555 Timer Calculator (Bistable Mode)
Explore the 555 in bistable (flip-flop) mode: set/reset inputs with no RC timing. Shows trigger thresholds, output current limits, and a live SET/RESET/Q timing diagram.
How to Use
- Enter the supply voltage Vcc — everything in the 555 is ratiometric to it.
- Trigger pin 2 below 1/3·Vcc to SET the output high.
- Pull reset pin 4 below ~0.7 V to RESET the output low.
- There is no RC timing in bistable mode — the output holds its last state until the other input fires.
- The timing diagram shows an example SET pulse → Q high → RESET pulse → Q low sequence.
Operating Notes
Formulas & Limits
History of the 555 Timer
The 555 was designed by Hans Camenzind at Signetics in 1971 and entered production in 1972. Camenzind prototyped the chip after his original pitch was rejected; he was paid a one-time fee and no royalties, despite the part going on to sell over a billion units a year — one of the highest-volume integrated circuits ever manufactured.
Bistable mode is the 555's fourth configuration, after astable (oscillator), monostable (one-shot), and Schmitt/hysteresis (comparator). In bistable mode, the chip behaves as a simple SR latch: the upper comparator is disabled by tying threshold (pin 6) to ground, and only the lower comparator (trigger, pin 2) and the direct reset (pin 4) control the internal flip-flop. No timing capacitor is needed — the circuit is entirely digital.
Though microcontrollers have displaced the 555 in most new designs, bistable-mode 555s still appear in industrial control panels, alarm latches, and safety interlocks because of their robust 200 mA drive capability, wide supply range, and lack of firmware — a reassuring property for safety-rated equipment that must work the same way every time, forever.
About This Calculator
Unlike astable and monostable modes, bistable has no RC timing to compute — the output holds whichever state was last commanded. This page instead reports the ratiometric voltage thresholds (1/3 Vcc for trigger, ~0.7 V for reset), verifies the supply is within the NE555's 4.5-16 V range, warns if the output load exceeds the 200 mA limit, and provides an interactive SET/RESET/Q timing diagram you can drive with on-screen buttons to see how the latch responds.
Everything runs entirely in your browser — no values are sent to a server. For CMOS operation (2-18 V supply, ~Vcc − 0.1 V high output, ~1 µA standby), substitute a 7555 / LMC555. Both pin-compatible variants work the same way in bistable mode.
Frequently Asked Questions
What is bistable mode?
Bistable (two-stable-states) mode uses the 555 as a simple SR latch — essentially an RS flip-flop. A short negative pulse on trigger (pin 2) sets the output high; a short negative pulse on reset (pin 4) returns it low. No timing capacitor is used.
Why use a 555 as a flip-flop instead of a 74HC74?
The 555 sources and sinks up to 200 mA directly — you can drive an LED, relay coil, or small motor with no external transistor. A 74HC74 needs a buffer above ~20 mA. The 555 also works from 4.5-16 V, which is convenient for higher-voltage set/reset lines.
How do I wire up the bistable?
Pin 1 (GND) to ground, pin 8 (Vcc) to supply. Pin 5 (control) gets a 10nF cap to ground for noise immunity. Pin 6 (threshold) is tied to ground — this disables the upper comparator so timing can't end. Pins 2 (trigger) and 4 (reset) are pulled high through 10kΩ resistors and taken to ground momentarily by pushbuttons or logic.
What's the minimum trigger pulse width?
Datasheet specifies 10 µs minimum for reliable triggering. Shorter pulses may miss, especially near the threshold. For clean operation use pulses of 100 µs+ from push-buttons (with RC debounce) or ~10 µs+ from logic gates.
Can I tie multiple triggers together?
Yes — OR multiple trigger sources with diodes to a common pull-up. Any low-going pulse then sets the output. Same for reset. This lets you build multi-source set/reset latches for interlock logic.
Common Use Cases
Latching Push-Button
First press turns an LED/relay on (SET), second press turns it off (RESET). Great for simple on/off control of lights, fans, or small pumps.
Alarm Latch
Sensor input triggers SET, stays latched until a manual reset button clears it. Common for intrusion alarms, overtemperature cutouts, and fault indicators.
Debounced Toggle
Combine with a monostable or Schmitt input to turn a bouncy mechanical switch into a clean latched output.
Manual Override
Automatic logic sets the output; a front-panel button resets it. Lets a human override an automated process without cutting power.
Power-On Latch
RC on the set pin fires once at power-up to put the system into a known state. Reset button returns to idle.
Relay Driver
Direct 200 mA output can drive small 5-12 V relay coils without an intermediate transistor — useful for simple industrial control panels.
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