Printronix T3204 Maintenance Manual page 93

Transistor switch Q2 is used to switch the gain of the emitter between HIGH gain and
LOW gain. When Q2 is turned on the resistor R147 is in parallel with potentiometer
R3. This represents the LOW gain mode. When Q2 is turned OFF, the resistor R107
is no longer in parallel with R2, this represents the HIGH gain mode. The HIGH gain
mode is used by the operating firmware to aid in distinguishing the difference
between the "out of stock condition" and the "label backing only" condition. In the
LOW gain mode the voltage difference between "out of stock" and "backing only" is
small and therefore hard to detect reliably. By switching to HIGH gain the voltage
difference between "out of stock" and "backing only" is amplified and is therefore
easier to detect.
The paper edge sensor circuits can be summarized as follows:
R132 sets the operating current for the transmissive LED.
R117 and R2 set the operating current for the reflective LED.
U33–8 LOW turns ON the transmissive LED.
U33–6 LOW turns ON the reflective LED.
R3 and R131 set the detector gain in the HIGH gain mode.
R3, R131 and R147 set the gain in the LOW gain mode.
Label Present Sensor (Optional)
The label present sensor circuitry consists of the phase-lock loop circuit U31,
inverter U33–4, diodes D2 and D3, resistors R81, R97, R80 and R79 and
capacitors C81, C65 and C3.
The label present sensor is used to sense whether or not a peeled label that
has been presented to the operator has been taken or is still waiting to be
taken. The label present sensor uses an infrared LED and a photo transistor
in a reflective mode. As soon as the label has been taken, the printer is able
to print and present the next label.
The input signal on U33–3 is a square wave that is generated by the Schmitt
Trigger formed by U30–1 and 2, R78, R89, R90 and C69. The frequency of
the square wave is about 4.6 KHz.
The output of the inverter (U33–4) is used to modulate the label sensor
emitter (LED) at the same rate as the square wave. The square wave signal
also serves as the oscillator input to the phase lock loop circuit (U31–6).
Since the phase lock loop frequency is set by the square wave input, this
circuit functions as a narrow band-pass filter that is tuned to the frequency of
the square wave input. Extraneous light signals such as ambient light will be
rejected by the band-pass filter unless the frequency of the ambient light
matches the frequency of the square wave oscillator.
Circuit Description 8–9