Building your own power & electronics — circuit diagrams

Build only extra-low voltage (12 V / 5 V) yourself. Anything with mains voltage (230 V in Europe, 120 V in North America) must be done exclusively by a qualified electrician. That also applies in a crisis, because in a blackout there is no emergency doctor to treat an electrical accident.

Basic tool kit (buy in advance): multimeter, wire strippers, crimping pliers + ferrules/cable lugs, soldering iron (12 V or gas soldering irons also work without mains power), solder, heat-shrink tubing, automotive blade fuses 3–30 A with holders for the outgoing circuits, MEGA/ANL fuses (40–100 A) for the main battery cables, cable 1.5 / 2.5 / 6 mm², insulating tape, cable ties.

Read offline (knowledge packages, once downloaded with DATEN-LADEN.bat): Electronics Q&A (electronics.stackexchange), Energypedia (solar/off-grid systems), Appropedia, Low-tech Magazine, iFixit (repair guides). All via 📚 Wikipedia & knowledge on the start page.

1. 12 V off-grid system (solar → battery → loads)

This is the most important diagram. It runs independently of the house wiring (and of any rooftop PV system with battery storage you may have) and supplies light, phone, radio, router and, if you have one, a satellite internet terminal.

• Order of connection: ① battery to the charge controller, ② then the solar panel. Reverse order when disconnecting. Otherwise the controller can be destroyed (with most models — check the manual).
• F1 and F2 as MEGA, ANL or MRBF fuses (blade fuses do not reliably interrupt the very high short-circuit current of a LiFePO4 battery). Place fuse F1 as close as possible to the positive terminal. A short circuit on a battery without a fuse makes cables glow — a fire hazard. Size: slightly above the controller's rated current (20 A controller → 25 A fuse).
• Always connect the inverter directly to the battery (with its own fuse F2), never to the controller's load output. 300 W inverter ≈ 30 A at 12 V → 40 A fuse, 10 mm² cable.
• Charge controller type: MPPT gets 15–30 % more out of the panel than PWM. It must match the battery chemistry: LiFePO4 and lead-acid charge differently.
• Battery: LiFePO4 (safe, 3000+ cycles, can be discharged almost completely) is the best choice. It must not be charged below 0 °C (buy a battery with low-temperature charge protection or keep it inside the house). Discharge AGM/lead-acid only to 50 %, otherwise the battery will soon be ruined. Never build packs yourself from lithium-ion cells taken from laptops or scooters (fire hazard).
• Car battery as a stopgap: inverter with clamps on the starter battery, engine running (car outside). Without the engine running, do not discharge below 12.2 V, otherwise the car will not start any more.

⚠️ If there is a PV system on your roof: never touch it. The panel strings carry 300–1000 V DC. That is instantly fatal, and an electric arc cannot be extinguished. Use a separate small panel for the off-grid system.

Calculating the cable cross-section

Cross-section (mm²) = 2 × length (m) × current (A) ÷ (56 × 0.36). This applies to copper with a maximum voltage drop of 3 % at 12 V.

Short runs matter more than thick cables: mount the battery, controller and fuse box close together.

Reading the state of charge from the voltage

Measure with the multimeter after the battery has rested for at least 2 hours without load and without charging. The values are guide values at about 20 °C.

LiFePO4 has a very flat curve, so the voltage only gives a rough indication. A battery monitor with a shunt (around €30–60) measures more accurately.

2. Charging a phone from 12 V

Simplest solution: a ready-made car USB charger (USB-C PD, 12–24 V input) in a cigarette-lighter socket on the fuse box (5 A fuse). It is robust and cheap — keep a few in stock.

DIY with a step-down module (e.g. LM2596 or XL4015, adjustable):

1. Connect the module to 12 V without the phone, measure the output with the multimeter.
2. Set it to 5.1 V with the trimmer potentiometer and fix the pot with a drop of nail varnish.
3. Only then solder on the USB socket: pin 1 = +5 V, pin 4 = ground. Connect D+ and D− to each other (standard "dedicated charging port"), otherwise many phones only draw 0.5 A. Choose a module rated for at least 2 A.
4. Fuse the input at 3 A, avoid reverse polarity (red = +).

3. LED light on 12 V with a series resistor

Formula: R = (U_max − sum of the LED voltages) ÷ current. Calculate with the highest voltage that occurs at the battery (lead-acid while charging up to 14.4 V): (14.4 − 9.0) ÷ 0.02 = 270 Ω. At 12.5 V the lamp will then shine a little more weakly, but the LEDs are never overloaded.

Simpler: ready-made 12 V LED lights or 12 V LED strips (resistors built in) connected directly to the fuse box. A 5 W LED light draws about 0.4 A, i.e. 5 Ah per evening (12 hours).

4. Bicycle dynamo → USB charging

This also works without sun and without petrol, but only delivers about 3 W. One hour of pedalling (on a roller/stand) yields around 2 Wh after losses, i.e. about 10–15 % of a modern phone battery — enough for emergency calls and texts. This is the last reserve, not the main source. Simpler and safer: a ready-made USB charger for hub dynamos from a bicycle shop.

• Without a load, a hub dynamo produces well over 6 V at high speed. Therefore: a 63 V electrolytic capacitor, step-down module LM2596HV (up to 60 V input; the normal LM2596 and the XL4015 only tolerate about 36–40 V) and the protection diode D5 (cathode to +).
• The TVS diode only catches short spikes. Under a sustained no-load condition it would burn out. So always keep a power bank connected and don't ride fast with no load connected.
• Best charge a power bank first, not the phone directly: when you stop, the voltage collapses, and the power bank smooths this out.
• Electrolytic capacitor: minus stripe to minus. Fitted the wrong way round, it can burst.

5. Generator to the house — concept only, only with an electrician

The transfer switch ensures that the generator is never connected to the grid at the same time. Otherwise the line outside is live while line workers are working on it, and when grid power returns the generator is destroyed. Installation according to the regulations of your country (Germany: DIN VDE 0100-551), only by a qualified electrician. Until then: connect appliances directly to the generator with an extension cable. Never plug a male-to-male cable into a house socket.

6. Faraday box (protection against surges/EMP)

Useful for spare devices that must work in an emergency: world-band receiver, two-way radios, an old phone, power bank, USB stick with this offline base, multimeter, small charge controller.

1. Metal container with a metal lid: galvanised dustbin, steel ammunition box, biscuit tin.
2. Insulate the inside completely: cardboard or foam, so that no device touches the metal.
3. Close the lid gap conductively: aluminium tape all the way round so that no gap remains.
4. Test: put an FM radio inside, switched on, and close the box. If the noise or the station goes completely silent, the box seals well. (When testing with a phone, a call often still gets through — that is a weak test.)

Honestly: whether and how badly a nuclear EMP damages household devices is disputed. The box costs little and also protects against lightning surges and moisture.

7. Improving radio reception (shortwave)

• Throw-out wire antenna: stretch 5–15 m of insulated wire as high as possible (tree, attic), clip one end with a crocodile clip to the telescopic antenna of the world-band receiver. On shortwave this brings in noticeably more stations, including foreign ones, for cross-checking as in the government comparison.
• During thunderstorms, disconnect the antenna and lay it outside.
• Medium wave (Germany): in Germany most medium-wave transmitters have been switched off, so a home-built crystal radio is hardly worthwhile there; better a crank or solar world-band receiver with FM, shortwave and DAB+. Check the situation in your country.

8. Repair instead of throwing away

• Check fuses first (multimeter on continuity). The most common "defect" in 12 V systems: a loose terminal, a corroded contact, a blown fuse.
• Clean contacts: glass-fibre brush or emery paper, then terminal grease.
• Guides: iFixit (offline) for phones, radios, tools; electronics questions offline in electronics.stackexchange.