Learning electronics: Release the Magic Blue Smoke

The Something Awful Forums > Discussion > Hobbies, Crafts, & Houses > Learning electronics: Release the Magic Blue Smoke

Cross_: Aug 22, 2008

I did some ultrasonics range sensing a long time ago- so take with a grain of salt:

SnoPuppy posted:

I have some 40 KHz ultrasonic transducers that I am using as a pulser/receiver pair. I am able to transmit and receive pulses just fine, but I am trying to get high range resolution and am running into some issues. It appears that my receiver continues to oscillate after the receive echos, giving me what looks like a longer pulse. This is bad, because two objects that are close together are not distinguishable (their echos overlap).

For timing pulse travel duration we used a comparator on the receiving end that would trigger upon reaching an adjustable threshold. Even if your transducers ring (and we did not notice such behavior with our devices) those signals would not exceed the maximum amplitude you get from the initial pulse. Of course if you are trying to detect phase shifts (and higher precision) then that would be more problematic. One issue we did see in our system was the reverse: the transmitter amplifier was slow to reach its peak output. We ended up priming it for a fraction of a second before closing the switch to the speaker.

quote:

Originally I was going to try to send a coded pulse out and convolve the echo with the transmitted pulse to compress my returns (similar to RADAR)

I have no clue what you are saying here, but if you want high speed readings I would recommend putting some kind of time code on top of the signal. It's secondary echoes (delayed reflections) that gave us huge problems.

The maxbotix sonar board has gotten some good reviews. Here's their FAQ:
http://www.maxbotix.com/MaxSonar-EZ1__FAQ.html

# ¿ Oct 8, 2008 01:22

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# ¿ May 19, 2024 23:36

Cross_: Aug 22, 2008

mtwieg posted:

These guys. Gotta drive and bias them with high voltage though, so be careful. Sensitivity will be pretty dependent on your bias voltage.

150v bias plus 300v p-p for the signal ? Woah.

# ¿ Oct 17, 2008 02:44

Cross_: Aug 22, 2008

kaptainkaffeine posted:

edit: I don't know how much voltage each signal is, but I can check later. Also, would an audio taper work more effectively here than a linear? I thought linear at first because I just want to control the mix, not the volume, but after a little reading I'm not so sure they're different.

My guess is that you want linear for balance and log for volume. 10k seems to be the typical value.
ET: Oh, here's some interesting stuff for you http://sound.westhost.com/pots.htm

Cross_ fucked around with this message at 01:48 on Oct 29, 2008

# ¿ Oct 29, 2008 01:46

Cross_: Aug 22, 2008

babyeatingpsychopath posted:

iRobot Create. Roomba without the vacuum.

If you want to be cheap you could even get a used Roomba- they all have the same serial port & protocol. Adding your own sensors would just be a bit harder.

# ¿ Jan 16, 2009 23:15

Cross_: Aug 22, 2008

Has anyone here experience with metal detector circuits or capacitance sensors ?
I am trying to add a sensor to my project in order to detect the presence of a metallic object above an electrode. The electrode should be ca. 3" x 3" and detection distance should be up to 1".
The past two weeks I have been playing around with an Omron B6TS capacitance sensor but unfortunately could not get the SPI to work properly even though the design looked nice. Basically I am looking for a replacement with somewhat increased sensitivity. Any recommendations ?

# ¿ Jan 19, 2009 21:28

Cross_: Aug 22, 2008

Nope, it doesn't have to be capacitance- all I want to know is whether my metallic object is within sensor range. However, following along the same idea I tried out the CapSense example found here: http://www.arduino.cc/playground/Main/CapSense
It's just timing how long it takes for the output voltage across the improvised capacitor to reach a reference value. While I did get some clear spikes they were limited to a distance of no more than 0.25". And since I was using a non-grounded laptop the values greatly fluctuated depending on whether I was touching the keyboard at the time :-( Even though this looks promising I don't know how reliable it would be. Will the system go haywire if a user touches the board enclosure? Can this be tweaked to get more sensitivity and thus increased range?

With this out of the way I am curious how your approach would work. Are you suggesting an RC/LC oscillator and then measuring the phase/frequency shift induced in the capacitor or coil? Would that provide improved resolution compared to the timing method above?

# ¿ Jan 20, 2009 21:59

Cross_: Aug 22, 2008

I tried to learn PIC programming, but having to buy a separate programmer and the language scared me away. Wish I had heard about PICAXE at that time.

Then I discovered arduino and it's been bliss ever since. Plug it in and start typing away with regular libc support. Where speed is needed you can bypass the wiring lib or go straight to assembly. With regards to cost the cheapest solution is to build it yourself: the atmega IC (168V/L) is $5 and the USB interface is $7. Add a couple of capacitors, a crystal, and you're done

Cross_ fucked around with this message at 04:23 on Feb 26, 2009

# ¿ Feb 26, 2009 04:17

Cross_: Aug 22, 2008

quote:

Is it possible to mix assembly in with arduino code? Always wanted to try it, since it would make them a lot more flexible.

Yes it is, but it ain't pretty. Lots of quotation marks, escape characters, etc.
In my cases where the arduino IO functions were too slow (lots of boundary checks, PWM-related functionality, etc) I replaced them with C register writes and that solved the bottleneck.

quote:

In general they're find as long as you don't mind the loss of certain I/O pins and you don't need fast serial communication.

I believe the SPI clock goes as fast as 4MHz. Not sure if that's fast enough.

Since you mentioned FPGAs, I almost picked up one of these boards last year:
http://www.digilentinc.com/Products/Detail.cfm?NavTop=2&NavSub=423&Prod=S3EBOARD

# ¿ Feb 27, 2009 03:54

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# ¿ May 19, 2024 23:36

Cross_: Aug 22, 2008

I came across a circuit to be used for touch sensing. Could anybody here walk me through how/why this is supposed to work ? The input is a 9v square wave. The potentiometer (top right) is supposed to set the trigger touch capacitance (lower right). Output is connected to a FET gate. What's that diode doing there?

code:

     20kHz square 0..9v
         .-----.
         |  G  |
         |_-_-_|--.--------.
         |     |  |        |
         '-----'  |       .-.
          |       '------>|'| 0..200 kOhm
         ===      |       | |
         GND      |       '-'
                  |        |
                  |       .-.
                  |       | |
                  |       | | 15 kOhm
                  |       '-'
                  |        |     47 kOhm
                   >|      |       ___
            PNP     |------'------|___|---.
                   /|                     |
                  |                      ---
                  |                      ---0..300 pF
            diode V                       |
                  -                       |
                  |     signal out        |
                  |                      ===
           .------o--------o             GND
           |      |
          .-.    ---
  470 kOhm| |    --- 0.047 uF
          | |     |
          '-'
           |      |
          ===    ===
          GND    GND

(Glorious ASCII art thanks to AACircuit)

# ¿ Aug 11, 2009 23:20

The Something Awful Forums > Discussion > Hobbies, Crafts, & Houses > Learning electronics: Release the Magic Blue Smoke