ESD protection of capacitive touch doorknob circuit?

Question

This is a follow-up to my question: How can I use a generic doorknob as a capacitive touch sensor?

I am now ready to move beyond the proof-of-concept stage, but I have not yet learned of a satisfactory answer for how to protect the microcontroller U1 in this circuit from ESD. I have tried adding a small-valued capacitor (C1 connected between the input pin and ground) on the theory that it might be enough to absorb a zap, and while it reduces the sensitivity of the circuit drastically I believe I can compensate in the software. But will it actually protect the microcontroller pin? And if so what's a reasonable capacitance value to use?

Answer 1

In short a small cap is not enough to protect the input from along wire with ESD exposure.

Suppressing a slow clock is not sufficient to detect a touch. Better put 10k in series with CMOS input if you expect some nasty 10k-25k V ESD to limit the current to a few mA.

Discharge protection requires a large series resistance and Ohms Law on ESD diode protection current is also affected by reactive impedance. Essentially a simple solution is using along wire, picks up lots of stray 60Hz instead of a 10Hz clock. which can be attentuated by the body, hand or finger to ground via a series resistance of 10k to shunt the signal and limit the ESD current into the protection diodes to the CMOS rails. switch normally open with the hand lower impdance to shunt the signal. If the knob cannot be grounded, then a pullup resistor with a series R for protection to inject.60KHz and clamp ESD will detect finger touch. 300pF is 10M near 60Hz.

Addittional info

You want your system to work under any condition, without any environmental restrictions or limits to tribolelectric generation. . Plan on at least +/-25kV and those devices rated for ESD use a smaller model for finger capacitance for discharge. You want it protected for Hotel Room arcs of an inch with Neoprene shoes walking over Nylon carpets, which can can easily generate a good 1" arc, which is roughly 25mm and at low humidity works out to be around 25kV or 1kV/mm.

Impedance Capacitance and leakage resistance tend to be inversely related with contact area and skin moisture content. Contact thru the tip of a resistor wire can be 300pF and 300kOhm. A finger tip to a knob can be 3nF and 30k while the whole hand on a metal door knob can be >30nF and <3k. But the discharge time is controlled by the ESR or effective series resistance of your body & skin when ionized air penetrates your dry skin to the sub-cutaneous level into the blood vesels and can be ~ few hundred Ohms, rather than 5k~500k. This results in picosecond rise time discharges.

Your skin may differ by more than an order of magnitude, but my RC time is around 30nF*3k ~=100us using a 1kHz LRC meter, which coincidentally is the time constant of most scope probes. Many amps of current by the finger can be delivered from a tiny arc but in a very short time. The shortest times are well below sensing by humans but can still damage semiconductors.

Answer 2

Take Nick's approach (mentioned in the comments) with the standard ESD approach used in most silicon IC. You want a device that has a fast response with a breakdown voltage under the ESD protection voltage of the IC. Under normal operation you are trying to detect a capacitance change so having a circuit which dumps the ESD event it fine. Realize that most likely an ESD even will still disturb your capacitive sensing so you may need to reset things, but the goal is to prevent destruction of the sensing.