Theory of Operation: W3 Control Head
Microphone cartridge M1 contains a capacitive transducer coupled to a FET amplifier. Microphone
audio is obtained from the output of MI and fed to Q6-b via C28, which provides either 0dB of gain in
portable mode or 20dB in mobile mode, during transmit. This gain change is achieved by the action
of Q5, which is turned on in mobile mode only, and bypasses R71. (Note that this gain is with respect
to a radio input impedance of 2k.
The on-/off-hook control circuit comprises U8 and Hall switch U9. This circuit is only active in mobile
mode, during which the output at U8-6 and -7 is driven high by U3. This provides supply volts for U9,
the output of which still remains in its normally high state, provided that the microphone is off-hook.
During the on-hook condition, when the microphone is retained in its hang-up bracket, a magnet in
the hang-up bracket activates the Hall switch, which in turn pulls U9-3 low. This is detected by U3,
which then sends an appropriate SB9600 message back to the radio. When the microphone is
removed from the hang-up bracket, U9-3 returns high, causing U3 to send another SB9600 message
back to the radio.
The backlight circuit comprises U3, U15, Q3, Q4, and LEDs D3 to D17. U3 enables the backlights,
as required, by placing a low at U3-16. This, in turn, turns on Q3 and Q4, causing the LEDs to turn
on. The action of D35 and R34 provides a constant current source for the LEDs over a supply
voltage range of 5.5 to 17 volts. U15 provides current balance between the individual pairs of LEDs,
and R34 controls the total current to about 60 mA.
The power supply consists of a 5-volt linear regulator, U1, which provides the necessary 5-volt
supply for all of the digital circuits. It has a low drop-out voltage to enable operation from an input
voltage of about 5 volts up to 17 volts. This regulator also contains internal current-Iimiting
protection, which will normally protect against damage from short-circuit faults. In addition, current is
supplied directly from the unregulated input to the backlight LEDs for power dissipation reasons.
D22 is provided as a status LED for programmable radio functions. It is activated by an SB9600
message from the radio which is detected by U3, resulting in a high at U8-2 and -3. In addition, the
PTT LED, D21, can provide flashing radio status messages and can also be activated at the same
time as D22.
The only jumpers that may normally be installed are JU10, JU11, and JU12. When these jumpers are
installed, the ground provided is passed on to U3. These jumpers indicate whether a speaker is
installed in the unit, the SB9600 serial-bus device address, and single- or dual-control head control,
as defined in the table below.
for portables and 600 for mobiles.)
Table 3-1. W3 Jumper Selection
SB9600 address = 6
No internal speaker
BS9600 address = 5
September 26, 2003