- on Test & Measurement
- Published in issue 12/2004 on page 0
The circuit presented here (in two versions) uses a light barrier as a position sensor. In the first version (Figure 1), the light barrier consists of an LED (D1) for the light source and a LDR (R2) for the receiver. The LDR is part of a CMOS oscillator that generates a digital signal (pulse waveform) whose high/low ratio (duty cycle) and frequency depend on the intensity of the impinging light. With the arrangement shown in the photo, the sensor is used together with a positioner. The light-dependent resistor is more or less obscured by a pendulum, so the duty cycle and frequency of the output signal depend on the position of the actuator. If the LED and pendulum are omitted, the light falling on the light-dependent resistor can also be used to directly determine the frequency and duty cycle.