Why are two 555 timers in separate sub-circuits cross-talking? (Poltergeist in the Breadboard)

Question

As an introduction to electronics, I am following Charles Platt's Make: Electronics (2nd edition).

Every circuit worked as expected, until Experiment 17: Set Your Tone.

In circuit on figure 4-37 (page 163), two 555 timers are used in astable mode (as oscillators). The first timer (low frequency) output is wired to the control pin of the second timer (audio frequency), as to generate a siren sound... at least supposedly!

Minimal example

I have been facing different "surprising" behaviours with this circuit, and while trying to work my way out of this situation, I extracted the minimal example below:

This circuit is two 555 timers in astable mode mounted as separate sub-circuits on the same breadboard. (I call them separate because, as far as I can tell, they only share the power rails). The left timer should have a frequency ~1/10th right timer (thanks to the left discharge resistor being 100K, compared to 10K on the right)

This behavior is what I observe if I only provide power to one timer at a time. But if I connect both at the same time (as shown in the picture above), then something I cannot explain happens:

• They both start flashing in sync, at a high frequency (but not exactly the second timer frequency, something a bit higher).

I cannot see how those two separate sub-circuits can talk... And from my understanding of experiment 17 in the book, they are not supposed to.

Questions

• Is there a malicious being living inside the breadboard, and how should I name it?

• Otherwise, what is the rational explanation behind this behavior?

Additional details:

• I provide 9V via some universal transformer which provide quite stable voltage.
• The timers are marked 99AG7ZM NE555P
• The ceramic capacitor in series with the control pin is 0.01μF (marked: 103), as recommended by the book.
• The electrolytic capacitor is 10μF 25V
• I tried replacing both timers with other timers (exact same model), resulting in exactly the same behavior.
• Initially, I built the two sub-circuits very close, and tried another build with more distance (as the pictures in the post).

Measures

Measuring the voltage across the electrolytic (timing) capacitor gives a value oscillating between ~3.1V and ~6V on each sub-circuit, which is exactly what is expected. It is true when only one sub-circuit is connected to the power bus.

Measuring the voltage of the same capacitor in the left subcircuit when BOTH timers are connected to the power bus gives a stable voltage ~3.27V (barely oscillating between 3.265 and 3.275). I cannot explain that either (yet I do suspect this is all the same problem).

Close-ups

Here is a closeup of the left subcircuit:

And the right subcircuit:

Answer 1

Why did you leave out C3?

From Charles Platt's Make: Electronics (1st edition):

There is a reason for everything in an electronic circuit (and Platt did explain why it is needed). You have learned this the hard way.

From p155:

Set your power supply to deliver 9 volts. It will be convenient for this experiment if you supply positive down the righthand side and negative down the lefthand side of the breadboard, as suggested in Figure 4-14. C3 is a large capacitor, at least 100 µF, which is placed across the power supply to smooth it out and provide a local store of charge to fuel fast-switching circuits, as well as to guard against other transient dips in voltage. Although the 555 isn’t especially fast-switching, other chips are, and you should get into the habit of protecting them.

The comments and other answers have said you should use a decoupling capacitor of 0.1µF. You can certainly try that, but you appear to have a long power supply cable and Platt has recommended 100µF. So start small and increase, as in experiment.

To get it to work, you will need to link the circuits via R7. IC1 fires IC2.

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Why did Charles Platt leave out C3 in his second edition?

From p99 from Charles Platt's Make: Electronics (2nd edition):

A universal adapter such as the one in Figure 3-1 is the most versatile option, providing a switchable range of outputs. Typically these will include 3V, 4.5V or 5V, 6V, 9V, and 12V. Universal adapters are intended to power small devices such as voice recorders, phones, and media players. They may not deliver a perfectly smooth or accurate DC output, but you should be able to smooth the power yourself with a couple of capacitors, as I will illustrate when we get to a project that uses the adapter.

Not sure if he ever does that later in the book. He considers four power sources: 9V battery; universal adapter with smoothing capacitors; fixed adapter with 5V regulator; and a benchtop power supply. He states one of the the last three are essential. Odds are the 9V battery gets discharged which impacts circuits working properly.

He is considering better quality power supplys, so he is eliminating the need for decoupling capacitors. This really depends on users implementation of the power source and wiring. He does not rule decoupling capacitors out, but rather differentiates between good power supplies and poorer.

10µF or 22µF on power entry and tight circuit will help regardless. 0.1µF in addition would not hurt.

Answer 2

When the oscillators operate and switch the LEDs on and off, they will be creating voltage ripples on the power supply as the current demand changes.

These voltage ripples can then cause other devices on the same power rail to malfunction.

As mentioned in the comments, add a decoupling capacitor across the power pins on each of the 555 timers. A value of 0.1uF should be sufficient. Connect them as close to the 555 power pins as possible.

This will smooth out the ripples which may be affecting your circuit.

Answer 3

They are both connected to same power supply with long wires. Wires have inductance.

The 555 oscillators oscillate and when they toggle state they can take a short but high current spike from the power supply.

As inductance prevents high frequency currents, the supply voltage at the chips can dip every time they toggle.

Dips in the supply voltage are common to both chips, so their operation is affected by the voltage dips, regardless of which chip caused the dip.

Effect is similar to e.g. two metronomes going into perfect sync when operated on a wobbly table which acts as a common vibrating platform for both metronomes.

The power supply can also be a switch mode power supply that can cause high frequency voltage ripple for the chips.

The breadboard busbars also have stray inductance and stray capacitance between them.

Most importantly, the chips have no power supply bypass capacitors between their supply voltage pins, which would act as a local energy storage to prevent the local supply from having a dip when the chip needs a pulse of current quickly. Even a 10nF on each chip would do, best to try a 100nF on each chip, and maybe a 10 to 100 uF at the breadboard power entry for bulk storage.

Answer 4

There is no poltergeist in the breadboard, the poltergeist is the breadboard and they are notorious for their parasitic values that can create headaches for designers .

The circuit looks like this:

^{simulate this circuit – Schematic created using CircuitLab}

The resistance and inductance are from the breadboard rails, not to mention nF's of cross capacitance between the rails and other rails. The inductance was estimated, it could and probably is much more. The inductance is the thing that kills the circuit, because when either of the timers need power, the inductance of the breadboard blocks it momentarily (probably microsecond range). This means that you really need an oscilloscope to check the rails for voltage drops and a bypass capacitor is needed.

Answer 5

I'd like to suggest you check out this video from EEVBlog! He's an excellent teacher and has an entire video series on bypass capacitors.

Answer 6

With the addition of decoupling and bulk capacitors, if you have problems with environmental noises you can use IC555 which is more immune than ne555 and the same pinout is for easy replacement.

Answer 7

As an electrical engineer, I'd say the inductors ( or adding a ferrite bead ) would possibly be beneficial. Also, measure the DC value of the power supply at the 555 chips themselves and make sure the P.S. values are nominal. Remember, long, thin wires from the power supply itself are your enemy. Lastly, read up on the subject of "ground bounce"