What is this phenomenon with test leads?

Thread Starter


Joined Sep 20, 2022

I have a beginner question about some behaviour I'm seeing with different connections between power (a 9V battery) and a simple 555 timer circuit which blinks some LED's.

When I connect the battery directly to the circuit I observe correct behaviour and there are no issues (see "Expected.png"), the LED's blink on and off as I want them to with the correct timing.

However, certain things cause the LED's to not blink correctly, they flicker for a fraction of a second before lighting fully. (see "WTF.png") and I observed the following behaviour which all centered around how the battery was connected.

Using direct battery connection (5cm battery connector) -> Gives expected result.

Using 30cm test leads with 22AWG stranded core -> Gives WTF result.

Using 30cm test leads with 22AWG solid core -> Gives expected result.

Measuring current from battery to breadboard with Fluke multimeter and Fluke test lead (measures about 40mA) - Gives WTF result.

As I don't yet know the name for this phenomenon I'm finding it challenging to search for it, so any pointers would be greatly appreciated!


Joined Aug 7, 2020
It’s called “decoupling”.
Most digital circuits take current from the supply only when they change state.The old 555 is one of the worst offenders.
This dips the voltage at the IC supply terminals due to the resistance and inductance of the supply connection.
To prevent it from happening a store of energy close to the IC is required to supply the current it takes whilst changing state. The store of energy is a “decoupling capacitor”.
For most ICs, 100nF ceramic is used.


Joined Aug 21, 2008
Your waveforms do not unambiguously show the problem. It is possible that for some reason the width of your pulses is changing with time, or that noise (such as the power line frequency) is mixing with your signal causing jitter with respect the incoming signal in you triggering circuit.

Make sure all of your ground are connected and that there is no undesirable noise on the trigger signal.