Beeping continuity testers have been around for a long time, but for PCB reverse engineering purposes they leave a lot to be desired. They respond to a “short circuit” of several ohms whereas one would much prefer to discriminate PCB trace and test probe resistance of less than 1Ω to avoid false alarms.

Then one would want the beep tone pitch to indicate a few milliohms of ΔR, to determine which side of a closed relay contact, transformer winding, fuse, or low-resistance current sense resistor is actually connected to the net of interest, especially when the copper traces are hidden under components.

When you are rapidly sweeping a wire broom across a PCB to find common net points, no time delay can be tolerated; the beep must sound instantly, and be extended to a noticeable duration. Secondary requirements are low current drain for long battery life, low test voltage, to avoid biasing semiconductor junctions, immunity to 50-60Hz pickup, tolerance to ESD and charged capacitors, and headphone operation to avoid sonically annoying colleagues in the lab or office environment (really, this thing sounds like a scalded cat).

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The fastest way to find all the points connected to a single net to which a clip has been attached at one point is to sweep a broom probe across the rest of the PCB while listening for the squawks. The probe (Figure 1a ) uses very fine (3 mil) phosphor bronze bristles to avoid physical damage to small surface-mount components. Pogo pins zero in on the specific device pins once the general areas are located with the broom, and are useful for their gold plating and sharp points, minimizing contact resistance. Their telescoping sections are soldered together to avoid adding unwanted ΔR movement to the measurement. Use multi-point pogo pins; you are less likely to accidentally skewer your hand than with a single-point one, while still making good low-resistance contact. If needed, a single-point pogo pin can be used on a separate probe attachment that lies flat on the bench for very fine-pitch surface mount IC pins, but be careful – these are very sharp.

Figure 1a  Broom and radial pogo pin

Figure 1b   Axial pogo pin at opposite end

To make the probes easy to handle while sweeping, you can use coiled cords – in this case, the four AWG26 conductors in the cord are paralleled to minimize resistance. The stationary probe can use a banana plug to attach to various sizes and types of grabber clips. Periodic alcohol cleanings will minimize the ΔR variations caused by the banana plug connection.

A typical reverse engineering setup is shown below (Figure 2 ).

Figure 2  A blue-box Milliohm Squawker fits nicely under an ergonomic microbench which raises the BUT (board under test) to close-up magnifying-visor eye level, with schematic capture software display in the same plane of view.

Figure 3   The Milliohm Squawker schematic (TinyCAD drawing)

The 9V battery is regulated to 5V for the low-level analog circuits. R4 sets the probe test current at 1mA, and R3 limits the test voltage to 10mV. R2 adds to the test lead resistance to ensure a positive offset voltage for U2, which is compensated by trimpot R8. This is necessary since U2 uses a single supply; its offset could be negative, and the test lead resistance is compensated later with other circuitry.

C3 removes 50-60Hz AC stray pickup, but discharges instantly into a short at the test probes for fast response. R5, D1, D2, & D4 clamp ESD and any voltages from charged BUT capacitors. U2 is a low-level comparator chosen for low current consumption and low input offset voltage, but is fast enough to respond to a broom sweep pulse of 1ms. It is available only in surface mount, so if one builds this with leaded components, an adapter board is needed. You can experiment with other fast low-power low-offset op-amps; I chose the LTC6240 simply because it responded fast enough in the LTspice simulation.

Trimpot R8 sets the beep threshold resistance: 1Ω is a good choice based on the long thin traces of a typical PCB. Neglecting U2’s input offset voltage, 1mV at U2’s negative input sets the threshold for the 1mA test current at 1Ω probe + R2 + PCB trace resistance. R8 can be set for different thresholds if desired.

A probe voltage of less than 1mV causes U2 to trigger the 100ms monostable U3A. This serves to extend the beep so it will be noticed during a fast broom sweep. U3A enables the U3B VCO, which drives the speaker with a 4% duty cycle. During idle (no beep) periods, U3B holds speaker driver transistor Q1 off; the low duty cycle ensures Q1 is mostly off so to minimize battery current drain. R14 isolates speaker current pulses from the battery to prevent any interaction between the speaker current and low-level analog circuits. C6 provides the current peaks necessary to drive the speaker to a reasonable loudness.

If headphones only are used, then Q1 will not be necessary; U3B can drive the headphones directly (a weakness in this speaker circuit is that even when headphones are used, there is current wasted through the volume control. Since most of the time the Squawker is quiet, I ignored this battery-wasting problem.)

The final version was built with SMT on a PCB with a solid ground plane, so I got away with sending the speaker return current through the plane. However, if you build this on vectorboard, keep the speaker return current separated from the low-level analog ground system with its own return path directly to the battery. The initial solderless breadboard version had all sorts of problems related to this.

U4A and U4B provide the tone pitch vs. ΔR feature. Capacitors C7 and C8 were found to be unnecessary in the PCB version, and are shown here as a ‘just in case of trouble’, CYA move. The 0-1mV across the probes is amplified by U4A, whose gain is set by trimpot R16. Normally, R16 is kept fully clockwise for minimum gain; I have found this to be quite adequate for easily distinguishing pitch tone changes down to 5mΩ ΔR. Trimpot R16 can be set for higher gain if it is ever necessary to increase the ΔR resolution; so far, I have not found this to be necessary. Do not overdo it – U4A can saturate on input offset if the gain is set too high.

U4B and front panel adjustment R20 let the user “zoom in” to the ΔR range of interest. U4B drives the VCO U3B control input to set the beep pitch. R20 sets the ΔR measurement window and adjusts out the resistance of the test probes, banana plug attachments, and BUT trace resistance. Start by shorting the probes together and tune R20 until the beep just starts to rise from its lowest pitch. A few more milliohms between the probes will cause a further increase in the beep pitch. If your circuit sniffing finds long BUT traces, readjust R20 to accommodate the increased trace resistance and lower the beep pitch back into its linear measurement range.

Eventually, you may reach a point where your net of interest ends in closed relay contacts or a transformer winding. Both sides of these will produce a beep, but the side with the lower pitch (lowest resistance) is where your net under test is connected.

You must hold the pogo pins firmly on the test points for minimum contact resistance. Note that temperature changes of the pogo pins will also result in pitch changes, so if you have just soldered the BUT or installed a new pogo pin onto the probe assembly, give them time to stabilize to room temperature. Also, do not touch the pogo pins during use. The warmth of your fingers will change the resulting beep pitch.

“What are the chances of a bare PCB being available for building one of these?”

“Reminds me of the “Short Sqeeker” I used to detect shorts. Made by Continental Specialties, that was renamed as Global Specialties to avoid confusion with an “Adult” novelty company, was circuit-powered and quite noisy! No adjustments. Found a set on

“Glen,nnWhere did you get the broom probe? Did you make it yourself?”

“I used to use a Wavetek Meterman SF10, but it beeped at 30 ohms, no volume control or headphones, and no indication of small delta-R so it was often impossible to figure out the real circuits. I don't think they make them anymore.”

“@BBell Sorry, PCBs are not available, we made only enough to satisfy our own in-house needs. But they do work on Vectorboard, just keep the speaker and headphone grounds isolated from the low-level analog grounds, connected at a single point at the battery.”

“Hi Martin, The broom probe is custom made to our design by a wire brush company. I found that phosphor bronze has lower contact resistance than stainless steel, is more flexible, and does not damage 0403 components on the bottomside of boards. I tried both 6mil and 3mil phosphor bronze bristles, much prefer the 3mil. However, if there are no fragile components 6 mil is a bit better if thin conformal coatings are present. Heatshrink holds the bristles together while still flexing, when the ends get too worn we simply trim the wires and heatshrink a bit shorter.”

“Phosphor bronze is also a popular material for making guitar strings :-)”

“I’m fortunate to have an IR imager at my company so I first image the PCB and look for a “hot” spot or trace. If none found I troubleshoot to find the circuit and then drive that circuit with a small DC current. Sometimes a via under a component with inadequate clearance can cause problems. Very fast and efficient method.”

“Hi Glennnice circuit but unfortunately most circuits I work on have some sort of coating which makes brush type probes no good. I use normal probes with needles or sawing pins soldered on them so that the sharp edge penetrates the coating.nMahmood”

“My favourite go-to devices for this (when I did this work regularly a few decades ago) were the HP 546A Logic Pulser and HP 547A Current Tracer. No noise – the output is light, not sound, and the current probe will follow the ‘shortest’; (highest current) circuit, as well as other paths. A salesperson kept trying sell us a ‘Huntron Tracker’ which could do this for far more money, but never made a sale with us… This has the advantage of only needing electrical contact at two points for the logic pulser, the current tracer is non-contact. It might be interesting to resurrect the idea with a pulser (easy to build!) and a modern Hall-effect sensor, along with a more informative display, perhaps an Arduino with an LCD… hmmm – sounds like a weekend project…”

“I once worked at a place where the technicians (and managers) swore by their Huntron Trackers. Until the day when a tech could not figure out why a power module would not work. The tracker told him the output filter cap was good. I showed him that an ohmeter revealed the filter cap had 40 ohms internal leakage. My oft-repeated adage ‘Never trust a Huntron’ stayed with him after that.”

“Nice to have an IR imager. But lacking one, a similar 'poor man's' approach is to spray the board with freeze-mist until a layer of frost forms, then watch the route where it thaws out first.”

“These are clever little devices like the Huntron Tracker, that you can't really appreciate until you use one and it solves your problem. For those more inclined to buy then build – I got one several years back from Probemaster.com, Model – 105- Audible Continuity Probe. It is hand held and works well on live and dead circuits!”

“Love it!! Seems to be well thought out and a very handy device.”

“Glen,nnI was just wondering what the name of the wire brush company you ordered the probe from was?”

“gordonbrush . com Give them your required dimensions and size of mounting hole(s). The 3 mil diameter phosphor-bronze bristles are held in a crimped-flat copper tube, they can figure out how many bristles needed to fill your dimensions. You do need to use heatshrink to support the bristles and keep them held together.”

“Is there a PCB or gerber file you can share?nTIA”

“Glen, I was wondering, in your box design you used a volume control knob. What component did you use to connect it to R12 to allow you to control the volume?”

“A bit off-topic but still:nOn the photo it shows that you are using the pink colored material to protect the board under test.nPlease note that this material is NOT SUITED if the board contains high impedance or high voltage circuits. The pink material is covered with a gell-like chemical which contaminates the board. This chemical is sensitive to moist and will degrade surface resistance significantly when exposed to moist. The surface resistance can go well below 1Mohm under such conditions. Rubbing firmly with an alcohol based cleaning fluid may remove the contamination. Just ‘wetting’ it will not clean it. Anyway, just to notify you all… you could be repairing a board while simultaneously making it dis-functional 🙁 Greetingz, Brody”

“About seven months ago, I embarked on writing a sequel to my first engineering book on PCB reverse engineering, and at the suggestion of a reader, decided to include this project in one of the chapters under Tools You Can Build. I searched for Glen’s email on line to obtain his permission but to my dismay learned that he had passed on a year earlier. In the end, I contacted Michael Dunn, the Editor-in-Chief, who graciously gave his approval. The book titled PCB-RE: Tools & Techniques has since been completed and published. I attributed the following words to Glen in the bio section containing his article: In memory of Glen Chenier, affectionately known as ‘zeeglen’ among his peers and fans, a great writer and contributor to several engineering magazines such as T&M, EDN and EE Times, sharing invaluable insights from his trove of practical knowledge and personal experiences through his unique writing style, and won himself the title of being an engineer’s engineer in the EE community. Though he is gone, his legacy will remain as a vital part of the electronic engineering community. RIP, Mr Chenier! Keng Tiong visio-for-engineers.blogspot.com”

“We miss Glen every day.”

“I designed and produce a Continuity Tester using just a few components and it has 2 probes. One detects resistances above 1k and the other detects very low resistances. The mechanical beeper on the board lets you know as the resistance increases, by the tone. The whole kit costs less than $3.00 but this website does not allow me to show the project or the circuit. Colin Mitchell TALKING ELECTRONICS.com”

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