Testing procedures to turn discarded yet functional PCB components into functional, low-cost DIY electronics kits.
Salvaging is not worth a hospital trip. Follow these non-negotiable rules before working on any discarded board.
DO NOT OPEN MICROWAVE OVENS EVER
Microwave ovens contain a lethal high-voltage capacitor (≈2,000–4,000V) that can retain a deadly charge indefinitely even when unplugged. The magnetron also contains toxic beryllium oxide ceramic. Microwaves are NOT salvageable. Leave them intact for professional e-waste recycling.
KEEP EXTREME CAUTION WHILE OPENING CRT TVs OR MONITORS
CRTs contain lethal voltages up to 25,000V–35,000V on the anode cap and can retain charge for weeks or months after unplugging. The vacuum tube can also implode violently, sending glass shrapnel flying. Only attempt CRT salvage if you have formal training in high-voltage safety. Discharge the anode to the chassis ground with a 10kΩ 10W resistor on an insulated stick—never a screwdriver. Wear safety goggles and heavy gloves. If in doubt, do not open it.
Power boards may retain lethal charge. Always discharge high-voltage capacitors with a proper bleed resistor or insulated screwdriver before handling. NEVER trust a "dead" board.
Excessive heat destroys semiconductor junctions. Limit iron contact to 3–4 seconds max per joint. Let the pad cool between attempts. If it won't come free, use more flux—not more heat.
Use copper desoldering wick (braid) and a solder sucker (desoldering pump) for clean removal. Apply flux to the wick, heat the joint through the braid, and let capillary action do the work.
Every component must pass before it enters a kit. Select a component type below to view the its testing tutorial. All procedures are for through-hole and large discrete components only.
Check for cracked housings, melted plastic, or corroded terminals. Discard if the actuator is stuck or damaged.
Set multimeter to continuity mode (or resistance Ω). Place probes across the switch terminals without pressing. Meter should read OL (Open Loop) or infinite resistance.
Press and hold the switch actuator firmly. Meter should read < 2Ω and beep (if in continuity mode). Release and verify it returns to OL.
Actuate the switch 20–30 times rapidly. Recheck continuity. If readings fluctuate or stick, the internal contacts are worn—discard.
Rotate the shaft by hand. It should spin freely with minimal resistance. Check for bent shafts, seized bearings, or excessive end-play. Listen for grinding or clicking.
Set multimeter to resistance (Ω). Measure across the motor terminals. Typical small DC motors read 5Ω – 60Ω depending on size and winding. Optical drive spindles usually fall in the 8Ω – 25Ω range.
Apply 1.5V – 3V DC briefly (use a AA battery or bench supply with current limit). The motor should spin smoothly in one direction. Reverse polarity to verify bidirectional rotation.
With motor running at rated voltage, no-load current should be modest (50–300mA for small motors). Stall current will be higher—do not stall for more than 2 seconds.
Examine the speaker cone for tears, punctures, or delamination. Check the surround (foam or rubber edge) for rot or separation. Gently press the cone—it should move freely without rubbing or scraping.
Set multimeter to resistance (Ω). Measure across the speaker terminals. Typical small salvaged speakers read 3.2Ω – 8.5Ω. This is DC resistance (DCR), not impedance—DCR is always lower than the rated impedance.
Touch a 1.5V AA battery briefly across the speaker terminals. You should hear a distinct "click" or "pop" as the cone moves. Reverse polarity—the cone should move the opposite direction.
If available, connect to a low-power audio source at moderate volume. Listen for distortion, rattling, or buzzing that indicates cone damage or voice coil rub.
Identify the cathode (negative) side—usually marked with a band on diodes or a flat side on LEDs. For LEDs, the shorter lead is typically cathode (manufacturing convention, but verify).
Set multimeter to diode test mode (symbol: ⏧). Connect red probe to anode (+), black to cathode (−). Silicon diodes should read 0.5V – 0.7V. LEDs will show a higher forward voltage (1.8V–3.3V depending on color) and may glow dimly.
Reverse the probes: red to cathode, black to anode. The meter should read OL (Open Loop) indicating the diode is blocking current in reverse.
For LEDs, connect a current-limiting resistor (220Ω–1kΩ) in series with a 3V–5V supply. The LED should illuminate brightly at its characteristic color. No light = dead LED.
Electrolytic capacitors: look for bulging tops, leaked electrolyte (brown crusty residue), or lifted vent scores. Ceramic/disc caps: check for cracks or chips. Any physical damage = immediate discard.
Use a multimeter with capacitance mode or an LCR meter. Measure the capacitance and compare to the marked value. Tolerance is typically ±20% for electrolytics, ±10% for ceramics.
High ESR indicates a dried-out electrolytic. Use an ESR meter or an LCR meter at 100kHz. Good electrolytics show ESR under a few ohms (varies by capacitance and voltage rating). High ESR = poor filtering performance.
Charge the capacitor to its rated voltage through a resistor, then disconnect and measure voltage drop over 60 seconds. A healthy capacitor should hold >90% of its charge. Rapid discharge indicates high internal leakage.
Check for burnt or discolored bodies, cracked paint, or corroded leads. Carbon film resistors may show a dark ring if overloaded. Metal film resistors should have uniform color bands with no scorch marks.
Read the color bands to determine nominal value. Common 4-band: Band 1-2 = significant digits, Band 3 = multiplier, Band 4 = tolerance (gold = ±5%, silver = ±10%). 5-band: Band 1-3 = digits, Band 4 = multiplier, Band 5 = tolerance.
Set multimeter to resistance (Ω). Measure across the leads. Compare to the color-coded nominal value. Allow for tolerance: a 1kΩ ±5% resistor is good between 950Ω and 1050Ω. Do not touch both probes with your fingers—body resistance will skew low-value readings.
For large wirewound or cement resistors (1W+), inspect the ceramic body for cracks. Measure resistance—these are typically low values (0.1Ω–100Ω). Wirewounds may read slightly high if the element has oxidized from overheating.
Identify Base (B), Collector (C), and Emitter (E) from the datasheet. Common TO-92 pinouts: flat side facing you, pins down—EBC (2N2222, BC337) or CBE (2N3904, BC547). Always verify with the exact part number.
Set multimeter to diode mode. For NPN: red probe to Base, black to Emitter → should read 0.5V–0.7V. Reverse probes → OL. For PNP: black probe to Base, red to Emitter → 0.5V–0.7V. Reverse → OL.
Repeat the diode test between Base and Collector. Same expectations as Base-Emitter: one direction shows 0.5V–0.7V, reverse shows OL. This confirms both PN junctions are intact.
Test between Collector and Emitter in both directions. A good transistor reads OL in both directions. Any finite reading indicates C-E leakage or breakdown—discard the transistor.
If your multimeter has an hFE socket, insert the transistor matching the pinout (E-B-C). Typical small-signal BJTs show hFE of 100–400. Very low gain (<50) or no reading suggests a weak or damaged device.
Common TO-220 / TO-247 MOSFETs: pins are Gate (G), Drain (D), Source (S) left-to-right with the front label facing you. Always cross-check with the datasheet—some devices have different pinouts or include an internal protection diode.
Most power MOSFETs have an integral body diode from Source to Drain. Set multimeter to diode mode. For N-Channel: red to Source, black to Drain → should read 0.4V–0.7V (diode forward drop). Reverse → OL. For P-Channel: black to Source, red to Drain → 0.4V–0.7V. Reverse → OL.
Test between Gate and Source, then Gate and Drain, in both directions. The Gate is insulated by a thin oxide layer—you should read OL in both directions on a good MOSFET. Any finite reading indicates a blown gate oxide (ESD damage or overvoltage).
Set multimeter to diode mode. Touch the red probe to Gate and black to Source briefly to charge the Gate capacitance. Now test Drain-Source: an N-Channel MOSFET should now conduct (low resistance) because the channel is enhanced. Discharge by shorting G-S with a finger or wire, and D-S should return to OL (or body diode reading).
Check the metal can for dents, cracks, or corrosion. HC-49/S crystals should have intact leads with no stress fractures at the glass seal. Ceramic resonators (3-pin blue/tan blobs) should have no chips or cracks.
Set multimeter to resistance (Ω) or continuity. Measure across the two terminals of a crystal, or between the outer two pins of a 3-pin resonator. You should read OL (infinite resistance). A crystal is not a conductive component—any low reading means it is cracked or contaminated.
If your multimeter has a capacitance mode, measure across the crystal terminals. A typical HC-49 crystal shows a few picofarads (3–10pF) due to the holder capacitance. wildly different values or OL suggest damage.
The only definitive test is to place the crystal in a known-good oscillator circuit (e.g., a Pierce oscillator with an inverter gate or a microcontroller with known-working firmware). If the circuit oscillates at the marked frequency, the crystal is good. No oscillation = dead or off-frequency crystal. A frequency counter or logic probe at the inverter output confirms oscillation.
Look for burnt or melted winding insulation, bulged cores, or scorch marks on the bobbin. Smell for the distinct "burnt transformer" odor—once smelled, never forgotten. Any charring = discard immediately.
Identify each winding by tracing the pins or using a schematic. Set multimeter to resistance (Ω) and measure across each winding pair. A good winding shows low resistance (typically 0.5Ω–50Ω for small signal transformers, up to a few hundred ohms for line transformers). OL on any winding = open (burnt) winding.
Test between every pair of windings (primary to secondary, secondary to secondary, each winding to the core/metal frame). You must read OL in all cases. Any finite reading indicates insulation breakdown—a shorted transformer is a fire hazard.
If you have an LCR meter, measure the inductance of each winding. Compare to the datasheet or expected values for the application. Significantly lower inductance suggests shorted turns inside the winding.
Rotate the shaft through its full range. It should turn smoothly with consistent resistance—no gritty spots, dead zones, or excessive wobble. Check the body for cracks and the terminals for corrosion. For trimmers, verify the adjustment slot is not stripped.
Set multimeter to resistance (Ω). Measure between the two outer terminals (CW and CCW ends). This should read the marked value (e.g., 10kΩ, 100kΩ) within tolerance (usually ±20%). OL = open track. 0Ω = shorted track.
Connect one probe to the wiper (center terminal) and the other to one outer terminal. Slowly rotate the shaft from one end to the other. The resistance should change smoothly and monotonically from ~0Ω to the total resistance. Any jumps, dropouts, or erratic readings indicate a worn or cracked resistive track.
For audio/log pots, at mid-rotation (50%) the resistance from wiper to ground should be roughly 10–15% of total for log taper, or 50% for linear. This confirms the resistive track taper is intact and not damaged.
Check the plastic housing for cracks, burn marks, or melted areas. Shake the relay gently—there should be no rattling (loose armature or broken spring). The pins should be straight and corrosion-free. Look up the datasheet to identify coil pins vs. contact pins.
Identify the coil pins from the datasheet (often pins 1-16 for 8-pin relays, or marked on the body). Set multimeter to resistance (Ω). Typical 5V relay coils read 60Ω–120Ω; 12V coils read 200Ω–500Ω. The value should be stable and within the datasheet range.
With no power applied, test the contacts. For an SPDT relay: the Common (COM) pin should show continuity (low Ω) to the Normally Closed (NC) pin, and OL to the Normally Open (NO) pin. Verify with your multimeter in continuity mode.
Apply the rated coil voltage (e.g., 5V or 12V DC) to the coil pins. You should hear a crisp "click" as the armature pulls in. While energized, recheck contacts: COM should now connect to NO (low Ω) and be open to NC (OL). Remove power and verify it returns to the de-energized state. Pitted or welded contacts will not switch cleanly.
For high-current relays, pass a modest current (e.g., 100mA) through the closed contacts and measure voltage drop. Good contacts show <100mV drop. Higher drop indicates contact oxidation or pitting.
A dead laptop, a non-working CRT TV, the printer that stopped printing — these are goldmines for students who can't afford components. Every board you donate keeps toxic e-waste out of landfills and puts tools into eager hands.
Email us at amorphn@nordra.me with details about your donation or collection of the kits and any queries about our mission (For instance: where to ship the PCBs?). You can also take the mission into your own hands in your own way alternatively.