www.jabolatorium.com The aim of this experiment was to design and build a simplified USB powered ESR spectrometer. A sample of TCNQ is placed in a coil which is part of an LC circuit of a Robinson oscillator operating at 42Mhz. The signal output of the Robinson oscillator which has gone through a band pass filter (1610Hz) is connected to a lock-in detector. The detected signal is then connected to the analogue input of an Arduino Nano. The frequency output of the Robinson oscillator, which has gone through a frequency divider and an ECL to TTL converter, is connected to the 16-bit hardware Timer1 of Arduino Nano. The second modulation signal (1610Hz) is produced by a direct digital synthesis. An external magnetic field (0-2mT) is produced by a miniature Helmholtz coil with an internal coil diameter of 12mm, and controlled through a coil driver by a PWM signal from an Arduino Nano. The ESR signal can be observed on a PC (RS232), any android device (Bluetooth), or a built-in 0.96 inch OLED display. Additionally the noise and the "butterfly signal" can be listened to, as an audio from a speaker with amplifier connected to the output of the bandpass filter.