Built-in project · no microcontroller

TLC555 blinker on 3.3 V

The 555 blinker, wire for wire, on a 3.3 V supply with a TLC555: the CMOS 555 runs from 2 V where an NE555 will not start, at the same 1.4 or 7.4 Hz. There is no microcontroller in it: the simulator solves the 8 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 $6.15. On Mokxi they're free.

The circuit runs here once the page has loaded.

What you'll learn

A 555 timer as an oscillator
How resistors and a capacitor set the blink rate
Why a CMOS 555 works at 3.3 V
Calculating frequency with 1.44 over (R1 + 2 R2) C

Difficulty: Beginner
Time: About 20 minutes
Parts: 10

How it works

A TLC555 timer is wired as an astable oscillator: it flips its output on and off by itself. A capacitor charges and discharges through resistors, and the timer watches its voltage and switches at set points. The output drives a red LED with a 220 ohm resistor.

A slide switch picks the second resistor. With 47k the blink is slow, about 1.4 Hz. With 4.7k it is fast, about 7.4 Hz. The 10k resistor and the 10 uF capacitor stay the same.

The TLC555 is a CMOS 555 that runs from 2 V, so it works from 3.3 V. An ordinary NE555 needs 4.5 V and will not start. With the LED on, the high output sits at about 2.46 V.

Parts list

10 parts, plus the jumper wires. They retail for about $6.15; on Mokxi they're free.

1 × Full-size breadboard
1 × Power, 3.3 V
1 × Resistor, 10k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
1 × TLC555 CMOS timer
1 × Resistor, 47k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
1 × Resistor, 4.7k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
1 × Slide switchIn Logic Lab Kit, Spaceship Panel Kit.
1 × Capacitor, 10 µFIn Power Bank Kit.
1 × Resistor, 220 ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
1 × LED, redIn 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
Resistor, 10k Ω pin 1	3.3 V
TLC555 CMOS timer pin RESET	3.3 V
TLC555 CMOS timer pin VCC	3.3 V
TLC555 CMOS timer pin GND	Ground
Capacitor, 10 µF pin 2	Ground
LED, red pin C	Ground
TLC555 CMOS timer pin DIS	Resistor, 10k Ω pin 2
Resistor, 47k Ω pin 1	Resistor, 10k Ω pin 2
Resistor, 4.7k Ω pin 1	Resistor, 10k Ω pin 2
TLC555 CMOS timer pin THR	TLC555 CMOS timer pin TRIG
Slide switch pin 2	TLC555 CMOS timer pin TRIG
Capacitor, 10 µF pin 1	TLC555 CMOS timer pin TRIG
Resistor, 220 Ω pin 1	TLC555 CMOS timer pin OUT
Slide switch pin 1	Resistor, 47k Ω pin 2
Slide switch pin 3	Resistor, 4.7k Ω pin 2
LED, red pin A	Resistor, 220 Ω 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

// The 555 blinker on 3.3 V, with a TLC555, the CMOS 555.
//
// The same circuit as "555 blinker", wire for wire, with two changes: the
// chip is a TLC555 and the supply is 3.3 V, an ESP32's or a Pico's. An NE555
// will not start on 3.3 V (it wants 4.5 V); a TLC555 runs from 2 V.
//
// The timing is the RC's, so it is the same as on 5 V:
//
//   f  =  1.44 / ((R1 + 2 R2) C)
//
//   slow:  10k + 2 x 47k  = 104k  with 10 uF  ->  1.4 Hz
//   fast:  10k + 2 x 4.7k = 19.4k with 10 uF  ->  7.4 Hz
//
// And its output reaches the rail: high is 3.3 V with nothing on it, where an
// NE555 on 5 V only gets to 3.3 V. With the LED and its 220 ohm on it, the
// output sits at about 2.46 V while it is high, through the TLC555's own
// output resistance (about 200 ohm at the top).
//
// Click the slide switch while it runs to swap R2.

Troubleshooting

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

The circuit does not blink on 3.3 V. This project
A standard NE555 needs about 4.5 V. Use the TLC555, which runs from 2 V.
The blink speed is not what you expect. This project
The speed comes from the resistors and the 10 uF capacitor. Check which resistor the slide switch has chosen.
The chip does nothing at all. This project
RESET must be tied to 3.3 V through the 10k resistor, and VCC and GND both need to be connected.
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.
The output does not oscillate or is stuck. TLC555 CMOS timer
Check the timing resistors and capacitor and the pin connections: trigger and threshold tie together in astable mode. Reset (pin 4) must be high.
The frequency is not what you expect. TLC555 CMOS timer
For astable mode f is about 1.44 / ((R1 + 2 R2) C). Use accurate parts, and keep R values above a few kilohms.
The input reads random values in one position. Slide switch
A switch only connects or disconnects. Add a pull-up or pull-down resistor, or use INPUT_PULLUP, so the pin has a defined level when the switch is open.

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