From 8191a4bf03130a49eaea20ab50df8f5a967883f0 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?King=20K=C3=A9vin?= Date: Thu, 22 Jul 2021 18:13:40 +0200 Subject: [PATCH] describe project in README --- README.md | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 59 insertions(+), 4 deletions(-) diff --git a/README.md b/README.md index eaea1ff..8897c48 100644 --- a/README.md +++ b/README.md @@ -1,11 +1,69 @@ -these are the hardware design files for **insert project name here**. +these are the hardware design files for the USB bug detector. purpose ======= +the purpose of the USB bug detector is to identify USB cables (or devices) with integrated circuits. + usage ===== +to use the USB bug detector, follow the steps provided on the back of the board: + +- insert a CR2025 or CR2032 coin cell battery in the USB bug detector for it to be operational +- press on the RESET button to arm the test +- the BUGGED LED should be off +- plug the USB cable in the USB receptacle +- if the BUGGED LED switches on, the USB cable embeds an integrated circuit (e.g. a bug) +- if the BUGGED LED remains off, the cable is same to use + +after the test, to save power, press again on the RESET button. + +to ensure that the USB bug detector works, after pressing on the RESET button, press on the TEST button. +this will simulate a USB plug with integrated circuit. +the BUGGED LED should switch on. +this also ensures the battery is not empty. + +fabrication +=========== + +the board has an optional READY LED (with accompanying resistor located next to it). +only populate it to debug the board. +the READY LED will we on when the BUGGED LED isn't. +this allows to always check if the battery is empty, but this also drains the battery even when not testing because an LED is always on. + +the first prototype of the board (v0.3) offer three populating options, depending on the available NOR gate part: + +- 1x SOIC-14 package: U1+C1 +- 2x SOT23-5 pachgae: U2+C2, U3+C3 +- 2x SOT-353 package: U3+C3, U5+C5 + +only populate one of the options, depending on the availability of the NOR gate part. +note: the Cx decoupling capacitor is not really necessary and is just present for good design. + +mode of operation +================= + +and NOR-gate-based SR-latch is used. +the RESET button triggers the R signal to reset the latch. +when a USB cable is plugged, power is provided by the battery to the cable. +if an integrated circuits is present, current will flow through. +a 1 k Ohm resistor on the low side (e.g. ground) will create a voltage. +if the plug draws more than 1.6 mA, the resulting 1.6 V will trigger the S signal to set the latch. +the BUG LED will indicate the send SR-latch is set. +the latch remains set until the RESET button is present. +thus, even if the plug stops drawing power, the LED remains on. + +this minimum 1.6 mA current draw is often caused by the decoupling/bypass capacitor required by integrated circuit, or accompanying voltage regulator. +when charging, the capacitor acts as a short for a small time, allowing current to flow. +on USB plugs with just resistors, or LEDs, the resulting current flow is not large enough. + +the TEST circuit simulates an integrated circuit by putting a 100 nF capacitor across the USB receptacle. + +because of the 1 k Ohm inline resistor, and limited 3.3 V provided by the battery, a maximum of 3.3 mA can be drawn by the USB plug. +this is often not enough to power up integrated circuit properly, particularly if they use a radio interface. +thus it is safe to use the USB bug detector on bugs, without activating it. + requirements ============ @@ -91,6 +149,3 @@ to export gerber files for PCB manufacturer (and photo preview + overview docume rake fabrication ~~~ -fabrication -=========== -