This paper presents a closed-loop optogenetic stimulation device to achieve online modulation of neurons. The device is designed to be mountable on small rodents in pre-clinical settings. Considering the size of rodents and the need for portability, a single-piece self-contained device is developed which allows real-time photostimulation based on detected neuronal states. It consists of three components: a neural recorder, a control algorithm, and an optogenetic stimulator. The neural recorder which is realized by analogue circuitry measures the neural signal. The on-off control algorithm analyses the neural signal and controls the stimulation of the target neurons. The optogenetic stimulator performs sampling and digitization of the detected neural signal, runs the control algorithm, and manages the operation of the light source. The configurable neural recorder is capable of 64 dB amplification in the frequency range of 300 Hz to 6 KHz. The outcome of bench testing of the device is reported. The device is portable and headmountable which makes it suitable for use with small rodents in pre-clinical trials.