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Actually the speed detection of the car is a difficult part.
Circuit where I introduce this time is the thing for the car that adopted the computer control method and the speed signal of the car is output as pulse signal( My car is this type ).
That it depends on the car, there is the type that moves the needle of the speed meter with the rotation of the magnet which is connected the wire that rotates with the propeller shaft. In the case of such a type, the speed detection adapter needs to be inserted in for the shaft and wire. ( I have not used )
Check well, because the speed detection method differs by the type of the car.
Although in the case of the pulse signal type, transmission method of the rotation of the wheel to the meter with the wire is same as the method of the explanation above.
In the meter there is the switch that turns on/off with the rotation of the wire instead of the electromagnet.
The photo transistor( transistor that detects light ) should be used for the detection of the rotation of wire.
The mechanical switch can not do the high speed switching in the long time.
It is rectangle wave() to appear as the signal. When it becomes a high speed the interval of rectangle wave becomes short. ( Frequency becomes high )
It is the point of the circuit in this time to transform the change of the speed of the car to the change of voltage by integration of this signal.
Circuit where I introduce in this page detecting the speed of the car and the door is locked. In addition the various application is possible.
Even 'Additional stop lamp control unit' that I am introducing in other page is controlling the flickering of the lamp by the speed of the car.