PC based frequency meter

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

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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