Blinky Hawk User Guide

1 · What it does

Three answers instead of one ambiguous zero

A normal voltmeter reading 0 V can't tell you whether you're on a genuinely connected net, or simply not touching anything — a floating probe, a clip that fell off, an open relay. Blinky Hawk resolves that ambiguity continuously:

Closed

Green flash + beep

The probes see a low-impedance path — your 0 V is a real zero on a connected circuit.

Open

Dim blue flash

The probes are floating — that 0 V means not connected, not "no voltage."

Voltage

Red flash + double beep

A live voltage is present. The continuity check suspends itself automatically until the voltage is gone.

It works by briefly interrupting an internal resistor bridge and watching how the resulting microsecond transient decays across the probes — high impedance decays slowly (open), low impedance decays fast (closed). The injected signal is tiny (never more than ~1.25 V behind hundreds of kilohms, worst-case ~6 µA), your meter stays in voltmeter mode the whole time, and detection runs many times per second. Full technical background is on the main project page.

2 · Quick start

From box to first detection

Plug Blinky Hawk into your meter. Its stackable banana plugs go straight into the meter's V / COM jacks — the same two jacks you use for voltage measurements.
Plug your test leads into the back of Blinky Hawk's plugs. The connection is purely parallel: your probes, your meter, and your workflow are unchanged.
Flip the toggle toward the meter. That one position both powers the unit and connects it to the measurement path. The switch has three positions — center is off, and the far side is a test load rather than a second on (details below).
Watch the power-on cue. The LED blinks green 1–4 times to report the battery level (1 = 0–25%, 4 = 75–100%).
Use your meter normally. Touch the probes together — green flash and a beep. Separate them — dim blue flashes. Probe something live — red flash and a double beep, and the meter shows the voltage as always.

Voltage modes only. Leave your meter in DC or AC volts while Blinky Hawk is on — that's the mode it's designed to live in, and it never asks you to leave it. Before using any other mode (resistance, continuity, diode, capacitance, or current), flip Blinky Hawk OFF: it sits in parallel with the input jacks, and its sense bias can disturb those measurements. OFF makes your meter fully stock again.

3 · The toggle

One switch: power, isolation, and a test load

The toggle is a triple-pole, three-position switch with a center-off. One pole switches the battery; the other two galvanically connect or disconnect both measurement leads. The center position is off, and the two ends do different things:

Position	Blinky Hawk	Across your probes	What it means
Toward the meter	On	≈ 1.1 MΩ	Normal operation. Blinky Hawk is powered and connected across the probes, detection runs continuously, and the meter may show a few millivolts of ghost voltage when the leads are open.
Center	Off	Nothing	Fully inert. The battery is disconnected and the detector is galvanically removed from the probe circuit — your meter behaves 100% stock, with zero ghost voltage and zero battery drain. You can leave Blinky Hawk fitted permanently and simply center the switch when you don't want it. This is the right position for the meter's non-voltage modes (Ω, continuity, diode, current). Note that it disconnects the battery from the charger too — the switch must be toward the meter to charge.
Away from the meter	Off	100 kΩ	Blinky Hawk stays off, but a 100 kΩ resistor is placed across the meter's inputs as a deliberate test load. See below.

The 100 kΩ position: identifying high-source-impedance voltages

A voltmeter reading tells you a voltage is present. It does not tell you whether that voltage can do anything — whether it is a real supply, a reading sitting behind a large source resistance, or a few picofarads of coupling. The difference shows up the moment you load it, and the three switch positions give you three known loads without changing anything else in your setup:

Switch	Load across the probes
Center (off)	Your meter alone — typically 10 MΩ
Toward the meter	1.1 MΩ (Blinky Hawk's bridge), in parallel with the meter
Away from the meter	100 kΩ

The 1.1 MΩ is two 500 kΩ divider legs plus a ~100 kΩ sense resistor, and it is what the probes see whenever the bridge is in its resting state — which is almost always. The bridge switches off only for the brief instant of each open/closed test.

Read the same point in each position and watch what the number does:

Barely moves → a supply rail. A stiff, low-impedance source: a real rail, a battery, an energized circuit.
Sags significantly → a high-source-resistance voltage. Real, but behind a large resistance — a resistive divider, leakage, or a high-value pull-up. It reads full scale unloaded and cannot hold up under any load.
Collapses quickly toward zero → a phantom voltage. Capacitive coupling from an adjacent conductor — see ghost voltage — not a source you can draw from at all.

If you want a number rather than an impression, one loaded reading is enough. With V_open measured in the center position and V_load measured across a known load R_load:

R_source = R_load × (V_open − V_load) / V_load

So a point reading 12 V in the center position and 6 V in the 100 kΩ position has a source impedance of 100 kΩ × (12−6)/6 = 100 kΩ. A point that reads 2 V open and 20 mV loaded is sitting behind roughly 10 MΩ — that is coupling, not a supply.

The math assumes the center-position reading is the true open-circuit voltage. Once the source impedance climbs into the megohms, your meter's own 10 MΩ is loading it too, and the calculated value reads low. For that range, treat the result as a lower bound.

The 100 kΩ position is for low-voltage work only. Two reasons. First, it is a deliberate load on the circuit under test — that is the whole point of it, but never leave the switch there for normal work, and think before using it on anything delicate. Second, the resistor has to dissipate V²/100 kΩ, which climbs quickly:

Across the probes	Current drawn	Dissipated in the resistor
12 V	0.12 mA	1.4 mW
48 V	0.48 mA	23 mW
110 V	1.1 mA	121 mW
120 V	1.2 mA	144 mW
240 V	2.4 mA	576 mW

The wattage rating of the resistor in the final production design is still pending. If you have a pre-production unit, assume it is a 1/8 W part, which hits its maximum at 110 V, unless I have told you otherwise.

It also changes your meter's other modes. In the 100 kΩ position an ohmmeter reads 100 kΩ and a continuity test sees a resistor rather than your circuit. Center the switch for Ω, continuity, diode, and current modes.

4 · Indicators

LED & beep reference

LED

Pattern	Meaning
Green flash (up to 2/s)	Leads closed — continuity.
Dim blue flash (up to 1/s)	Leads open / floating.
Red, red (up to 2/s)	DC voltage, red probe positive — continuity checking suspended.
Red, blue alternating	DC voltage, red probe negative.
Red, blue, green cycling	AC voltage.
1–4 green blinks at power-on	Battery level in quarters (1 = low, 4 = full).
Slow dim-red blink	Charging over USB. Normal alerts are suppressed while charging.
Slow dim-green blink while plugged in	Charging, and the battery is effectively full.
Dark between flashes	Normal — alerts are brief rate-limited flashes to conserve battery, not steady lights.

Beeps

Single beep the moment the leads close, then a short reminder beep about once a second for as long as continuity holds.
Two or three beeps when a voltage is detected, with the rhythm telling you which kind: short-short for DC+, short-long for DC−, and three shorts for AC. This is a priority alert — it sounds immediately, even mid-continuity-beep.
Rate-limited: an intermittent connection can't machine-gun the buzzer; beep sequences are capped at 2 per second.
Quiet mode, no tools: hold the probe tips together (or clip them) while flipping the power switch on — audio stays muted until the next power cycle. The LED continues to work normally.
Every beep behavior — including disabling the buzzer permanently — is reconfigurable over USB; see the firmware manual.

Voltage type: AC, DC+, or DC−

When Blinky Hawk finds a voltage, it also tells you what kind — without you touching the meter. The LED steps through a short color sequence, and the buzzer changes rhythm. Both carry the same answer, so it works whether or not you are looking at it.

What is on your probes	LED sequence	Beep
DC — red probe positive	Red, red	short-short
DC — red probe negative	Red, blue	short-long
AC	Red, blue, green	short-short-short

Red always comes first. Whatever the type turns out to be, the first flash of the sequence is red, so "there is voltage here" reads immediately and the type follows on the flashes after it. DC+ is simply red twice — which is exactly what every voltage looked like before this feature existed, so the alert you already know has not changed.

Polarity is relative to your probes. DC+ means the probe in your meter's V jack is at the higher potential; swap the probes on the same battery and it reads DC−. That is the useful part. You can tell which side of a connection is the hot one without looking at the meter, and reversed polarity announces itself as a different rhythm rather than as a minus sign you have to notice on a display.

What it can see. Type detection is built around mains frequency, so 50/60 Hz sits comfortably inside its range. Confirmed on the bench: 120 VAC; AC as small as 3 V peak-to-peak at 40 Hz; and 10 V peak-to-peak signals at a spread of frequencies up to about 4 kHz. By 5 kHz it no longer identifies AC reliably.

Out of range means the wrong label, never a missed alert. The type test runs only after the voltage alert has already been decided, and it cannot change whether that alert happens — only which pattern plays. AC that is too fast or too small to identify still alerts; it just reports one of the DC patterns instead. Treat the type as useful extra information, not as a safety verdict: the thing that matters, red, is there either way.

If you would rather have the single red alert back, !SET,VCLASS,0 followed by !SAVE turns type detection off permanently. The color sequences, the beep rhythms, and the sensitivity of the AC-versus-DC decision are all adjustable over USB — see the firmware manual.

5 · Interpreting results

What the verdicts mean at the bench

Probes separated by	Meter shows	Blinky Hawk says
Copper — same net	0 V	Closed
Air gap / floating probe	0 V	Open
Open relay or switch	0 V	Open
GPIO pin driven low	0 V	Closed
GPIO pin, MCU unpowered	0 V	Open
Capacitor (unenergized)	0 V	Often Closed while it charges *
Transformer winding-to-winding	0 V	May read Closed (reactive coupling) *
Anything energized	the voltage	Voltage — check suspended, and the pattern says AC, DC+, or DC−

* The one honest limitation. Blinky Hawk detects a low-impedance path to a fast injected transient. The vast majority of the time that means a genuine wire-to-wire connection — but reactive components are the exception. A capacitor passes the transient and can read "Closed"; a transformer can couple it through interwinding capacitance and read "Closed" even when a megohmmeter shows the windings galvanically wide open. Near capacitors, transformers, and large inductors, treat a green "Closed" as "low-impedance path", not "guaranteed DC connection."

The reverse direction is trustworthy everywhere: a blue "Open" verdict means the probes genuinely see a high impedance — there is no low-resistance connection between them.

6 · Ghost voltage

Why your meter shows a few millivolts with open leads

With Blinky Hawk switched on and the leads open, your meter will typically display a small offset — around 5 mV DC (tens of mV on AC ranges, depending on the meter). This is the detector's tiny sense bias, visible only because nothing else is driving the probes.

It is cosmetic, not a measurement error. The bias sits behind very high source resistance, so probing any real circuit pins the reading to the true value.
It disappears during real measurements. The instant a voltage is detected, the bridge stops toggling entirely.
It's removable. Center the toggle and the detector is galvanically gone — the meter reads exactly as it would bare.
You can prove it's ghost voltage. Switch to the 100 kΩ position: a coupled voltage collapses under that load, a real one barely moves. See section 3.
Higher sensitivity settings use more bridge resistance, which increases ghost voltage slightly; the factory setting is chosen to keep it minimal.

7 · Sensitivity

Sensitivity (most users can skip this)

Under the hood, sensitivity is a genuine tradeoff: more bridge resistance produces a stronger detection signal (better discrimination of high-resistance circuits) but slightly more ghost voltage on the host meter. Your unit is calibrated and set at the factory, and most users should simply leave it alone.

The unit stores four sensitivity settings and uses one of them. The factory choice is tuned for everyday use with minimal ghost voltage.
There is nothing to adjust on the outside of the unit — no switch, no dial. Changing the setting means connecting it to a computer over USB and sending one command, which takes effect immediately with no reboot or reprogramming.
What each of the four settings means is itself reprogrammable, so one can be re-purposed for a specific application. Both are covered in the firmware manual.

Early unit with DIP switches? A single pre-production unit has a 4-way DIP switch on the board for this instead. If yours is that one, the switches work as two pairs that must move together — the correct pairing is printed on the PCB next to the switch, and the factory position is 10. See the hardware revision 2 appendix in the firmware manual.

8 · Power

Battery & charging

Battery: internal 320 mAh single-cell rechargeable lithium battery.
Runtime — roughly 60 hours of ordinary use. Blinky Hawk spends almost all of its time in a low-power state, waking about sixteen times a second to check the leads, and it drops into that state about a second after the leads go open. Average draw is around 2 mA that way, against about 11 mA fully awake. A charge lasts about 60 hours of normal working use before the gauge reads empty — and only about 24 hours if the leads are left connected to something so the unit never gets to rest. Leaving it awake costs far more than leaving it switched on. The gauge deliberately calls empty at 3.7 V, with real charge still in the cell, because below about 3.6 V the analog baseline drifts enough to cause false alerts.
Zero drain when off: the center position disconnects the battery outright, so a switched-off unit holds its charge indefinitely. The 100 kΩ position leaves the battery disconnected too — it is a passive load, and draws nothing from the cell.
Charging — the toggle must be toward the meter: plug a USB-C cable into the port on the unit with the switch in the on position. The same switch poles that make the center position fully inert also disconnect the battery from the charger, so a switched-off unit ignores USB power entirely. While charging, the LED shows a slow dim-red blink, switching to a slow green blink when the battery is effectively full.
Don't use it while charging. Detection alerts (LED and beeper) are deliberately disabled whenever USB power is present, for two reasons: many USB power supplies are electrically noisy enough to cause a stream of false alerts, and the USB cable can couple your meter to an earth ground you may not want in your measurement. Charge it idle, unplug, then work. (For bench and diagnostic sessions the alerts can be forced back on over serial with !ALERTS,1 — see the firmware manual — but the same noise and grounding caveats apply.)
Power-on battery gauge: the 1–4 green blinks at every power-up are your fuel gauge; one blink means it's time to charge. The gauge reads empty at 3.7 V, which is deliberately above the point where the cell is actually flat — see below for why.
A low battery announces itself as false alerts. Below about 3.6 V the detector's internal reference drifts far enough that simply moving the leads can trigger a voltage alert on a dead circuit. So: if Blinky Hawk starts calling voltage when you wiggle the leads, charge it — that is a flat battery talking, not your circuit. This is why the gauge is calibrated to read empty while there is still charge left in the cell: "empty" means "stop trusting it", not "it is about to switch off".

9 · Safety

What it can take, and what to keep in mind

It sits beside your meter's inputs. Blinky Hawk connects in parallel across the same jacks your probes use — whatever your probes touch, it sees too. Its front end is built for that exposure: detection has been verified working from 0 V through ±1 kV, including −1 kV applied from a megohmmeter; beyond its sensing range the front end simply saturates behind its own series resistance and clamp protection — there is nothing fragile exposed to the probes.
It is gentle on circuits. The injected test signal can never exceed the internal 1.25 V reference, and worst-case current into a dead short is about 6 µA, falling to nano-amps into high impedances — where sensitive electronics would actually care.
Your meter's ratings still govern. Blinky Hawk does not change your meter's CAT rating, and it carries no independent safety certification of its own — observe the category and voltage limits of the meter and leads you use, exactly as you would without it.
It is an indicator, not a lockout. Use it to catch unplugged-probe mistakes faster — but always verify de-energization per your normal safe work practice before touching a circuit. In particular remember the reactive-parts caveat in section 5.

10 · Troubleshooting

FAQ

Symptom	Explanation / fix
Meter shows a few mV with nothing connected	Normal ghost voltage while the detector is on and the leads are open. It vanishes on any real measurement; center the toggle to remove it entirely.
No LED activity at power-on	Battery is empty — charge over USB-C. (A single green power-on blink means it was nearly empty.)
LED only does a slow red blink	The unit is on USB power and charging; alerts are intentionally disabled while charging (USB supply noise causes false alerts, and the cable can earth-ground your meter). Unplug to resume working.
Plugged into USB but the battery won't charge	The toggle must be toward the meter to charge — the center and 100 kΩ positions both disconnect the battery from the charger as well as from the meter.
Meter reads 100 kΩ on every resistance measurement	The toggle is in the far position, which puts a 100 kΩ test load across your probes. Center it. See section 3.
No beeps, LED works	Audio was muted for this session (leads were touching at power-on) — power-cycle with the leads apart. If it was disabled permanently via configuration, re-enable it (`!SET,BEEP,1` + `!SAVE`).
The LED flashes blue or green while a voltage is present	Working as intended — that is the voltage type being reported. Red then blue is DC with your probes reversed; red, blue, then green is AC. See section 4.
A known AC source reports as DC	Most likely above the frequency the type test covers (about 4 kHz), or too small a signal. It still alerts as a voltage — only the label is wrong. See section 4.
False voltage alerts when I move the leads on a dead circuit	Low battery. Below about 3.6 V the voltage detector gets twitchy enough that lead movement alone sets it off. Charge it; the behavior goes away. See section 8.
"Closed" on a part I know is isolated	Almost certainly a reactive path — capacitor or transformer interwinding capacitance. See the limitation note in section 5.
Detection seems too eager / not eager enough	Try one of the other three sensitivity settings, or retune the thresholds, over USB per the firmware manual.
Meter reads normally but no open/closed verdicts, LED blue	Check that both banana plugs are fully seated in the meter's V/COM jacks and the toggle is toward the meter — with the detector isolated the leads always look open.

Stuck on something not listed here? Email Nick@OIHDesigns.com.

Three answers instead of one ambiguous zero

From box to first detection

One switch: power, isolation, and a test load

The 100 kΩ position: identifying high-source-impedance voltages

LED & beep reference

LED

Beeps

Voltage type: AC, DC+, or DC−

What the verdicts mean at the bench

Why your meter shows a few millivolts with open leads

Sensitivity (most users can skip this)

Battery & charging

What it can take, and what to keep in mind

FAQ

Configuration over USB