Pinout diagram
Summary specifications
Full pin reference
| Pin | Name | Group | Direction | Levels | Description |
|---|---|---|---|---|---|
| 1 | GND | Ground | Power | 0 V | Ground reference. Tie directly to the supply 0 V rail. |
| 2 | TRIG | Analog | Input | 1/3 VCC threshold | Trigger input. A falling edge below 1/3 VCC sets the output high and starts the timing cycle. |
| 3 | OUT | I/O | Output | 0 V to VCC (approx) | Output. Sources or sinks up to 200 mA; sits near GND when low and near VCC when high. |
| 4 | /RESET | Control | Input | TTL, active low | Active low. Reset. Pulled low to force the output low and end the timing interval; tie to VCC if unused. |
| 5 | CTRL | Analog | Bidirectional | 2/3 VCC | Control voltage. Overrides the internal 2/3 VCC reference; bypass to ground with 10 nF when unused. |
| 6 | THRES | Analog | Input | 2/3 VCC threshold | Threshold input. Rising above 2/3 VCC resets the output low and discharges the timing capacitor. |
| 7 | DISCH | I/O | Open-drain / open-collector | open-collector | Discharge. Open-collector transistor that drains the timing capacitor to ground; not a push-pull output. |
| 8 | VCC | Power | Power | +4.5 V to +16 V | Positive supply. Decouple with 100 nF close to pin 8. |
Common uses and repair diagnostics
The NE555 is the ubiquitous 8-pin timer that, with two resistors and a capacitor, produces a stable square wave or a single timed pulse. It appears in retro computers as a reset generator, a debounce circuit, a clock source and a missing-pulse detector, and the same pinout covers the bipolar NE555, the CMOS TLC555 and the low-voltage LMC555.
In retro hardware the 555 most often generates the power-on reset pulse: a capacitor on the THRES pin charges through a resistor until it crosses 2/3 VCC, holding RESET low for a few milliseconds after switch-on, after which DISCH releases and the CPU runs. The same circuit, with the values swapped, debounces a push-button or stretches a short pulse into something a slow peripheral can catch.
In astable mode the 555 becomes a free-running clock, and it is still the quick way to drive a speaker, an LED flasher or a sample-and-hold strobe. Pair it with the 555 Timer & LED Calculator on this site (/bench-tools/555-timer-led-calculator) to pick the timing resistors and capacitor before you breadboard it. The bipolar NE555 and the CMOS TLC555 share this pinout, but the CMOS part draws far less supply current and swings its output rail-to-rail, which matters for low-voltage battery gear.
- Measure pin 8 (VCC) and pin 1 (GND) first; the 555 will not trigger reliably below 4.5 V on the bipolar part.
- In astable mode, scope pin 3 (OUT) and pin 7 (DISCH). A flat output with no discharge activity points to the timing capacitor being open or the CTRL pin floating.
- Tie pin 4 (RESET) to VCC if it is unused. A reset left floating can randomly pull the output low and look like an intermittent timing fault.
- Check pin 5 (CTRL) sits at roughly 2/3 VCC; if it reads 0 V or VCC the internal divider has failed or an external modulation signal is shorting it.
- If the output drives an inductive load, confirm pin 3 is not being pulled below ground; back-EMF can punch through the output stage and kill the timer.