Built-in project · no microcontroller

NE556 two blinkers

The two 555 timers in one NE556, each an astable with its own LED: 1.5 Hz and 3.2 Hz from one chip on one 5 V supply. There is no microcontroller in it: the simulator solves the 11 parts as a circuit, so it runs the moment you press Run, and nothing needs compiling.

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Beginner About 20 minutes to build and run. Runs in your browser. Free, and no account needed.

The parts retail for about $6.10. On Mokxi they're free.

The circuit runs here once the page has loaded.

What you'll learn

The 555 astable oscillator
Frequency from two resistors and a capacitor
Two timers sharing one chip
Timing node with THR and TRIG tied together
Why bigger RC values blink slower

Difficulty: Beginner
Time: About 20 minutes
Parts: 13

How it works

An NE556 is two 555 timers in one 14-pin chip, sharing power and ground. Each timer is wired as a classic astable oscillator with its own resistors, capacitor and LED. Both RESET pins are tied to 5 V.

Timer 1 uses 1k and 47k with 10 uF, and blinks at about 1.5 Hz. Timer 2 uses 1k and 22k with 10 uF, and blinks at about 3.2 Hz. The formula is f = 1.44 / ((RA + 2 RB) C).

The two timers do not know about each other. Change one capacitor and only its LED changes speed.

Parts list

13 parts, plus the jumper wires. They retail for about $6.10; on Mokxi they're free.

1 × Full-size breadboard
1 × Power, 5 V
1 × NE556 dual timer
2 × Resistor, 1k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
1 × Resistor, 47k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
2 × Capacitor, 10 µFIn Power Bank Kit.
2 × Resistor, 220 ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
1 × LED, greenIn Starter Kit, Halloween Prop Kit, Logic Lab Kit.
1 × Resistor, 22k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
1 × LED, yellowIn Starter Kit, Halloween Prop Kit, Logic Lab Kit.

Wiring table

Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.

Part and pin	Connects to
NE556 dual timer pin 1RESET	5 V
NE556 dual timer pin 2RESET	5 V
NE556 dual timer pin VCC	5 V
Resistor, 1k Ω (1) pin 1	5 V
Resistor, 1k Ω (2) pin 1	5 V
NE556 dual timer pin GND	Ground
Capacitor, 10 µF (1) pin 2	Ground
LED, green pin C	Ground
Capacitor, 10 µF (2) pin 2	Ground
LED, yellow pin C	Ground
Resistor, 1k Ω (1) pin 2	NE556 dual timer pin 1DIS
Resistor, 47k Ω pin 1	NE556 dual timer pin 1DIS
NE556 dual timer pin 1TRIG	NE556 dual timer pin 1THR
Resistor, 47k Ω pin 2	NE556 dual timer pin 1THR
Capacitor, 10 µF (1) pin 1	NE556 dual timer pin 1THR
Resistor, 220 Ω (1) pin 1	NE556 dual timer pin 1OUT
NE556 dual timer pin 2THR	NE556 dual timer pin 2TRIG
Resistor, 22k Ω pin 2	NE556 dual timer pin 2TRIG
Capacitor, 10 µF (2) pin 1	NE556 dual timer pin 2TRIG
Resistor, 220 Ω (2) pin 1	NE556 dual timer pin 2OUT
Resistor, 1k Ω (2) pin 2	NE556 dual timer pin 2DIS
Resistor, 22k Ω pin 1	NE556 dual timer pin 2DIS
LED, green pin A	Resistor, 220 Ω (1) pin 2
LED, yellow pin A	Resistor, 220 Ω (2) pin 2

The notes that come with it

There is no microcontroller in this one, so nothing is compiled: the simulator solves the circuit itself. The file in the editor is the notes that come with it.

sketch.ino

// An NE556: two 555 timers in one chip, two LEDs blinking at their own rates.
//
// The NE556 is two NE555s in a 14-pin chip sharing VCC (pin 14) and GND
// (pin 7). Timer 1's pins are 1 to 6 (1DIS, 1THR, 1CTRL, 1RESET, 1OUT,
// 1TRIG) and timer 2's are 8 to 13 (2TRIG, 2OUT, 2RESET, 2CTRL, 2THR, 2DIS).
// Each is wired as the ordinary astable, RA from 5 V to DIS, RB from DIS to
// the timing node (THR and TRIG together) and C from the node to ground:
//
//   f  =  1.44 / ((RA + 2 RB) C)
//
//   timer 1:  1k + 2 x 47k = 95k with 10 uF  ->  1.5 Hz  (the bottom LED)
//   timer 2:  1k + 2 x 22k = 45k with 10 uF  ->  3.2 Hz  (the top LED)
//
// Both RESET pins are tied to 5 V. The two timers do not know about each
// other: change one capacitor and only its LED changes speed.

Troubleshooting

The usual mistakes with this circuit, and the fix for each.

An LED stays on or off This project
Check that THR and TRIG are joined together on that timer's capacitor, and that RESET is on 5 V.
Both blink at the same rate This project
Check that timer 1 uses the 47k resistor and timer 2 the 22k resistor. The rates differ because of those values.
The timer does not oscillate, or one half works and the other does not. NE556 dual timer
The two halves have separate pins. Check you wired the right half. Power is VCC on pin 14 and GND on pin 7. Tie the reset pins to VCC when not used.
The frequency is off or unstable. NE556 dual timer
For astable mode the frequency is about 1.44 divided by ((R1 plus 2 times R2) times C). Use a 10 nF capacitor from each control voltage pin to ground, and stable parts.
The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure.
A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground.
An electrolytic capacitor heats up, bulges or pops. Capacitor
Electrolytics have a polarity. The stripe marks the negative leg, which goes toward ground. Also keep the voltage under the rated value, such as 10 V or 16 V.
An RC circuit charges much slower or faster than expected. Capacitor
The time constant is R times C. A 10 k resistor with a 100 uF capacitor gives 1 second, and about 5 seconds to charge fully. Check that you read the value on the part right.

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