One MOSFET, one bridge, one transient — and the measured data behind
every claim on the product page.
Instead of switching to an ohmmeter, the circuit breaks a biased divider for an
instant and watches how the node settles. The decay rate is the impedance between the
probes — which is how you get a continuity answer without ever leaving voltmeter mode.
First, a clarification
Two versions of the same idea
The same bridge trick is built two different ways, tuned for opposite goals. It
is worth knowing which one you are looking at, because the data further down this page comes
from both.
The product
Blinky Hawk
A compact detector that clips onto the meter you already
own. It is tuned for invisibility: the injected signal is deliberately
small so it barely disturbs the host meter's reading, which costs signal but keeps ghost
voltage down to a few millivolts.
A self-contained meter with its own display and its own external
ADC, tuned for information: a bigger reference and a much larger swing, so it can
resolve the rich in-between impedance states rather than just open-or-closed.
Not a product. It is a bench
concept piece — but it is the better instrument for characterizing the effect, so several
of the measurements below were taken on it.
Where the data comes from. Charts labelled
standalone were captured on the concept piece, whose larger signal makes the
underlying behavior easiest to see. The physics is identical in both builds; only the
amplitude differs, and the
comparison further down puts numbers to that difference.
The requirement
What the circuit has to do
A voltmeter reports one number for two completely different situations, and nothing in the
reading distinguishes them:
What's really happening
A normal meter says
Probes on the same net (truly connected)
0 V
Probe floating / not contacting anything
0 V
At best
Wasted troubleshooting time chasing a “dead” circuit that was never really probed.
At worst
You trust a “0 V” that is really a floating probe — and reach into
a circuit that is actually live.
Today's fix
The ohms-mode dance: power down → switch to Ω → read → switch back → power up.
Slow, repetitive, and easy to leave in the wrong mode.
The solution, in one sentence
A voltmeter that continuously and automatically
runs a continuity check between the probes whenever voltage is below a threshold (< 1 V) —
and alerts you the instant it sees a low-impedance path between the probes.
No mode switch. No exposed ohmmeter. No guessing what “0 V” means. It has to clear a short list of
hard requirements:
Survives ±1 kVDoesn't disturb the readingNo perceptible delayCompact & low power
The rest of this page is how that gets built.
How it works
One MOSFET, one bridge, one transient
The parallel front end (1.25 V LM4060 reference).
A MOSFET (Q8) sits in series with the sense leg of a biased voltage divider (1.25–2.5 V).
Two ADC taps straddle a bridge resistor — one tracks the bias, the other tracks the input.
Normally ON → the circuit behaves like an ordinary voltmeter ~99% of the time.
Toggle it off for an instant → the bridge breaks, creating a transient on the probes.
The transient's decay = the impedance between the probes. That's the whole signal.
Series resistors form the divider and limit current; Schottky diodes clamp the
varying node once the bridge is broken; a small resistor pair sets the probe-side amplitude (smaller =
less ghost voltage).
The key insight: break the bridge, watch how it settles
The decay rate is set by (probe impedance × node capacitance). Higher impedance → slower decay.
Read direction and rate, not an exact voltage:
Probes (bridge broken)
Differential decay
Verdict
Closed / low-Z
Fast decay
Leads Closed
Open / floating
Slow decay
Open Leads
High-Z / in-between
Intermediate decay
Maps to circuit impedance
The data
The decay maps cleanly to impedance
This is what the ADC actually samples on the standalone build (an ADS1015). Toggle the bridge and
the post-toggle decay gets slower the higher the probe impedance — even a coarse 17-sample capture orders
every decade from 10 kΩ to 200 MΩ (open) without overlap. A faster
ADC only sharpens the separation.
Post-toggle differential into the ADC, one trace per bridge resistance (standalone, ADS1015).
What the circuit presents to the world — the safety story
A fair question for anything that injects a signal: could it harm what you're probing? Measured front-end
data says no. The voltage at the probes only rises toward the reference as the leads open (it can never
exceed the reference), and the current is worst near a short — about 6 µA — collapsing
to single-digit nano-amps as impedance rises, exactly where a sensitive part would care.
Voltage presented to the probes (left) and resulting transient current into the circuit (right),
for both the 2.5 V and 1.25 V front ends.
≤ ref
Probe voltage
Peaks ~0.6–0.9 V and can never exceed the reference.
~6 µA
Worst-case current
Near a short — dropping to nano-amps when impedance is high.
~flat
Into the ADC
~1.0 V at 1.25 V ref, ~1.6 V at 2.5 V ref, independent of probe R.
The two builds, side by side
What the tradeoff actually costs
Here is the price of that difference in goals, measured. The
standalone concept piece runs a 2.5 V reference and gets a
~340 mV span between a short and 10 MΩ — enormous headroom for
resolving in-between states. Blinky Hawk deliberately gives most of that
up, working with about 10 mV, because the same signal that carries
information also lands on the host meter's display as ghost voltage.
Settled differential vs probe resistance — the quantity the firmware thresholds on.
Standalone version
Blinky Hawk (parallel)
Purpose
Standalone voltmeter that detects closed leads
Ternary Open / Closed / Voltage, minimal ghost V, compact
ADC
ADS1015 on an isolated rail
Internal MCU ADC
Voltage reference
2.5 V (LM4040)
1.25 V (LM4060)
Permanent sense-divider
1 MΩ
1 MΩ
Ghost voltage on host DMM
−170 mV
−5 mV DC (~80 mV AC)
Signal differentiation (10 M → short)
~340 mV
~10 mV
Power draw
—
~2 mA idle (low-power polling), 11 mA awake
What's “ghost voltage”? The few-millivolt offset the host meter displays when the leads are
open. It's cosmetic, not a measurement error: the detector sits behind very high source resistance, so probing
any real circuit pins the reading to the true value — and the instant a voltage is detected, the bridge stops
toggling entirely. Ghost voltage is just the small price of leaving the detector clipped on, visible only when
there's nothing to measure.
Robustness
Survives real-world use and abuse
Detection keeps working from 0 V all the way to ±1 kV, because it lives
behind the meter's protected voltmeter inputs — nothing fragile is ever exposed. Push the input
harder and the sense rail simply saturates while the protection diodes do their job; there's nothing
delicate to damage. Here it is still indicating during everyday use on a live 120 VAC outlet, and
under genuine abuse with −1 kV applied from a megohmmeter.
Live 120 VAC — detection still indicating.−1 kV applied — behind the protected inputs.
The hardware
Real builds, validated on lots of silicon
These aren't breadboard toys. The standalone build runs on an ADS1015 / ADS1115; Blinky Hawk,
the minimalist parallel build, has been validated on a Renesas RA4M1, an
RP2040 (XIAO), and a SAMD21 (QT Py). Every ADC + MCU combination tried
functions as expected.
The family: a parallel detector clipped onto a commercial DMM, plus standalone units.
Standalone — 2.5 V ref, external ADC, live on-device readout.Blinky Hawk (parallel) — 1.25 V ref, the MCU's own ADC.
