Electronic On/Off; Emergency - Motorola MCS 2000 Service Instructions Manual

Electronic On/Off

Emergency

(Refer to schematic
Page 10-26 for
reference)
UNSW 5V is only used in a few areas which draw low current and requires 5 V
while the radio is off.
Unlike older radios whose on/off feature was controlled only by the
mechanical on/off switch, this radio has circuitry which allow radio software
and/or external triggers to turn the radio on or off without direct user action.
Some examples of this usage are 1) automatic turn on when emergency switch
is engaged; 2) automatic turn on when ignition is sensed and off when
ignition is off; 3) radio deaffiliation, with certain features the radio can be
programmed to stay on for a brief period after the user has "turned it off".
During this "extra on" period the radio may transmit a signal to a base station
letting the station know that the radio has been turned off.
Q0511 is used to provide SW B+ to the various radio circuits. Q0511 acts as an
electronic on/off switch where pin 4 acts as the control pin. The switch is on
when pin 4 is low and off when pin 4 is high. When the radio is off Q0510 is
cutoff and the voltage at Q0511-4 is at A+. This effectively prevents current
flow through Q0511 from pins 2/3 to 5/6/7/8. When the radio is turned on the
voltage at the base of Q0510 is high (at least above 3V) and Q0510 switches on
(saturation) and pulls down the voltage at Q0511-4 to ~0.07V. With FET
Q0511 now "enabled" current flows through the device. This path has a very
low impedance (less than 1 ohm) from pins 2/3 to 5/6/7/8. This effectively
provides the same voltage level at SWB+ as at A+.
The electronic on/off circuitry can be enabled by the microprocessor, the
emergency switch, the mechanical On/Off switch on the control head, or the
ignition sense circuitry. If any of the 4 paths leading to the base of Q0510
provides a high voltage through diodes CR0510 or CR0511, the electronic
"ON" is engaged.
The emergency switch, when not engaged, normally grounds the base of
Q0414. When the emergency switch is pressed this connection is opened and
the base of Q0414 is pulled high by R0437. This causes the collector of
transistor Q0414 to go low (.2V), thereby setting the EMERGENCY IN input of
U0510 low. U0510 is a dual 555 timer configured as two "pulse stretchers", one
for the Emergency input to electronically turn on the radio and the other for
the mechanical On/Off switch on the control head to cause the radio to
electronically switch on.
With regard to emergency, when U0510-8 EMERGENCY IN is above 1/3 of the
supply voltage at U0510-14, that causes the device to internally ground
U0510-13, which in turn holds capacitor C0513 in a discharged state. The
device also keeps U0510-9 EMER TRIG at a low voltage. When EMERGENCY
IN goes below 1/3 of the device supply voltage, U0510 disables the discharge
mechanism at pin 13 allowing the capacitor C0513 to charge through R0513.
The charging rate is set by the values of R0513 and C0513. When the voltage
at pin 12 goes above 2/3 supply voltage, the device sets pin 9 EMER TRIG high
which in turn switches on Q0510 enabling the radio to turn on to begin
emergency operation. U0510 will keep pin 12 high for a period set by R0513
and C0513 which for 200K/1 uF equals 220 ms.
While SW B+ is on for 220 ms, the microprocessor starts execution, reads that
the emergency input is active, and sets the B+ CNTL output to a logic high.
This high will keep Q0510 switched on through CR0510 beyond the 220 ms
period after which U0510-9 will return to a low level. This operation allows a
momentary press of the emergency switch to power up the radio. When the
Controller Section Theory of Operation 7-3