What equipment do I need in order to alter the frequency (hertz) of a direct electrical current(e.g. from a dc battery)?
The name of the circuit that you are looking for is “Oscillator”. A battery powered oscillator is cheap and easy to build. Simple variable frequency oscillator (source http://www.electronic-circuits-diagrams.com/oscillatorsimages/oscillatorsckt1.shtml) This is a very simple circuit utilising a 555 timer IC to generate square wave of frequency that can be adjusted by a potentiometer. With values given the frequency can be adjusted from a few Hz to several Khz. To get very low frequencies replace the 0.01uF capacitor with a higher value. The formula to calculate the frequency is given by: 1/f = 0.69 * C * ( R1 + 2*R2) The duty cycle is given by: % duty cycle = 100*(R1+R2)/(R1+ 2*R2) In order to ensure a 50% (approx.) duty ratio, R1 should be very small when compared to R2. But R1 should be no smaller than 1K. A good choice would be, R1 in kilohms and R2 in megohms. You can then select C to fix the range of frequencies. More in these links Article
