Adjustable pulse generator

A slow LED astable with two frequency ranges obtained by replacing the capacitor with power off. Test points compare capacitor waveform, output and duty cycle.

Documented designs awaiting construction and prototype tests. Values are calculated from nominal components; no frequency, duration, audio power or stability has been measured. File checks are not electrical tests.

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.

With CT of 10 µF, nominal frequency varies from 0.626 to 4.8 Hz. Replacing it with 1 µF gives 6.26 to 48 Hz. High duty cycle stays above 50% and varies with RV1. STOP holds RESET low.

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.

With power off, identify sockets and check values. Keep ICs out of their sockets during continuity checks.

Wire J1, grounds and supply capacitors. Check for shorts and correct polarity.

Wire NE555 pins from the netlist, then CCONT and the timing network.

Join pins 2 and 6 at TIMING. RA connects V5 to DISCHARGE; RB and RV1 then lead to TIMING. Join potentiometer W and B.

Fit CT of 10 µF, positive to TIMING, then STOP and the LED. Keep the alternate 1 µF capacitor separate; it replaces CT rather than adding in parallel.

Check every net, switch power off, insert ICs with the correct orientation and prepare test points.

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.

Observe TP2 and TP3 with a ×10 probe. After several cycles, measure period, high time and low time at both RV1 extremes.

Compare nominal calculations with measurements. TP2 moves approximately between V5/3 and 2 V5/3; OUT does not necessarily reach the positive rail. An LED cannot measure these levels.

Switch off, discharge CT and replace it with 1 µF. Repeat measurements; nominal frequency increases tenfold. Check STOP and record startup transients separately from steady operation.

Record levels, settings and instruments. These test points are for this dossier measurements, not an unknown input or a transmitter.

Excess current or heating: switch off and check rails, orientation, polarity and shorts before trying again.

No activity: verify the actual PDIP8 pinout, V5 and GND, then RESET, TRIG and THRES. Measure these before replacing the IC.

No oscillation: check RESET, the joined pins 2 and 6, the RA/RB/RV1 path and CT connection to GND.

Unexpected range: measure RV1 with power off, check wiper joined to B and verify CT value. Duty cycle variation is normal in this circuit.

Erratic behaviour: shorten wiring and check local decoupling, grounds and contacts. Sound or an LED cannot rule out parasitic oscillation.

RBtotal includes RB and the used part of RV1. The equations describe steady operation of the classic circuit, not 50% duty cycle. TP3 is not a 50 Ω source or a universal logic output.

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.

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

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.

tH ≈ 0.693 (RA + RBtotal) CT; tL ≈ 0.693 RBtotal CT

RA = 10 kΩ; RBtotal = 10…110 kΩ

CT = 10 µF → f ≈ 0.626…4.8 Hz; DH ≈ 52.2…66.7%

CT = 1 µF → f ≈ 6.26…48 Hz; DH unchanged

TP2 ≈ VCC/3…2 VCC/3; TP3: high-impedance measurement only

Texas Instruments NE555 — SLFS022K, March 2026

https://www.ti.com/lit/ds/symlink/ne555.pdf

Disclaimer

These dossiers are supplied as is for educational and experimental purposes. Calculations, simulations and file checks are neither measured performance nor certification or qualification of the assembled device.

The person building or using the device must check the schematics, components, connections and protective measures, follow manufacturer instructions and applicable regulations, and have the completed device validated before use. Depending on the device, electrical voltages, radiofrequency energy and heat can cause electric shock, burns or fire. Tests must be conducted by a competent person, starting at low power with suitable protective measures.

To the extent permitted by law, ON4CRD and the contributors do not guarantee fitness for a particular purpose or results, and disclaim liability for damage arising from construction, modification or use. This notice does not limit any mandatory rights or any liability that the law does not allow to be excluded.
