Two light intensities difference convert into frequency modulator for

Abstract

The present invention includes two photodiodes in parallel connected to some transistors and an adjustable current source. The photocurrent difference from the two photodiodes combines with the current form the current source to be a positive value. This current then charges a capacitor. An automatic comparator circuit detects voltage of the charging capacitor. The Q output of the RS flip-flop is low and the transistor is off to charge the capacitor if the voltage of the capacitor is below a preset value. The buffer output is at low voltage in this case; The Q output of the RS flip-flop is high and the transistor is on to force discharging of the capacitor and the buffer output is at high level when the capacitor voltage reaches a preset value. By repeating these conditions, frequency modulated output can be obtained. The frequency value is related to the two light intensities difference and the current source combined current. When light intensity varies, the frequency varies accordingly. The current source can compensate the imbalance of dark currents between the two photodiodes. It can also decide the central frequency of this circuit. The present invention provides a new method to form two light intensities difference frequency modulator suitable for the control of frequency in a photoelectronic system.