Astable Multivibrator Circuit Diagram
This is a free printable astable multivibrator circuit diagram: download the diagram as SVG or open it and print to paper or PDF.
An astable multivibrator circuit diagram shows an oscillator with no stable state — it continuously switches between HIGH and LOW output, generating a square wave without any external trigger. The most common implementation uses the 555 timer IC configured with two resistors (RA, RB) and a capacitor (C) to set the frequency and duty cycle. Astable multivibrators are used for LED flashers, tone generators, clock sources, and PWM signal generation.
The 555 timer IC (NE555, LM555) in astable mode is the most popular astable multivibrator configuration. The IC contains a voltage divider (three 5 kΩ resistors setting thresholds at 1/3 VCC and 2/3 VCC), two comparators, an SR flip-flop, and an open-collector discharge transistor. In astable mode, the capacitor C charges through RA + RB and discharges through RB alone, creating a continuous oscillation.
Circuit connections: VCC to pins 8 and 4 (reset held HIGH). Output at pin 3. Trigger (pin 2) and threshold (pin 6) are connected together to the top of capacitor C. Capacitor C connects from the pin 2/6 junction to GND. Discharge (pin 7) connects to the junction of RA and RB. RA is between VCC and pin 7. RB is between pin 7 and pin 2/6 (the capacitor top). A 10 nF bypass capacitor from pin 5 to GND improves noise immunity.
Operation cycle: When C is charging (output HIGH), the flip-flop keeps pin 7 (discharge transistor) open. C charges through RA + RB toward VCC. When C reaches 2/3 VCC, the threshold comparator trips the flip-flop, pulling pin 7 LOW. C now discharges through RB toward GND. When C falls to 1/3 VCC, the trigger comparator resets the flip-flop, releasing pin 7. C begins charging again through RA + RB. The cycle repeats indefinitely.
Frequency formula: f = 1.44 / ((RA + 2×RB) × C). The factor 1.44 = ln(2)^−1 (derived from the RC exponential charging/discharging between 1/3 and 2/3 VCC).
Period: T = 1/f = 0.693 × (RA + 2×RB) × C.
High time (output HIGH): tH = 0.693 × (RA + RB) × C. Low time (output LOW): tL = 0.693 × RB × C. Duty cycle: D = tH / T = (RA + RB) / (RA + 2×RB). The duty cycle is always > 50% in the standard configuration because both RA and RB contribute to charging but only RB to discharging.
Achieving 50% duty cycle: Add a diode (e.g. 1N4148) in parallel with RB (cathode toward pin 7, anode toward pin 2/6). During charging, current bypasses RB through the diode, making tH = 0.693 × RA × C. During discharging, the diode is reverse biased and tL = 0.693 × RB × C. Setting RA = RB gives D = 50%.
Design example — 1 Hz LED flasher: Target f = 1 Hz, choose C = 10 μF. RA + 2RB = 1.44/(1 × 10×10^−6) = 144 kΩ. Choose RB = 68 kΩ, then RA = 144k − 136k = 8 kΩ → use 10 kΩ. Resulting f ≈ 1.44/(206k × 10μF) ≈ 0.7 Hz. Adjust RB downward to fine-tune.
BJT astable multivibrator (discrete): Two NPN transistors (Q1, Q2, e.g. 2N2222 or BC547) cross-coupled through two RC timing networks form a two-transistor astable. When Q1 is ON, Q2 is OFF. C2 charges through R2 until it forward-biases Q2, turning Q2 ON and turning Q1 OFF via the cross-coupling. Period = 0.693 × (R1C1 + R2C2). For equal components: T = 1.386 × RC.
Applications: LED blinking indicators, audio tone generators (attach a small speaker or buzzer to pin 3), PWM motor speed control, clock signals for digital counters and shift registers, and strobe lights.
Output current capability: The 555 can source or sink up to 200 mA at pin 3, enough to drive an LED directly (with a current-limiting resistor) or a small relay coil. For higher current loads, use a transistor driver stage.
You can build and simulate an astable multivibrator in the free circuit diagram editor at circuitdiagrammaker.com — place the 555 symbol, add RA, RB, and C, and visualise the oscillation waveform.
How to wire astable multivibrator circuit diagram
- Choose target frequency and duty cycle Decide on the oscillation frequency (e.g. 1 Hz for a visible LED flasher, 1 kHz for an audible tone) and whether 50% or asymmetric duty cycle is needed.
- Calculate RA, RB, and C Fix C (e.g. 10 μF for low frequency, 10 nF for audio). Then RA + 2RB = 1.44/(f × C). Assign RB for the desired duty cycle and solve for RA = (1.44/(f×C)) − 2×RB.
- Connect the 555 timer Wire VCC to pins 8 and 4. Connect pins 2 and 6 together and to the top of C. Connect C bottom to GND. Connect discharge (pin 7) to junction of RA and RB. Add 10 nF decoupling on pin 5 to GND.
- Add output load For an LED, connect a 470 Ω current-limiting resistor from pin 3 to the LED anode, LED cathode to GND. For a buzzer or relay, add a transistor driver.
- Power up and verify oscillation Apply supply voltage and observe the LED blinking or measure the square wave on pin 3 with a multimeter in frequency mode or an oscilloscope.
- Measure frequency and duty cycle Use an oscilloscope to measure tH and tL. Calculate actual f = 1/(tH+tL) and D = tH/(tH+tL). Compare with design targets.
- Adjust component values if needed Increase C to reduce frequency, decrease C to increase frequency. Adjust RB relative to RA to shift duty cycle.
Specifications
| Oscillation frequency | f = 1.44 / ((RA + 2×RB) × C) |
|---|---|
| Period | T = 0.693 × (RA + 2×RB) × C |
| High output time | tH = 0.693 × (RA + RB) × C |
| Low output time | tL = 0.693 × RB × C |
| Duty cycle (standard) | D = (RA + RB) / (RA + 2×RB) > 50% |
| Supply voltage range (NE555) | 4.5 V – 15 V (CMOS TLC555: 2 V – 15 V) |
| Output current capability (pin 3) | Source/sink up to 200 mA |
| Threshold comparator voltages | 2/3 VCC (upper) and 1/3 VCC (lower) |
| Timing component range | RA, RB: 1 kΩ – 10 MΩ; C: 500 pF – 1000 μF |
| IC part numbers | NE555, LM555, SA555 (bipolar); TLC555 (CMOS, lower power) |
Safety warnings
- Keep pin 4 (reset) firmly tied to VCC; a momentary LOW on reset immediately stops oscillation — use a pull-up resistor if reset is connected to an external control signal.
- Do not exceed the maximum supply voltage (15 V for NE555) — overvoltage destroys the internal comparators; use TLC555 for 3.3 V logic-level systems.
Tools needed
- NE555 or TLC555 timer IC
- Resistors RA and RB (1 kΩ – 1 MΩ range, from formula)
- Capacitor C (from formula; film type for precision)
- 10 nF decoupling capacitor (for pin 5)
- DC regulated power supply (5 V or 9 V)
- Oscilloscope or multimeter with frequency measurement
Common mistakes
- Omitting the 10 nF decoupling capacitor on pin 5 (control voltage) — noise coupling into pin 5 shifts the comparator thresholds and causes erratic frequency.
- Not connecting pin 4 (reset) to VCC — a floating or LOW reset pin keeps the output permanently LOW and the circuit will not oscillate.
- Using too small an RA (below 1 kΩ) — when pin 7 pulls LOW during discharge, the discharge transistor must absorb the full RA current, potentially overloading the internal transistor.
- Using electrolytic capacitors without the correct polarity — the negative terminal must go to GND; reverse polarity slowly damages the capacitor.
Troubleshooting
- 555 output stays permanently HIGH or LOW
- Cause: Pin 4 (reset) is not connected to VCC, or the timing capacitor is shorted or open. Fix: Confirm pin 4 is tied to VCC. Test the capacitor for correct value and absence of shorts. Replace the capacitor if suspected faulty.
- Frequency is far from calculated value
- Cause: Capacitor value is incorrect (wrong marked value or high tolerance), or there is significant stray capacitance on the PCB. Fix: Measure C with an LCR meter and recalculate. Use 1% film capacitors for precision timing. Keep PCB traces short near pin 2/6.
- Output waveform is distorted or not reaching full VCC swing
- Cause: Output load current exceeds 200 mA, pulling the output voltage down. Fix: Add a transistor buffer between pin 3 and the load. Ensure load current stays within the 555's 200 mA output specification.
Frequently asked questions
How does an astable multivibrator circuit diagram using a 555 work?
The 555's internal comparators monitor the capacitor voltage. When C charges to 2/3 VCC, the output goes LOW and the discharge transistor discharges C through RB. When C falls to 1/3 VCC, the output goes HIGH and C charges again through RA+RB. This cycle repeats automatically, generating a continuous square wave.
What is the frequency formula for a 555 astable multivibrator?
f = 1.44 / ((RA + 2×RB) × C), where RA and RB are in ohms and C is in farads. For example, RA = 10 kΩ, RB = 68 kΩ, C = 1 μF: f = 1.44 / ((10k + 136k) × 1×10^−6) = 1.44 / 0.146 ≈ 9.9 Hz.
Can a 555 astable circuit produce a 50% duty cycle?
Yes, by adding a diode (1N4148) in parallel with RB: cathode toward pin 7, anode toward pin 2/6. During charging, the diode bypasses RB so tH = 0.693×RA×C; during discharging, the diode is reverse biased and tL = 0.693×RB×C. Setting RA = RB gives exactly 50% duty cycle.
What is the difference between an astable and a monostable 555 circuit?
An astable (free-running) multivibrator has no stable state and oscillates continuously. A monostable (one-shot) has one stable state and produces a single pulse of defined width T = 1.1×R×C when triggered by an external pulse. An astable needs no trigger; a monostable needs a falling-edge trigger.
What is the difference between bipolar NE555 and CMOS TLC555?
The bipolar NE555 can source/sink up to 200 mA and operates from 4.5–15 V. It has higher quiescent current (~6 mA). The CMOS TLC555 operates from 2–15 V, consumes only ~170 μA quiescent, but has lower output current capability (~100 mA). Use TLC555 for battery-powered designs.
What values of RA and RB are safe for the 555 timer?
RA should be at least 1 kΩ to limit the discharge current into pin 7 when the discharge transistor turns on. Maximum practical values are about 10 MΩ. For RB, a minimum of 1 kΩ is recommended. Excessively high values (>10 MΩ) cause frequency instability due to leakage currents.
Can I use a 555 astable circuit to drive a motor or relay?
The 555 can directly drive small loads up to 200 mA. For a relay coil, add a flyback diode across the relay. For motors above 200 mA, use a transistor or MOSFET driver between pin 3 and the motor, with the 555 providing only the gate/base drive signal.