This circuit is based on the 555 timer chip. It's set up as a free running oscillator with a squarewave output to drive the light emitting diode or laserdiode. With the parts shown, the frequency is about 24 Hz and the duty cycle is close to 50%. The driver transistor can be almost any TO-220 case. For use with a laser diode, carefully select the resistor so laser current wont be exceeded.  For modulation, the circuit on the left shows a CDS cell used to vary the frequency depending on the ambient light levels. The 470k resistor limits the range as the cell changes resistance from about 1k to 100k ohms from light to dark. For fine tuning, the one on the right has a dip switch and resistor array to change the frequency in small steps.

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