I Built a 3-in-1 Power Tester for Almost Any Electronic Device | 150W AC + Variable DC + 12V

I don't buy new components if a working second-hand one is sitting in a scrap market for a tenth of the price.

That's not a slogan — it's just how I build. In local scrap and second-hand markets near me, real, working parts show up constantly: SMPS units, laptop and mobile chargers, routers, motors, speakers, buzzers, FM radios, extension boards, switches, sound boxes. A 10–20A 12V SMPS that costs ₹1000–2000 (~$12–23) new turns up here for ₹50–100 (~$0.60–1.20). A laptop or mobile charger for ₹50 (~$0.60) or less. The only problem is that nobody there can tell you if it actually works. You buy blind, or you don't buy at all.

So I built something to close that gap: a pocketable device that lets me test almost anything on the spot, before I pay for it — AC devices, low-voltage salvage, and standard 12V electronics, all from one 55Wh battery pack I built myself.

This isn't a fancy microcontroller-driven project, and I want to be upfront about that. There's no code, no display beyond a simple voltmeter, no app. What it is, is a finished, working tool that I use every week, that saved me real money, and that turned a stack of untested scrap into a running collection of laptop chargers, SMPS units, routers, and motors — instead of a pile of gambles. I'd rather bring you something simple that actually works than something advanced that looks good on a bench and never leaves the house.

Everything below is priced at what I actually paid, in Indian Rupees (₹), with an approximate US Dollar ($) equivalent for makers outside India. (Conversion is approximate — local prices for these modules will vary by region.)

1. 3x 21700 Li-ion cells, 5000mAh each (3S pack) — main energy storage — ₹550 (~$6.30)
2. 3S/20A BMS board — charge/discharge protection, cell balancing — ₹50 (~$0.60)
3. XL4015 buck converter + multi-turn potentiometer — variable DC output — ₹200 (~$2.30)
4. 150W inverter module (12V → 220V AC) — AC output — ₹250 (~$2.85)
5. AC socket — 220V output receptacle — ₹50 (~$0.60)
6. 3-digit digital voltmeter module — live voltage readout — ₹50 (~$0.60)
7. MH-DL18S battery indicator module — 4-segment battery % gauge — ₹80 (~$0.90)
8. 10A inline fuse + holder — main rail protection — ₹50 (~$0.60)
9. PVC enclosure + paint — housing — ₹150 (~$1.70)
10. 2x LEDs + 2x 1KΩ resistors — per-rail status indicators — ~₹10 (~$0.10)
11. 2x DC barrel jacks (₹10 each) — variable DC & 12V output ports — ₹20 (~$0.25)
12. 2x rocker switches (₹10 each) + 1x AC-rated rocker switch — branch ON/OFF — ₹70 (~$0.80)
13. Heatshrink, wire, misc. — insulation & wiring — ₹60 (~$0.75)

Total: ~₹1500–1600 (~$17–18)

For context: a single working 10–20A SMPS from the second-hand market costs about the same as a fifth of this entire build. One afternoon of testing pays for itself many times over.

Before touching a soldering iron, it's worth understanding why three outputs instead of one.

Almost everything I encounter in the second-hand market falls into one of three buckets:

1. Devices that run on mains AC — laptop chargers, SMPS units, soldering irons, serial-light controllers.
2. Devices that need a specific, non-fixed voltage — small motors, broken speakers, toys, LEDs, and random salvaged parts with no label.
3. Devices that expect a steady ~12V — routers, Wi-Fi gear, TVs, set-top boxes. This is the single biggest category of consumer electronics, and a ±0.5V tolerance is fine for nearly all of it.

One fixed-voltage power bank can't cover all three. That's the entire reason this is a 3-in-1 device instead of a simpler single-rail one — every output rail exists to answer a real category of device I run into.

I sized the pack at ~55Wh on purpose, not because I couldn't go bigger, but because the actual use pattern is short bursts: power on, check if the LED lights up or the motor spins, power off, move to the next item. Each test is 30–60 seconds. A 55Wh pack comfortably covers a full day of that kind of walk-and-test use.

Before getting into individual wiring steps, here's the full circuit exactly as built — every branch, every tap point, every test voltage labeled.

I want to be upfront that this is a hand-drawn, custom/DIY diagram, not a software-generated schematic. A few of the modules I've used — the 150W inverter board, the XL4015 buck board, and the MH-DL18S battery indicator — are repurposed, off-the-shelf modules that don't have standard footprints or symbols in common PCB/schematic design tools. There's no clean way to represent them in something like KiCad or Eagle without inventing symbols for boards that were never meant to be schematic components in the first place.

So instead of forcing a "professional-looking" schematic that wouldn't actually be accurate, I drew this by hand, directly from the wiring on the physical prototype — color-coded per branch (AC, variable DC, 12V direct), with every BMS tap point labeled by its stage voltage (0V / 4.2V / 8.4V / 12.6V) so it can be verified with a multimeter as you build. It's not polished, but every wire and connection shown here is exactly what's in the working unit — nothing simplified or left out.

Refer back to this diagram as you go through the wiring steps below; each step covers one branch of it in detail.

Three 21700 cells are wired in series (3S) into a 3S/20A BMS board.

I chose 21700 cells over the more common 18650 for two reasons that matter directly for this build: 21700s hold more capacity in the same form factor, and — more importantly for an inverter load — a typical 18650 is rated around 1C continuous discharge, while these 21700 cells deliver up to 3C. The 150W inverter branch pulls real current in short bursts, and that extra discharge headroom is what keeps the pack safe and stable under an AC load instead of sagging or heating up.

Tap points are labeled by stage voltage so you can verify with a multimeter as you go:

1. 0V — pack negative
2. 4.2V — top of cell 1
3. 8.4V — top of cell 1+2
4. 12.6V — full pack positive (fully charged)

Break out a 5-pin balance connector (JST-XH style) from these same taps for external balance charging.

This module drives the 4-segment battery-percentage LED gauge and reads pack voltage straight off the main rail.

If you're replicating this with a different pack size: the module has jumper pins (S1–S8) that must be shorted to match your configuration — short the 3S pins for a 3S pack as built here, or the 4S pins for a 4S pack. This is what makes the same module reusable across different battery chemistries and pack sizes.

Pack positive routes through a 10A inline fuse into the BMS output rail. From there, power splits into three branches — and every branch gets its own rocker switch placed immediately after the BMS, before the module it feeds.

This ordering matters more than it looks. If the switches were placed after each module instead, an "off" inverter or buck converter would still be sitting there drawing quiet standby current from the pack all day. Switching at the BMS side means an off branch is genuinely disconnected — zero draw, not just disabled. For a device meant to survive a full day of on/off field testing, that difference adds up.

Pack negative forms a common ground bus (heavy black wire loop) returning to all three modules and to the 12V IN/OUT jack.

BMS output → AC ON/OFF rocker switch (LED indicator via 1KΩ resistor) → 150W inverter module → AC socket.

This branch is what lets me plug in and test laptop chargers, SMPS units, and soldering irons directly — watching for the charging LED or fan spin-up to confirm a unit is alive before I hand over any money. It also doubles as a genuine 150W travel inverter when I just need to fast-charge a phone or run a small AC tool away from a wall socket.

⚠️ This side carries lethal 220V. Insulate every AC wire, terminal, and the socket housing fully before closing up the enclosure, and never work on this section while the inverter is powered.

Tested live: powered a bulb, a hot glue gun, and a router directly off the AC socket --inverter held steady under each load, confirming the branch works for real AC devices, not just on paper.

BMS output → Variable DC rocker switch (green LED via 1KΩ resistor) → XL4015 buck converter → 3-digit voltmeter (for live readout) → variable DC barrel jack.

The multi-turn potentiometer gives fine control — on this build, roughly 1.25V up to 9V depending on input headroom — which covers most of the unlabeled, unknown-voltage salvage I run into: bare motors, broken speaker drivers, LED strips, toys. Crocodile clips or plain wire leads off this jack make it easy to probe a part with no connector at all.

Tested live: dialed the pot across its full range and ran a small DC fan, LEDs, and a small light — smooth speed/brightness response at every setting confirms the buck converter and voltmeter readout are accurate across the range.

BMS output → 12V IN/OUT rocker switch → DC barrel jack, bypassing both the inverter and the buck converter entirely.

This is the single most-used port on the whole device, because ~12V is what most consumer electronics actually run on — routers, modems, set-top boxes, old solar setups, TVs. It also works as a charging input if I ever need to top up the pack from an external 12V source.

Tested live: Powered an LED COB and a 775 DC Motor directly off the 12v rail --instant,steady power with zero lag, confirming this branch bypasses the inverter/buck stage cleanly.

Mount all modules into a PVC enclosure, keeping the AC section physically separated and fully insulated from the low-voltage side. Paint or finish the case as you like — mine is a simple painted PVC box, functional over decorative, because the priority here was a tool I could actually carry and use, not a showpiece.

794 grams, weighed and confirmed — under the 800g target.

This number isn't a spec-sheet flex; it's a practical requirement that came directly from how this device gets used. It doesn't sit on a bench all day. It goes into a bag or gets carried by hand while I'm walking through a second-hand market, sometimes for a long stretch at a time, until I actually find the item I'm looking for in that pile of scrap. If the device were heavy, that changes fast — a couple hundred extra grams doesn't sound like much until it's been in your hand or on your shoulder for an hour of walking and testing.

So weight was treated as a real design constraint from the start, right alongside cost and output count — not something checked at the end. The 21700 cells, the compact PVC enclosure, and keeping only the essential modules (no extra display, no unnecessary shielding, no oversized heatsinking) all fed into keeping this at a weight that's actually comfortable to carry for hours, not just technically "portable" on paper.

This is where the device earns its keep. In practice, a market visit looks like this:

1. AC branch — plug in an untested SMPS or laptop charger, watch for the power LED.
2. Variable DC branch — probe an unlabeled motor or speaker with crocodile clips, dial in voltage, listen/watch for response.
3. 12V branch — plug in a router or set-top box, confirm it boots normally.

Each test takes under a minute. A single visit might mean 15–20 quick tests, and the 55Wh pack handles that comfortably in one charge.

Real-world validation shown in steps 6-8 above (AC, Variable DC, 12V) carried over directly into second-hand market testing below.

Indoors, it works as:

1. A portable workbench power supply for Arduino and general electronics projects — no separate bench PSU needed, since this covers variable DC, fixed 12V, and AC in one unit.
2. A mini UPS for routers, switches, and set-top boxes — the internal 12V pack can power them directly for 2–3 hours with zero power loss, no inverter stage involved at all for this use case.
3. A fast AC charger on demand — the 150W inverter branch handles phone chargers or small tools when a wall socket isn't convenient.

Outdoors, in the market, it works as:

1. A low-power tester for LEDs, small lights, speakers, and small motors via the variable DC branch.
2. A mid-power tester for larger motors, bigger lights, and old 12V solar gear — the category that covers most consumer electronics.
3. A 150W AC tester for old appliances, chargers, and SMPS units — the highest-value, highest-risk category to buy blind.

That's three real use cases outdoors and three more indoors from the exact same device. It's not a single-purpose gadget — it's closer to a portable bench power supply that happens to also solve a very specific problem I have every week.

1. Verify polarity at every BMS tap point with a multimeter before first power-up.
2. Never omit the 10A main fuse — it's the primary protection against a wiring fault on the main rail.
3. The AC side carries lethal 220V. Fully insulate all AC wiring, terminals, and the socket housing before closing the enclosure, and never work on it while the inverter is live.
4. If you build this with a different pack size (e.g. 4S), make sure the BMS, the inverter's input voltage range, and the MH-DL18S jumper setting are all rated for that configuration first.
5. Use only protected 21700 cells from a reputable source, and stay within the BMS's rated continuous current (20A here).

I know this project won't look as advanced next to microcontroller-driven builds in this contest, and I'm okay with that. What I built is something a low-budget or middle-class maker can actually replicate for about ₹1500–1600 (~$17–18), using it to avoid spending 5–10x that on new parts — while also pulling working components out of the e-waste stream instead of letting them get scrapped. It's not a prototype sitting on a shelf. It's a tool I use, in the same market I built it for, every single week.

If that counts for something in a contest about battery-powered builds, I hope it's this: a project judged not by how advanced it looks, but by how much it actually gets used.

Related Links

1. GitHub repo (README + circuit diagram): Humanix Tech Lab
2. YouTube build video: I Built a 3-in-1 Power Tester for Almost Any Electronic Device | 150W AC + Variable DC + 12V
3. More projects: youtube.com/@humanixtechlab

A Final Note

Thank you for reading through this build. If you're a fellow maker working with a tight budget, I hope this shows that a "simple" project, built around a real need and used every week, can be just as worthwhile as an advanced one sitting on a shelf. If you end up building your own version, I'd love to know what modifications you made for your own local market.