With development of vehicle techniques, a lot of mechanical vehicle parts have been replaced with electronic parts, and the vehicle horn has been through several modifications, a major change of which is that an electronic oscillator has been used to replace mechanical contacts so as to extend lifespan of the horn and decrease interference introduced by the electromagnetic radiation. Up to hundreds of patents have been invented in this regard. But, most of them have not been put into practice since they fail to achieve a driving circuit, frequency of which is consistent with resonant frequency of the horn's acoustic system including horn diaphragm and resonance cavity.
Because the sounding diaphragm of the horn has inherent resonant frequency just as a tuning fork, a key point to the electronic horn is that signal frequency produced by the horn's electronic circuit should be consistent with that of the sounding diaphragm and the acoustic system, so as for the horn to produce optimum sound and achieve higher horn efficiency and better stability.
Currently, ways of adjusting frequency for electronic horn can be classified into three categories:
1. adjusting resistance to change oscillating frequency;
2. changing oscillating frequency by means of frequency synthesis;
3. using microcomputer chips to directly generate a signal having a particular frequency.
Among these, the second and the third categories of ways can produce a signal frequency, which is in good match with that of horn diaphragm, but they result in higher cost. The first category of ways can produce a signal having the same frequency as the diaphragm, but the frequency might change under operation due to the resistance change since the adjustable resistor comprises a contact which, under vehicle vibration, may inevitably lead to a resistance change.
Taking the first category of way for example, there is an alternative solution, in which an adjustable resistor may be used to find the match point and then replaced with a fixed resistor. By this way, no resistance change of the fixed resistor would be found when the vehicle is vibrating. However, the adjustable resistor could hardly be replaced by fixed resistors with the same resistance. For example, the adjustable resistor is tested to have a resistance value of 4.85 kΩ when the driving circuit is in a good match with the horn diaphragm. But, fixed resistors have generally discrete values such as 4.7 KΩ, or 5.1 kΩ, no fixed resistor being found to have exactly the value of 4.85 kΩ. The adjustable resistor with the tested resistance has to be replaced with a fixed resistor having approximate but different value, which might lead to a dramatic change in frequency and thus a drop of horn performance. Even if a fixed resistor having exactly the same resistance as the adjustable resistor has been found, the horn assembled with the fixed resistor may suffer a great change in performance from the test due to distributed capacitors along leads in the adjustable resistor.
Therefore, the problem in respect of stability and reliability has not been solved for long time. The U.S. Pat. No. 5,266,921, titled as ‘Method and apparatus for adjusting vehicle horns’ and granted in Nov. 30, 1993 uses laser to trim resistor to avoid the problem of resistance change. But, this method has its own defect, i.e., the frequency can not be adjusted to the match point. Because a point can only be learned to be the match point only after it is passed. If the match point is passed, it means over-tuned, otherwise it is mis-tuned.
Therefore, the adjusting methods mentioned above have bad industrial applicability and can not be well practiced in mass production.