.020 = 0.048W → tiny resistor
Efficiency: Only 20% of supply voltage wasted in resistor
vs. single LED wasting 73%
Maximum series LEDs is limited by supply voltage. On 12V with white LEDs (3.2V each), you can fit 3 in series (9.6V) with 2.4V headroom. Four in series (12.8V) would not light at all.
Failure mode: If one LED in a series string opens (burns out), the whole string goes dark. If one shorts, the others get slightly brighter (but the string keeps working).
Parallel Arrays
In parallel, each LED needs its own resistor. Never wire LEDs in parallel without individual resistors — small manufacturing differences in forward voltage cause one LED to hog all the current and burn out while the others stay dim.
(+) ─┬─── R₁ ─── LED₁ ───┬─ (-)
├─── R₂ ─── LED₂ ───┤
└─── R₃ ─── LED₃ ───┘
Each resistor is calculated individually using the single-LED formula. This uses more resistors and wastes more power but means one failed LED does not kill the rest.
Polarity and Identification
LEDs only conduct in one direction. Reverse polarity does not damage an LED at typical voltages (it simply does not light), but at high reverse voltages it will fail. Always connect the anode (+) to the more positive voltage.
Identifying anode and cathode:
- New LEDs: The longer lead is the anode (+). The flat side of the plastic dome indicates the cathode (−).
- LEDs already mounted on circuit boards: Look for a small mark, dot, or flat edge on the LED body indicating the cathode. Traces on the PCB usually mark polarity.
- Unknown polarity: Connect briefly to a battery through a 470-ohm resistor. If it lights, you have the polarity right. If not, reverse it. No damage either way at typical voltages.
Scavenging the Best LED Sources
Not all LED salvage is equal. Prioritize by utility:
LED strip lights (12V): The single most useful salvage item. Sold as rolls of adhesive-backed flexible strips, they contain LEDs and resistors already wired for 12V operation. Cut at marked cut points (every 3 LEDs typically), connect directly to a 12V battery. No additional components needed. One 5-meter roll of 60 LED/m strip draws about 1.2A and produces 600–900 lumens — enough to light a room. Collect every roll.
LED household bulbs: Inside a standard A19 LED bulb is a circuit board carrying multiple small SMD LEDs wired in series-parallel arrays, plus a driver circuit. The driver converts 120/240V AC to constant current DC for the LEDs. For 12V battery use, bypass the driver: crack open the bulb housing, find the LED board, measure its operating voltage (usually 30–60V DC for 8–12 LED strings), and use a boost converter or rewire individual LEDs with proper resistors.
Flashlight assemblies: Most modern flashlights contain a 1W to 10W LED clamped to a heat sink, driven by a small boost or buck circuit. The LED itself can be unsoldered and reused. Many flashlight circuits already accept 3.7V Li-ion or 3×AA = 4.5V, making them compatible with direct battery use.
Car LED assemblies: Vehicle LED headlights, taillights, and interior lights are designed for 12V DC. Most contain current-limiting resistors internally. Test with a 12V battery — if it works in a car, it works from your battery bank. Very robust and weather-resistant.
Traffic signal modules: These are high-power LED arrays (typically 10–20W) in sealed housings, already wired for common voltages. Excellent for area lighting.
Heat Management
LEDs convert roughly 60–70% of their input power to light and 30–40% to heat. This heat concentrates at the LED junction. If the junction temperature rises above its rated maximum (typically 85–125°C), the LED degrades rapidly and fails early.
For small indicator LEDs (20 mA), heat is negligible. For power LEDs (1W+), a heat sink is mandatory.
Improvised heat sinks:
- Bolt the LED to a thick aluminum plate (cut from a cookware bottom, computer heat sink, car engine part)
- Attach to any large metal surface that can dissipate heat to air
- For the best contact, use thermal compound (white paste from inside old CPUs) between the LED base and the heat sink
- Without thermal compound, a rough aluminum surface still conducts better than nothing
Rule of thumb: If you cannot hold your finger on the heat sink for more than 2 seconds, the LED is running too hot. Either add more heat sink area or reduce current slightly via the resistor.
Practical Deployment Examples
Single room lighting — 12V battery, 4-hour night use:
- Three white power LEDs (3W each) wired in series: 3 × 3.2V = 9.6V forward voltage
- Series resistor: (12 − 9.6) / 0.700 = 3.4 ohms, 2W rating
- Current draw: 700 mA = 0.7A
- Power: 0.7A × 12V = 8.4W
- Light output: approximately 750 lumens — comfortable room lighting
- Nightly energy: 8.4W × 4h = 33.6 Wh — achievable from a single car battery
Corridor/safety lighting — always on from solar:
- Single 0.5W warm white LED
- Low current, dim but sufficient to prevent tripping
- 12mA draw, negligible battery impact
- Use a current mirror or simple resistor to set current precisely
Workshop task light:
- Salvaged LED strip, 50 cm length (30 LEDs at 60/m density)
- Direct 12V connection, no additional parts
- Output: approximately 400 lumens, directed at work surface
- Draw: 0.6A at 12V = 7.2W