Watermelon raw and ripe discriminator circuit

Every year when watermelons are on the market, many people often have unnecessary disputes with melon sellers due to lack of experience in intuitively judging whether the watermelons are raw or ripe . It would be a very good idea to design a circuit
that could accurately determine the rawness and ripeness of two melons.
    The circuit of the watermelon raw and ripe discriminator is shown in Figure 1 49. Its core device is
a 2-input 4-NAND gate integrated circuit IC, in which Dl, DZ and D4 are all connected
as inverters (NOT gates).
    The Dl input level is determined by the voltage division of the watermelon resistance on the battery GB measured by RP and the probe .
Since the water and ion concentrations of raw melons and ripe melons are
different, their conductivities are sometimes also different. Ripe melons have low resistance while raw melons have
high resistance. When probes a and b are inserted into the ripe melon, the resistance of the melon is small. The DJ input terminal is low level (less than V / 2), and the high level output by Dl
is added to the input terminals of D2 and D3 respectively. On the one hand, D2 inverts the output: low power, and both light-emitting diodes (LEDs) The terminal obtains a certain potential
and emits light. On the other hand, the D3 and D4 audio multivibrators are made to work, and the piezoelectric ceramic chip emits light. "Beep-one"
sound. When probes a and b are inserted into the raw melon, the resistance of the melon is relatively large, the Dl input terminal is at a high level (greater than vJim/2), and its low-level nano output terminals "seal" the D2 and D3 input terminals
respectively , causing LEb to No light or B silent, depending on whether LED and 9B light up and make sound. , you can judge
the rawness and ripeness of the measured watermelon. ． ．