NE555 — three bench projects

Reference setup

NE555P in PDIP8, isolated bench supply set to 5.00 V with current limiting. Use short connections, a multimeter and an oscilloscope with suitable probes. These are bench projects; none of their outputs controls a transmitter.

Closing KEY1 turns Q1 off: RESET rises and the NE555 oscillates. Opening KEY1 turns Q1 on: RESET goes low. The LM386 stays powered with gain pins open. Key clicks are possible because no shaped envelope is provided.

Circuit schematic

Identical labels indicate the same electrical net even when a wire is not drawn between cells. Every terminal is identified. Capacitor + signs show polarity; NC marks an unconnected pin. The JSON netlist is the wiring reference.

Bill of materials

Pinout and layout

NE555 PDIP8, top view: 1 GND, 2 TRIG, 3 OUT, 4 RESET, 5 CONT, 6 THRES, 7 DISCH, 8 VCC. Check the notch, package and manufacturer pinout before insertion. Potentiometer terminal names describe their function, not a universal physical order.

Place CDEC close to pins 8 and 1, and CCONT close to pin 5. Keep TIMING and TRIGGER wiring short. Audio load returns must not share the sensitive input return path. No qualified PCB is supplied.

Assembly steps

1. With power off, identify sockets and check values. Keep ICs out of their sockets during continuity checks.
2. Wire J1, grounds and supply capacitors. Check for shorts and correct polarity.
3. Wire NE555 pins from the netlist, then CCONT and the timing network.
4. Wire Q1, RBASE, RRESET and KEY1. The key connects the base to GND; it never connects the audio output to another device.
5. Add CIN, RPAD, RSH, RV2 and the complete LM386 stage. COUT positive goes toward pin 5; RZ and CZ are in series. Initially fit RTEST instead of SP1 and set volume to minimum.
6. Check every net, switch power off, insert ICs with the correct orientation and prepare test points.

First power-up

1. Set 5.00 V and an initial 50 mA current limit. Power up; stop for persistent limiting, rapid heating or incorrect voltage. Connect probe grounds only to TP0/GND.
2. With KEY1 open, measure RESET near 0 V and OUT low. RESET above 0.3 V requires checking Q1. Close KEY1: RESET should exceed 4 V and the electrical tone appears at TP3.
3. Adjust RV1 and compare TP2 and TP3 with nominal frequencies. At minimum volume, check AMP_DC near V5/2 and DC after COUT approaching zero once the capacitor charges. Stop if idle current exceeds 30 mA.
4. After idle checks, use a 150 mA limit for audio tests. Increase volume slowly with RTEST fitted. Check for clipping and oscillation; keep TP7 below 1 V RMS. Switch off before replacing RTEST with SP1.
5. Record levels, settings and instruments. These test points are for this dossier measurements, not an unknown input or a transmitter.

Troubleshooting

1. Excess current or heating: switch off and check rails, orientation, polarity and shorts before trying again.
2. No activity: verify the actual PDIP8 pinout, V5 and GND, then RESET, TRIG and THRES. Measure these before replacing the IC.
3. Continuous or missing tone: check Q1 orientation and RESET with KEY1 open and closed. A weak RESET pull-down is not a substitute for this transistor.
4. Correct TP3 but no sound: check the attenuator, wiper, LM386 pins and COUT. For distortion or squeal, reduce volume and inspect supply, decoupling and RZ/CZ.
5. Erratic behaviour: shorten wiring and check local decoupling, grounds and contacts. Sound or an LED cannot rule out parasitic oscillation.

Calculations and assumptions

The fixed divider limits the signal before the volume control and amplifier. Calculations assume at most 5 V peak-to-peak from the NE555, nominal gain 20 and a typical 50 kΩ LM386 input. Calculated power in 8.2 Ω depends on waveform; the speaker is not a pure resistor.

f ≈ 1.44 / [(RA + 2 RBtotal) CT]

RA = 4.7 kΩ; RBtotal = 6.8…16.8 kΩ; CT = 68 nF

f ≈ 553…1157 Hz; DH ≈ 56.1…62.8%

Rload ≈ 2.2 kΩ ∥ 10 kΩ ∥ 50 kΩ; RPAD = 100 kΩ; Av ≈ 20

IC ≤ 5 mA + 1.5 mA; IB ≥ (5 − 0.85)/3.9 kΩ

PTEST = VRMS² / 8.2 Ω; VRMS ≤ 1 V → PTEST ≤ 0.122 W

Nominal analytical values exclude full tolerances, capacitor leakage and device variation. A calculated range is not a manufacturer guarantee. Record actual measurements on the blank worksheet.

Measurement worksheet

Record supply voltage, current, fitted components, test points and waveforms. The supplied worksheet is blank and contains no prefilled results.

Supply and current

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Frequency / duration / duty cycle

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Output levels and load

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Instruments, observations, decision

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Dossier files

• Circuit schematic · SVG
• Netlist and design · JSON
• Calculations and assumptions · JSON
• Bill of materials · CSV
• Localized instructions · TXT
• Measurement worksheet · CSV

Manufacturer sources

• Texas Instruments NE555 — SLFS022K, March 2026
• Texas Instruments LM386 — SNAS545D, August 2023
• onsemi 2N3904 — NPN transistor

French source and agent-reviewed English pivot; other languages use machine translation with field and technical-token checks. No professional linguistic review is claimed. Schematics and netlists are shared across languages.