Timer IC NE555P, DIP-8, 4.5-16V, 200mA output

This tutorial shows how to use the NE555P integrated circuit as an independent timer, without a development board and without a program. The circuit works on its own, powered at 5 VDC, and in the example we use it as an astable oscillator that makes an LED blink about once per second.

The NE555P is used in projects for flashing LEDs, delays at power-on or power-off, tone generators, simple PWM control and debouncing. The times and frequencies are set exclusively by the values of the resistors and the capacitor connected to it.

How it works:

Inside the NE555P there is a divider made of three equal 5 kohm resistors, which creates two thresholds: 1/3 of the supply voltage VCC for the TRIG input and 2/3 of VCC for the THRES input. Two comparators compare the voltage on these inputs with the thresholds and control a flip-flop. The flip-flop sets the OUT output to high or low level and controls an internal transistor, which connects the DISCH pin to ground.

When TRIG drops below 1/3 of VCC, the output goes high and the discharge transistor turns off, so the external capacitor starts charging through the resistor. When THRES exceeds 2/3 of VCC, the output goes low and the transistor discharges the capacitor. The thresholds are proportional to VCC, and so is the charging speed, which is why the time does not depend on the supply voltage as long as it stays constant during the interval.

In monostable mode, a short pulse on TRIG produces a single pulse at the output, with a duration of t = 1.1 x R x C. For example, for R = 100 kohm and C = 10 uF we get t = 1.1 x 100000 x 0.00001 = 1.1 s.

In astable mode, the circuit oscillates continuously, with two resistors RA and RB and a capacitor C. The frequency is f = 1.44 / ((RA + 2 x RB) x C), the time with the output high is tH = 0.693 x (RA + RB) x C, and the time with the output low is tL = 0.693 x RB x C. For RA = 1 kohm, RB = 68 kohm and C = 10 uF we get tH = 0.478 s, tL = 0.471 s and f = 1.05 Hz.

The table below shows the logic states of the circuit.

Connection:

The table describes the astable circuit used in the usage example, powered at 5 VDC from a 5 VDC 1 A mains adapter, through the DC jack adapter with screw terminals. The chip is placed on the breadboard with the notch facing left, and pin 1 is the first pin at the bottom left, looking at the chip from above. Numbering goes counterclockwise, so pin 8 is at the top left. Mount the electrolytic capacitor C1 with the negative (stripe) to ground and the positive to pins 2 and 6, and the ceramic capacitor C2 is marked 103 (10 nF).

Power the circuit at a voltage between 4.5 V and 16 V and respect the polarity. With a 5 V supply the output is compatible with TTL inputs. At higher voltages the output rises close to VCC, so do not connect it directly to a 3.3 V logic input. The output can source or sink up to 200 mA. For inductive loads, for example a relay coil, mount a diode in parallel with the coil. In our example, the 220 ohm resistor limits the LED current to about 10 mA, which is safe for a 5 mm LED.

Configuration:

The NE555P has no buttons, jumpers, potentiometers or memory. The operating mode and the times are set exclusively by how the pins are connected and by the values of the external components. At every power-up the circuit restarts the cycle from the beginning, so there are no settings to save.

For monostable mode, connect pins 6 and 7 together and choose R and C for the desired duration. A short press of the button pulls TRIG below 1/3 VCC, the output goes high and stays that way for the time t. The pulse on TRIG must be shorter than the time t, otherwise the output stays high as long as the button is pressed.

For astable mode, first choose C, then RB for the time tL and RA for the difference between tH and tL. Because tH is always greater than tL, the duty cycle cannot go below 50 percent with the basic circuit. If you want a different ratio, mount a diode in parallel with RB, with the cathode toward pin 6. In this case tH depends only on RA and C, and tL depends only on RB and C.

The RESET pin (pin 4) stops the timing when pulled to ground and puts the output low. When you do not use it, connect it to VCC.

Usage example:

The example builds an LED that blinks about once per second. Mount the circuit from the connection table on a breadboard, with RA = 1 kohm, RB = 68 kohm, C1 = 10 uF electrolytic, a 10 nF capacitor on pin 5, a 5 mm red LED with a 220 ohm series resistor and a 5 V power supply.

The calculation gives tH = 0.693 x 69 kohm x 10 uF = 0.478 s and tL = 0.693 x 68 kohm x 10 uF = 0.471 s. The period is about 0.95 s, so the frequency is about 1.05 Hz.

After connecting the mains adapter to the screw terminals of the jack adapter, check the polarity of the supply and the position of capacitor C1 before powering up. Then power the assembly. The LED should light for about half a second and turn off for about half a second, repeatedly. If you want a slower rhythm, increase C1 or RB, and for a faster one decrease them, recalculating with the formulas above.

Common problems:

The LED does not blink and stays off. The most likely cause is pin 4 (RESET) left unconnected or connected to ground, which keeps the output low. Connect pin 4 to +5 V. Also check that pin 8 receives power.

The LED stays on without blinking. Pins 2 and 6 are not connected together or capacitor C1 is not connected correctly. Check the connections and the capacitor polarity, with the negative to ground.

The circuit does not work at all or gets hot. The chip is probably rotated and the pins are numbered wrongly. Turn off the power, check the position of the notch and the pin numbering, and rebuild the circuit.

The frequency differs from the calculated one. Electrolytic capacitors and resistors have tolerance, so the real values may differ. Measure the components or adjust the value of RB until you get the desired rhythm.

A relay driven by the output behaves erratically. The coil produces voltage spikes when it is switched off. Mount a diode in parallel with the coil and power the relay from a source sufficient for its current.

Limitations and tips:

The NE555P operates from 4.5 - 16 V and at temperatures between 0 and 70 °C. The output can source or sink up to 200 mA, so for larger loads use a transistor or a relay. For frequencies above 100 kHz the signal shape degrades, and for such applications a CMOS version is recommended.

Timing accuracy depends mainly on the tolerance of the resistors and the capacitor, so use low-tolerance components for precise timings. Electrolytic capacitors have leakage and high tolerance, so for very long or stable times choose other capacitor types.

With the same basic circuit you can further build a tone generator with a small speaker, a timer for lighting or a debouncer for a button, changing only the values of the resistors and the capacitor.