Car glass hail melting circuit b

It is an automated ice circuit for automobile cab windshields. As a passenger, the author once personally witnessed a scene where thick ice formed on the car glass
and was opaque. The driver could not shovel it off with a shovel, causing the bus to stop running for half a day. In order to solve this problem, this circuit is designed in this example
. When the glass freezes or frosts, the ice and snow will melt automatically to ensure the driver's clear vision and avoid accidents. This circuit can also be used in
other places where dehumidification is required.
    In the figure, Rs is the heating resistance wire, which is placed on the icing windshield, as shown in Figure 7-68a. H is the icing humidity sensor , and Figure T68b shows
its control circuit. R transistors VT1 and VT2 are Schmitt trigger circuits . The collector load of VT2 is the coil winding of relay K. Ri, R:
are the base resistance of VT1, RD is the equivalent resistance of the humidity sensor H. When there is no ice, adjust the resistance values ​​so that VTI is turned on and VT2
is turned off.
    Once frozen, the equivalent resistance RD value of the humidity sensor
H drops to a certain value, R2∥RD decreases, causing VT1 to cut off, VT2 to conduct, and the VT2 collector load relay K coil to energize, and its normally open contact 2. Turn on the heating power E, and the indicator light is on, the
relay contacts are connected, the electric heating wire R is energized, and the ice and snow on the windshield is heated and melted. After melting, RD∥Rz returns to
the resistance value when it is not frozen. VT1 and VT2 return to their initial state, the indicator light goes out, the resistance wire is powered off, and heating stops, thus realizing automatic
ice melting control.