PV152
2-29
PRO1200 CIRCUIT DESCRIPTION
CIRCUIT DESCRIPTION
The capacitor C5 acts as a buffer for delivering the high
current pulses. The battery can not supply high cur-
rents, due to its internal resistance.
Backlight LED driving
Transistor Q8 drives the 12 LED’s via their load resistors
R18..R21. The On/Off status is imposed by IC1, output
RA2, which handles the switch-on criteria.
When the Jack cable connection is installed, transistor
Q8 is shunted by the saturated transistor Q7 (see above)
through D19, causing the backlight to be lit continu-
ously.
Indicator LED driving
Transistor Q6 drives LED D22 via resistor R28. The latter
needs to have a rather small value because the driving
pulses are very short.
3.5mm Jack Cable Connection
The ‘hot’ conductor of the Jack cable, is driven by tran-
sistor Q6. This line is supplied on the RS232 Communi-
cation Interface by the +9VSB. Resistor R26 determines
the amplitude of the current pulses, detected on the
series resistor of the pulse detection circuit built around
transistor Q52 on the RS232 Com. Interface. Series di-
ode D23 ensures that transistor Q7 is only driven if the
jack connection is installed, and not via R28/D22.
Field Service
The IC1 RESET (pin 28): needed to wake up the
microcontroller. A downward pulse should be visible at
pin 28 each time a key is pressed (except while an ad-
dress entry is expected, or when the backlight is still
lit).
Oscillator (pin 26,27): as soon as IC1 wakes up, its os-
cillator should start .
PPM Ouput (pin 6): after a debounce time of about 20ms,
a train of 13 pulses of 10us should be present at pin 6,
repeating itself every 130ms. (Except while the IC waits
for an address entry, or after pressing the backlight key).
