Handy Dandy Little Circuits #19

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This circuit started as a sinewave oscillator and I was having trouble taming the little beast . The original circuit made use of a thermistor as feed back controlling element . I was going to discard the circuit when I decided to see what happens when I substitute a photocell for the thermistor . With the ambient surrounding light the circuit was not even oscillating since the photocell resistance is very low it was effectively shorting the R/C feed back circuit but when I passed my hand over the cell there was an instantaneous oscillation of a nice square wave. I coupled the output to an amplifier(LM386) and at low volume setting I obscured the photocell and was immediately blasted by the sound output . Adjust RV1 for oscillation .

There could be so many applications for this simple circuit that I leave it up to you and your imagination of what to do with it . For applications on the LM386 See page # 10

The basic ciruit below is one section of an equalizer used in audio system . The table shows the values of C1 and C2 for the center frequency of your choice . For best results op-amps such as LM358, LF353,TL084,OP-08 should be used.

You must use a split power supply . With the TL084 which is a quad op-amp four sections can be build using a simple voltage divider as the ground point made up with two resistors and a filter capacitor, the same would apply using an additional TL084 for a total of eight sections whith its own voltage divider ,all grounds must be connected together.