Service
Manual
OLS-81-
10
9
Circuit Description
Nominal current setting
The nominal current setting comprises the voltage-controlled current source (U4.2/Q6/R25)
Diode D2 is the reference.
Modulation
The send diode DS-1 is modulated by
Q5. Q5 is driven at the same rate as the modulation
frequency by U1 pin 17. Depending on the base potential of Q5, the current flows through Q5
or through DS-1. The current and so the source can be switched on by DS-1.
Keypad controller
The keypad controller
U1 is a customized gate array. Four of the five keys are connected to it:
CW, 270 Hz, 1 kHz and 2 kHz. The gate array detects which key has been pressed, turns the
appropriate LED on and applies the appropriate modulation frequency to output F-OUT.
The keypad controller also controls turn off and turn on, permanent mode and the battery low
warning (see Section "Battery Monitoringw).
ONIOFF control
The "main switchw of the instrument is formed by the two, parallel MOSFETs U3.1 and U3.2. Q3
is an auxiliary power-on switch.
When the ONIOFF key is pressed, Q3 is turned on, the DC/DC converter U2 receives current
and starts up, the 5 V voltage supply builds up and the gate array receives a reset. When a reset
is detected, the gate array connects output OFF-OUT
(pinl7) to high and so turns on U3.1 and
U3.2. The instrument is now on.
If the ONIOFF key is held down, capacitor C8 charges via R15 and Q4. From a certain level
(ONIOFF pressed
>
2 s), the gate array detects PERM-IN low at its input and the instrument
.
goes into the permanent mode. This means: the PERM LED is turned on and the instrument's
battery saver facilrty is disabled.
If the PERM-IN input remains high (ONIOFF pressed
<
approx. 2 s), the instrument will turn
itself off automatically after 20 minutes to save the batteries. The PERM LED does not come on.
Turning off the instrument: if the ONIOFF key (instrument turned on) is pressed, the gate array
receives a pulse via Q4 and
C9 at input CPFF (U1 pin 11). The OFF-OUT output goes LOW
and U3.1 and U3.2 are disabled. The instrument is turned off.
Battery monitor
Voltage regulator U2 contains a comparator (U2 pin 1
=
input) which acts
as
a battery monitor.
Output U2 pin
2
(BLINK-IN signal) goes low when the battery voltage drops below 2.2 V. As a
warning, the gate array causes the active LEDs to flash.
5
v
supply
The DCIDC converter U2 will operate from a battery voltage of approx. 2 V and is design to
operate up to approx. 3.5 V.
Cutout circuit (from series
E)
Comparator U5 acts as a cutout for the OLS-8.
If
the send diode current exceeds 30 mA, Q5 is
turned on and so limits the current through the send diode. In the case of the OLS-10, current
limiting of this kind is not necessary and is deactivated Qumper BR4).
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