A logic probe
A 555 clock with a logic probe on its output and another on a column nothing is driving: pulsing, and the floating input a voltmeter cannot find. There is no microcontroller in it: the simulator solves the 6 parts as a circuit, so it runs the moment you press Run, and nothing needs compiling.
Teaching? Assign it to your class in one click.
Beginner About 15 minutes to build and run. Runs in your browser. Free, and no account needed.
The parts retail for about $5.45. On Mokxi they're free.
What you'll learn
- How a 555 astable makes a square wave
- What a logic probe shows: high, low, pulse, or nothing
- Why a floating input has no real level
- Contention: two drivers fighting over one wire
- Difficulty: Beginner
- Time: About 15 minutes
- Parts: 6
How it works
A 555 timer is wired as an astable with two 10k resistors and a 100 nF capacitor. The capacitor charges and discharges, and the output pin flips high and low about 480 times a second.
The first logic probe sits on that output. It flips too fast to follow, so the PULSE lamp stays lit with the frequency beside it. The second probe sits on a column that nothing drives. Its lamps stay dark, which is different from reading low.
A voltmeter on that loose column would still give you a number, but the number means nothing. The probe tells you the truth by showing nothing.
Parts list
6 parts, plus the jumper wires and 2 bench instruments. The parts retail for about $5.45; on Mokxi they're free, and so are the instruments.
- 1 × Full-size breadboard
- 1 × Power, 5 V
- 2 × Resistor, 10k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
- 1 × NE555 timer
- 1 × Capacitor, 100 nFIn Power Bank Kit.
- 2 × Logic probe
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 (1) pin 1 | 5 V |
| NE555 timer pin RESET | 5 V |
| NE555 timer pin VCC | 5 V |
| NE555 timer pin GND | Ground |
| Capacitor, 100 nF pin 2 | Ground |
| Logic probe (1) pin GND | Ground |
| Logic probe (2) pin GND | Ground |
| NE555 timer pin DIS | Resistor, 10k Ω (1) pin 2 |
| Resistor, 10k Ω (2) pin 1 | Resistor, 10k Ω (1) pin 2 |
| NE555 timer pin THR | NE555 timer pin TRIG |
| Resistor, 10k Ω (2) pin 2 | NE555 timer pin TRIG |
| Capacitor, 100 nF pin 1 | NE555 timer pin TRIG |
| Logic probe (1) pin TIP | NE555 timer pin OUT |
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.
// A logic probe, and the fault a voltmeter cannot see.
//
// A 555 timer wired as an astable, 10 k, 10 k and 100 nF, so its output on
// pin 3 is a square wave of about 480 Hz. Two probes:
//
// the left one on the 555's output. Far too fast to watch flip, so H and
// L share the time and PULSE stays lit, with the frequency
// beside it: the probe stretches every edge to a fifth of a
// second, and at this rate there is never a gap.
// the right one on column 34, which is wired to nothing at all. Every
// lamp is dark. Not low: dark.
//
// That is the difference the probe exists for. Put a voltmeter on a floating
// CMOS input and you get a number, and the number means nothing: nothing is
// driving the pin, so what is on it is whatever leaked there. The probe says
// so by saying nothing, and no other instrument on this bench does.
//
// The thresholds are not the probe's. What each lamp is showing is the level
// the *driver* declared, so a 3.3 V board's pin is a clean H here for the same
// reason it is one on the board. There is no TTL/CMOS switch to get wrong,
// because there is no number inside the probe to be wrong. The voltage under
// the lamps is there for the case with no declared level at all: a divider, a
// capacitor charging, an analog node on its way somewhere.
//
// Things to try. Wire column 34 to the ground rail and the right-hand probe
// goes from dark to L. Wire it to the 5 V rail instead and it goes to H. Wire
// it to both at once and it goes to ?: two drivers disagreeing, which is
// contention, and which is a real fault a real probe shows the same way.
Troubleshooting
The usual mistakes with this circuit, and the fix for each.
- The probe on the 555 output shows PULSE instead of H or L This project
That is correct. At about 480 Hz the output switches too fast for the probe to show a steady level. - The right-hand probe stays dark and you expected L This project
Dark means nothing is driving the column. Wire it to the ground rail to see L, or to the 5 V rail to see H. - 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 nothing or stays high. NE555 timer
Pin 8 goes to the supply, pin 1 to ground, and RESET (pin 4) must be tied high. Floating reset is the most common mistake. - The frequency is unstable or wrong. NE555 timer
Tie pin 5 to ground through a 10 nF capacitor. In astable mode f is about 1.44 divided by (R1 + 2 x R2) x C. Watch the polarity of electrolytic caps. - 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.
Change it and keep it
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