PC based frequency meter

Posted on Feb 6, 2014

The simple circuit offers no isolation, but the simplicity of the circuit combined with cost of sound cards and the fact that diodes 1 and 2 add a good measure of protection makes this circuit ideal for the average user. Notice the physical hardware includes a 20MHz crystal oscillator module. This is not required to make the circuit work, but is i

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ncluded for calibration purposes. ( Stop the doubt before it makes you go batty ). The prescaler consists primarily of a down counter. AC coupling is provided together with a DC trigger adjustment. An opto coupler provides the signal isolation. The power supply could be a complete hybrid 5Volt to 5Volt isolation unit, or as in this case a component based design. The transformer used is a base drive transformer savaged from a 11W energy efficient lamp. If I remember correctly mine comes from a Phillips unit. The circuit is not particularly efficient, and as a result a simple shunt regulator can be used on the output. I use R9 to detect the shunting current. In my case it sits at 12mA.

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