This circuit will enable you to cycle a string of Christmas lights , a beacon, or any other high or low voltage components when use with a relay of the proper rating.
Gates A and B of CMOS IC CD4001 are used as an oscillator the ouput of which turns on/off the transistor which in turn activate the relay. The relay coil resistance must be hight enought to limit the current drawn from the transistor to no more than 100mA and the relay contacts must be rated for the anticipated voltage and current . A high gain medium power transistor may be substituted.

R/C set the oscillation ,choose a large capacitor for C and a variable resistance for R and adjust to the desired frequency then substitute for a fixed resistance. If you have a set frequency in mind use the simple formula as shown.

I used this simple transistor tester circuit for many years it is fast and easy to use. You can even match transistors by their tones wether PNP,NPN and even power transistors. Enclose it in a small plastic box along with some external means of using leads for testing power transistor . Use a 9v batt.