GeoffT

Member Since: March 22, 2010

Country: United States

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EE with over 30 years experience designing opamp circuits, sensors, amplifiers, servos and safeties.

  • Primary protection against electric shock is provided by the resistors in series with the arm and leg leads. SF should confirm that those resistors also are rated for >36V insulation resistance and that the clearances of the leads are sufficient for that voltage. Although SF customers are generally savvy, my rule for design of such devices is that they should not conduct even if an idiot user touches 120V and the device is earth-grounded.

  • I must disagree with other posters' enthusiasm for McGuckin Hardware. My mistreatment at the hands of their security, when I had stolen nothing, was not just unprofessional, it was un-American! As a former store detective, I say so from experience. SF is blameless for associating with them, but I will not attend.

  • A PCB was being calibrated on a pogo-pin vacuum tester, but was not repeatable and passed PCBs failed in system. It turned out unequal forces on the (large) board could generate piezoelectric currents in a high-impedance node, sufficient to swamp the cal. This could show up today testing designs using modern accelerometers or cap sensors. (The solution at the time was foam pads under the unsupported areas).

  • An EKG connection is one of low resistance to the human body, close to the heart. A fault current much less than a milliamp can kill, if the frequency and phase are unlucky. No one should modify anything not designed for this purpose for this purpose!

    That said, you can make a homebrew EKG, just design from the ground up for safety. You would want battery power and optical isolation for the human-contact circuit’s interface to anything wall-powered. There are alternative instrumentation amplifiers in friendly DIP packages. You may also find some packaged circuits prewired for the EKG application.

  • The TL084 opamp has a 3MHz gain-bandwidth product. The 150K resistors in the circuit are, for this topology, about 10X the value for which stability would be assured. Users should check IC2 pins 7 and 8 for signs of ringing or oscillations at about 1MHz. If so at pin 8, add a few pF in parallel with R11. If oscillation is seen at pin 7, try a few pF in parallel with R8 and R9, but check for non-linear oscillation due to interaction with the diodes.

    Also, the TL084 only has proper specifications for +/- 5V or greater supplies.

  • The 3.0V threshold is for only 250uA drain current, and increases typically 100mV at 0degC, so 3.1V. A 3.3V supply at -5% tolerance is only 3.135V. Even if the driving device had zero drop from supply to logic output HIGH, the small divider at the FET gate reduces applied voltage to 3.104V, leaving essentially no extra voltage to enhance the current beyond 250uA. Compounding the issue, the e-textile application often uses thin wires having more resistance, so drop in the GND wiring further subtracts from Vg-s.

    The saving grace is that the “typical” Vg-s(th) is only 1.7V, so the worst-case scenario is rather unlikely. This circuit would be flagged in a design review for a critical application.

  • All CMOS devices are type HC, not HCT, so 3.3V control signals would be near the minimum spec value for recognition as a logic “1”. It would probably work, but temperature and unit-unit variations could cause mysterious errors over a population.

  • Probably not. The MOSFET is only rated for 30V and the resistor divider would slow the switching, causing extra power dissipation.

  • This MOSFET is not specified for gate “ON” voltage < 4.5V. Therefore any Arduino variant which outputs 3.3V is unsuitable for directly driving it. Normal unit-unit variation in threshold voltage would let most units work at 3.3V and room temperature with light loads, but would not be good design practice for mass production. Also, the gate is unprotected against ESD. The resistor divider would help, if a five cent zener had been placed from gate to GND. Use thorough ESD precautions when connecting, especially as gate damage can show up much later.

  • This product description should highlight PROMINENTLY that the connector is micro-B, not mini-B as other Teensy products use. Even the Teensy site graphics show mini-B; you have to notice exception text for the Teensy 3.0. While I have a landfill worth of mini-B units, I’ll have to make a second order for the micro-B cable.

No public wish lists :(