LCD-LED DisplayLights and Display Board Circuits

Reflection Light Barrier with Delay Schematic Circuit Diagram

This circuit is applicable for verifying if a refrigerator door is closed securely, among other uses. It functions by emitting a beam of light through an LED, which, when the door is properly shut, is reflected back. An optical sensor (CNY70) measures the light intensity. If the sensor doesn’t receive the expected amount of light, the buzzer activates after approximately a minute. Once the door is closed (and the CNY70 receives sufficient light again), the buzzer deactivates. The circuit operates on a power supply of approximately 12 mA at 12 V.

Reflection Light Barrier with Delay Schematic Circuit Diagram

Potentiometer P1 regulates the sensitivity of the sensor, ensuring reliable operation at a distance of one centimeter. Increasing the current through the LED can extend this range slightly. The delay can be fine-tuned using C3, while C4 provides additional filtering for the reference voltage. Without this filtering, the buzzer might produce a ‘chirping’ sound. The well-known NE555 is employed to drive the buzzer, operating with a duty cycle of 2:1, enhancing audibility.

The CNY70 sensor is designed as a reflective sensor, incorporating an infrared emitter and phototransistor in a package that blocks visible light. It finds applications in optoelectronic scanning and switching devices, such as index sensing and coded disk scanning (optoelectronic encoder assemblies).

This sensor emits an infrared beam through the IR transmitter and measures the reflected light’s intensity through the phototransistor. The output voltage signal varies based on the light intensity detected by the sensor.


  • Package type: leaded
  • Detector type: phototransistor
  • Dimensions (L x W x H in mm): 7 x 7 x 6
  • Peak operating distance: < 0.5 mm
  • Operating range within > 20 % relative collector current: 0 mm to 5 mm
  • Typical output current under test: IC = 1 mA
  • Emitter wavelength: 950 nm
  • Daylight blocking filter
  • Lead (Pb)-free soldering released
  • Material categorization: For definitions of compliance
  • No setting required
  • High signal output


  • Optoelectronic scanning and switching devices i.e., index sensing, coded disk scanning, etc. (optoelectronic encoder assemblies for transmission sensing).

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