DMDX Help.


Test Hardware


    In order to thoroughly test DMDX and the devices it will be using test hardware will need to be constructed. See Test Modes. While an oscilloscope or storage scope (digital analyzer and so on) certainly help the presence of a high precision timer built into modern CPUs means with a bit of feedback from some external test hardware DMDX can do all testing that needs to be done without external measuring devices.  And while I grant in this day and age perhaps it would be easier to just go and buy a 'scope there's actually not a lot that can be tested without constructing at least the photosensitive portion of this device and once you've gone that far the crystal controlled oscillator for testing input device variability isn't much more hassle.
    The circuit has two modes of operation, one where it free runs using the crystal timebase and one where it triggers from the optoisolator (depending on the switch's position.)  The banana plug optoisolator output is for hooking to the backs of switches as opposed to using the solenoid to push them.  You have to figure out which side of the switch is ground or negative when doing so, usually just trying one orientation then the other is sufficient, I've certainly never felt the need to replace the optoisolator with a reed relay.  Using the optoisolator is my preferred mechanism for testing as the solenoid takes at least 11 ms to charge up and then there's the time taken and variability incurred as the plunger depresses.  The BNC output is for hooking a scope up to as you're adjusting the sensitivity of the phototransistor.
    I've never been super happy with the sensitivity of the phototransistor (NTE 3036), it's kinda hard to get it to trigger on LCDs and dimmer CRTs.  It's listed as a high sensitivity part so perhaps the biasing could do with a tweak, perhaps it's load resistor R2 needs to be 22k, perhaps R1 needs to be eliminated, perhaps the 10 meg base current limiter is too big, who knows.  I've never really tinkered with it too much because as the circuit stands it's just sensitive enough for every use I've made of it.  I recommend making the 10k trimpot on the NTE 3036 a 20 turn device.  I've mounted the NTE 3036 in then end of a black pen cap and with a couple of suction cups it can usually be stuck to the screen for a while.  My circuit might have a 6.2 meg resistor in there with the crystal instead of the 22 meg that's listed, don't think it makes much difference but if yours doesn't oscillate that's probably why.   The whole circuit runs off 12V DC, I've used an old 63W PC supply.

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