3.5.3 Circuit Explanation
(1) Liquid Surface Detection Circuit
1) Oscillator (X1, Q1, etc.)
Colpitts Oscillator Circuit using ceramic oscillator (X1), oscillates a wave form of 400 kHz.
2) Limitter (D1)
Diode (D1) clips the 400 kHz wave form to 1.2 Vp-p.
3) Amplifier 1 (U1)
Non-inverting amplifier amplifies the 400 kHz wave form and adds the 400 kHz signal to the
probe.
4) Buffer (Q2, 3, 4)
This circuit receives signal of probe capacitance change. It also imposes the guard-shield at
the same potential with the probe (low impedance).
5) Detection Circuit (D5, C11, R15)
400 kHz wave form is peak-detected.
6) Highpass Filter (Q6, R18, C13)
The wave form after de-modulation is differentiated, and only the differential part is extracted.
7) Amplifier 2 (U2)
Differentiated wave form is amplified.
8) Comparator Circuit 1 and 2 (U3)
Liquid surface signals of touching and detaching the surface are compared with the reference
voltage (approximately, + 2.6 V, – 2.6 V).
9) Flip-flop (U4-8, 9, 10, 11, 12, 13)
Liquid surface detection signal is latched by flip-flop.
(2) Probe Crash Detection Circuit
Probe crash is monitored by a micro-switch in the probe holder (normally ON). The micro-switch
turns OFF when probe is pushed back and the probe crash signal is generated. The probe crash
signal is latched by flip-flop.
3.5.4 Settings and Adjustment
Setting and adjustments are not necessary.
CA-500 Series S/M 3-9 Revised December 2001 8
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