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4.8 Utilizing the External Trigger
In some VOD and data recording applications, it may be desirable to have the MicroTrap™ VOD/Data Recorder begin to record exactly
when a specific external event occurs. For the specific event to trigger the MicroTrap™ VOD/Data Recorder, and subsequently correspond
to time zero on the VOD graph, the EXT TRIG port is used.
The EXT TRIG function operates on the simple concept of MAKE or BREAK circuits. Both types of trigger capitalize on the fact that the
EXT TRIG port outputs a constant voltage after the START button is pressed. The traditional MAKE circuit is triggered when the open circuit
is closed by the event of interest, even if only momentarily. The traditional BREAK circuit is triggered when the complete circuit is broken,
even if only momentarily.
To create a MAKE or BREAK trigger using the EXT TRIG port, connect a BNC Adaptor to the port and subsequently connect a “duplex”
(two wired) cable to the adaptor. A good example of a duplex wire that is fit for duty as a trigger line is the lead line used for initiating
electric detonators in the field.
An example of a MAKE trigger would be to twist the duplex wire with itself, leaving the insulation on each wire, and then run it over the
tip of a detonator or around a section of detonating cord, so that upon detonation of the explosive, the insulation is stripped and the two
wires are forced into contact with each other under pressure from the explosive. This momentary contact will complete the MAKE circuit
and thus trigger the MicroTrap™ VOD/Data Recorder. The MAKE trigger is especially dependable because the explosive itself generates
an electrically conductive plasma in the vicinity of the detonation front, that will assist in arcing the current from one wire to the other.
An example of a BREAK trigger would be to wrap a duplex wire around an object that is expected to deform or expand during the event
of interest, and then twist the bare ends of the wires together so as to complete the circuit. When this object deforms or expands, the
wires will disconnect and cause the circuit to open - triggering the MicroTrap™ VOD/Data Recorder. Beware that even a momentary break
in the trigger circuit will cause the MicroTrap™ VOD/Data Recorder to trigger. Avoid disturbing a BREAK trigger once the START button
has been pressed.
The procedure for using the DAS™ Data Acquisition Suite Software to program the MicroTrap™ VOD/Data Recorder for MAKE or BREAK
external triggering is detailed in the DAS™ Data Acquisition Suite Software Manual.
4.9 Additional Information on Memory and Triggering
4.9.1 Memory
The MicroTrap™ VOD/Data Recorder has a large circular memory containing 4,194,000 data points (doubled with optional memory
upgrade) that are allocated across each of the tests into which the MicroTrap™ VOD/Data Recorders memory has been divided. The total
number of tests and selected recording rate dictate the total record time available for each test. Selection of the number of tests to be
recorded and the Recording Rate is done using the DAS™ Data Acquisition Suite Software.
The DAS™ Data Acquisition Suite Software automatically calculates and displays the Total Recording Time per Test and the Pre-Trigger Time.
For those Operators with additional interest in recording times:
A general equation to calculate the Total Recording Time (Pre-Trigger Time + Post-Trigger Time) is:
Total Recording Time (seconds) = 4,194,000 points / ((# of tests) x (recording rate))
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