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Milliohm Squawker ideal for PCB reverse engineering

11 Jun 2014  | Glen Chenier

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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 minimise battery current drain. R14 isolates speaker current pulses from the battery to prevent any interaction between the speaker current and low-level analogue 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 analogue 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. ∆R 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 ∆R. Trimpot R16 can be set for higher gain if it is ever necessary to increase the ∆R 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 ∆R range of interest. U4B drives the VCO U3B control input to set the beep pitch. R20 sets the ∆R 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 stabilise to room temperature. Also, do not touch the pogo pins during use. The warmth of your fingers will change the resulting beep pitch.

About the author
Glen Chenier contributed this article.

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