When fault finding or trouble shotting on EM machines (AC voltage) it can be difficult testing coils on score reels, ball in play & player stepper units etc. which are located in the back box. I use this test light instead of a multimeter as it gives a positive visual that a coil or contact is being energised.(especially if the score motor is pulsing a coil on the score reels on reset, or a relay is counting up etc) It has many uses for testing circuits & may also be used for DC voltages of 15v or below, however observe correct polarity. (You could lower the resistor value if you require a brighter LED, don't go below 1k .