Infrared proximity sensor circuit guidance  - Page 1
Forum section: electronics / projects

author: jakobhalskov
author_rank: Newbie
timestamp: August 23, 2017, 05:05:02 pm
content: Hi everyone,I am trying to design a simple Infrared reflectivity sensor, which is going to detect when I wave my hand over my home made alarm clock. Distance: 0 - 15 cm.After being through several designs and iterations while still running in to problems I would like to get some feedback on how to improve the circuit/method.The circuit is inspired by the following app note and article:https://www.maximintegrated.com/en/app-notes/index.mvp/id/4622http://www.nutsvolts.com/magazine/article/security-electronics-systems-and-circuits-part-5The idea is that the transmitter side generates a 10 kHz signal using a 555 timer, which controls an infrared transmitter LED.The receiver detects the reflected light, high pass filters and amplifies it. This is then fed through and envelope detector and compared with a set level (the sensitivity).Initially I had problems with the transmitter causing ripple on the VCC line, which affected the receiver so it amplified the noise on the power rail. It is almost solved by the decoupling capacitor/resistor R10 and C6.It is still quite unstable. By that I mean the amplified signal changes amplitude over time even though the IR TX and RX diode is pointing to a flat surface at a fixed distance.Furthermore the output of the amplifier gets weaker when the sensor is in a room with "DC" (0 or 50Hz) infrared light from the sun and incandescent bulbs. If it is very near my incandescent bulb it cannot detect the 10Khz ir light at all.I am stuck and don't know to improve the design/method to get a more reliable detection of: is there something in front of the sensor or not.Thanks!Jakob Halskov

author: ace1903
author_rank: Regular Contributor
timestamp: August 23, 2017, 06:41:17 pm
content: I have designed similar circuit and had similar problems.I solved it by using dedicated ir receiver IC (TSOP1738 or similar, do not remember well).That IC has built in filters and auto gain circuit. Filters and AGC together remove DC offset caused by direct sunlight.It is difficult to make that with discrete components. Also helps if you use diode with narrow radiation angle.If you take similar approach also check with schmith trigger characteristic tuning. Positive feedback should be optimized manually during development after that pot should be replaced by resistor.I also used LM567 instead of envelope tracker.

author: Benta
author_rank: Super Contributor
timestamp: August 23, 2017, 07:21:36 pm
content: Visit Vishay. They have great IR proximity parts and good app notes.No affiliation.

author: jmelson
author_rank: Super Contributor
timestamp: August 23, 2017, 08:22:42 pm
content: Put another decoupling filter between C6 and D1. Since the reverse-biased photodiode will be drawing VERY little current, a 100K Ohm series resistor should greatly help remove interference.If the emitter and sensor/amp are too close together, possibly a shield can be placed between them.Jon

author: jakobhalskov
author_rank: Newbie
timestamp: August 28, 2017, 07:45:03 pm
content: Hi again!Ace1903:For now I am trying not to use active IR Receiver with another formfactor than normal LEDS. Mostly due to practical and aesthetic reasons (to fit it in the enclosure I have built).It might be my last resort if I cannot get it working reliably.Benta:Thanks for the heads up. I found a general one about infrared diodes - not that specific on the circuitry needed.After reading it I might be worried if the IR photodiode is saturated by the ambient infrared light in my bedroom (from a 60W incandescent bulb) and therefore is not able to detect the 10 kHz signal properly.Jmelson:Thanks for the tip about adding a series resistor before the photodiode. I will try that and see how it affects the signal and sensitivity.General:I am in doubt of how much output power (current through my IR transmitting LED) I need to detect the reflected light at 15 cm.Thanks,Jakob

author: Benta
author_rank: Super Contributor
timestamp: August 29, 2017, 08:25:58 am
content: Visit this page:http://www.vishay.com/optical-sensors/Note that the proximity sensors usually also have an ambient light sensor built in for compensation. That takes care of your 60 W bulb :-)

author: jakobhalskov
author_rank: Newbie
timestamp: August 29, 2017, 10:34:00 am
content: Quote from: Benta on August 29, 2017, 08:25:58 amI do currently have a highpass filter to get rid of the DC light from the bulb, but it seemed like the receiving photodiode got saturated by the light from the bulb.I'll get back when I have checked the link outThanks

author: jakobhalskov
author_rank: Newbie
timestamp: September 06, 2017, 07:04:14 pm
content: I figured out the receiving IR photodiode got saturated by the ambient light and the incandescent bulb. It is alleviated by lowering the load resistor, which converts the photo current into voltage.It seems to make the system work now

author: MasterT
author_rank: Frequent Contributor
timestamp: September 06, 2017, 10:31:51 pm
content: I notice, that your circuits is similar to publish in nuts&volts. Personally, I wouldn't be using inverting amplifier configuration, that has low input impedance. Another circuits from maxim-ic is preferable, IMHO.One more things, since transmitter and receiver located in the same device, you don't have to design a nice filter for 10 kHz, use synchronous detector driven by main clock from 555.