(d) The Q output, pin 10 of U103, delivers one 160msec long positive
pulse per each QRS recognition. This signal activates the beeper through its volume
control R160 and emitter follower Q104, and also proceeds to the heart rate meter and
alarm section.
(e)
The same pulse is used to fire the defibrillator in the sinc mode.
(f) The 160msec pulse will appear inverted at the output of the first
NAND gate (U104) as long as pin 2 stays at a logic 1, which is the normal condition.
(g) A fire inhibiting signal is detected when the HOLD is depressed which
pulls pin 2 of U104 low.
(h) Q105 can be driven only if the defibrillator is switched to the sinc
mode. This applies +12v to the collector of Q105 and through the network composed of
R164, C131, and R165, drives pin 5 of U104 to a logic 1. The first signal which pulls the
collector of Q105 down fires the defibrillator when this mode is selected.
(8)
Heart rate meter and hi-lo alarm.
(a) Three seven-segment displays are used (U107, U108, and U109)
with the leading zero blanking for the hundreds. For reasons of conserving battery energy,
the display is strobed at approximately 60 percent duty by the oscillator frequency from
U106.
(b) The analog gate U111 drives the base of the pass transistor Q107
which controls the power to all the common anodes of the display. From pin 27 of U106, a
ramp is derived with a resolution of better than 1.0mv referred to the input (pins 30 and
31) of U106, and a repetition rate of 3 to 4 times per second. The actual update of the
display board is controlled by a 2 second oscillator U119 connected to pin 1 (Hold) of
U106.
(c) After amplification, C145 is charged to the peak value of the ramp
and is presented to 2 voltage comparators (sections of U114) with hysterisis. One of these
comparators is detecting high heart rate, and the other detects low heart rate. They do
this by adjusting their respective reference voltages with the Hi limit and Lo limit set
controls. These are combined with a set of switches that not only enables the alarm
function but also starts the chart recorder timer.
(d) The resulting positive going alarm signal is overriding the QRS
beeper volume control by driving Q104 directly, which in turn drives the QRS Alarm
beeper into full volume.
(e) The alarm signal drives pin 12 of U104 where also the firing pulse
from the defibrillator appears. Either one will generate a low at pin 2 of U105, a 15 second
timer.
MD0362
2-17
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