Hack session tonight

[Steven Pickles]

Jon might be a few minutes late (bus dependent), and I am taking yet another night off.

pix

[Steven Pickles]

I did however get some "hacking" done in absentia. This is an animated gif (90's retro!) I made on the DSO Quad showing the difference in input signal when touching/not-touching a capacitive sensor.

The blue line is the "interrogation signal" and the yellow line is what would be coming in to the sensing pin on the Arduino. When touching the sensor (in my case it was just a bit of wire), the yellow signal gets delayed by your capacitance and slews to the right. The idea is you measure the time difference between the blue vertical line (start of the interrogation signal), and about the middle of the yellow diagonal line, and this tells you if the capacitive sensor is being touched (long delay = more capacitance = high probability of meaty fingers).

By the way, I didn't have anywhere near as much resistance (~30kOhm) on this sensor as you normally would (~1MOhm), so the effect of touching it is quite subtle.

I've done this in code lots of times but was always curious what the actual signal looked like. Here 'tis.

pix

[Ken]

That image is very nifty, pix.

I haven't ever done either, but would it be easier to generate a narrower spike, and sense its occurrence?
The right resistance and pulse width, and you would see a pulse or no pulse.  That could be used to trip a trigger/interrupt or some such.
(How I'd do it if I was using discrete logic/circuitry to sense, in which case any AC can be used as the source, including mains.)

Ken.

--
You received this message because you are subscribed to the Google Groups "HackerSpace - Adelaide, South Australia" group.
To unsubscribe from this group and stop receiving emails from it, send an email to hackerspace-adel...@googlegroups.com.
To post to this group, send email to hackerspac...@googlegroups.com.
Visit this group at http://groups.google.com/group/hackerspace-adelaide?hl=en.
For more options, visit https://groups.google.com/groups/opt_out.
 
 

[Steven Pickles]

I guess you normally make the interrogation pulse as short as you can, but if it's arbitrarily short you might run the risk of not charging the capacitor (you) up enough to reach your trigger voltage on the input pin. On the arduino I'm guessing you would do something like, set interrogation pulse high, time until the input signal is detected (with some timeout incase of connection failure) and report, turn off interrogation pulse, wait for input signal to go low (or some arbitrary delay), then repeat ad infinitum.

By the way, it was the first time I have saved data with the DSO Quad, I didn't realise it saved screenshots. Useful for making animations like this tho ;)

pix

[Ken]

That's the idea.
Without a finger on the sensor, pulses occur into the input.
With a finger dampening it, no pulses are seen.

I haven't tried saving any images with my cheapie DSO yet, but I know it can.
Isn't it bloody marvellous how much waveform data they measure and display, for the measly price they are now.
My last oscilloscope was a PMG/AOTC/Telecom Aust/Telstra chuckout Tectronix valve jobbie.  Left it with Peter for disposal to a collector or rubbish bin.
(I'm old, but not quite ready for collection yet.)

Ken.