A simple inventer and NAND gate can be connected to yield a highly compact and reliable digital frequency detector. This circuit can detect frequencies up to 3 MHz with 50% duty cycles. When a frequency, j,, appears at the input, points A and B detect a logical high de level. Thereupon pointE increases the latch sets and the LED lights. If the input frequency is absent and if the voltage is either at a constant high or low level, points A and B will be complementary and pointE will decrease.