Light sensitive circuit

Thread Starter

Lokheshkumar A

Joined Oct 16, 2016
13
Hi everyone,
I currently working on a project called laser based distance measurement beyond 50m. I'm using laser as source and BPW34 as a receiver to implement TOF measurement. The problem is depending on light intensity, photodiode response is changing i.e. Time value is perfect when photodiode gets hit by laser directly but time changes when when a part of light gets hit on the photodiode. So, I guess depending on the intensity of light rising edge time is changing. Any suggestions ?? I am currently using transistor to get digital logic in receiver. Any modifications to be done in circuit ??
 

Sensacell

Joined Jun 19, 2012
3,432
Show your schematic?

Optical TOF measurement is cutting edge stuff, requiring really serious high speed design techniques.

The photo diode selected is glacially slow, in the 100 ns it takes for the diode to respond, light travels 30 meters...

The rise time of your photo diode amp also needs to be screaming fast, otherwise all you are measuring is the slow rise time of your photo diode circuit.
 

Thread Starter

Lokheshkumar A

Joined Oct 16, 2016
13
Show your schematic?

Optical TOF measurement is cutting edge stuff, requiring really serious high speed design techniques.

The photo diode selected is glacially slow, in the 100 ns it takes for the diode to respond, light travels 30 meters...

The rise time of your photo diode amp also needs to be screaming fast, otherwise all you are measuring is the slow rise time of your photo diode circuit.
Each resistor is 1Mohm.
What you said is perfectly right. My problem is rise time isn't same for all intensities striking on photodiode, depending on intensity values are changing significantly. What can be done for that ?!
 

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AlbertHall

Joined Jun 4, 2014
12,345
Optical TOF measurement is cutting edge stuff, requiring really serious high speed design techniques.
I worked on cable fault locators which worked by sending a pulse down the wire and looking for reflections from any discontinuities in the cable. Electricity travels about two thirds the speed of light in a cable so the time measurement problem is similar. The equipment I worked on did the timing using one TTL chip and the rest was slow cmos. The technique was patented (by A. C. Cossor) and used two crystal oscillators - 1MHz and 1.001MHz - and the timing came from the drift between the two being 1nS per cycle, giving timing measurement to 1nS resolution.
 

Sensacell

Joined Jun 19, 2012
3,432
Do more research on the subject- the fundamentals.

Your circuit is too slow by many orders of magnitude, not even in the ballpark, you need to re-think the project.
 

Thread Starter

Lokheshkumar A

Joined Oct 16, 2016
13
Hello everyone,
I'm currently involved in a project of measuring distance beyond 50m and upto 100m. Here is my setup, transmitter and receiver at one end and prism(reflector) on the other end. I'm using PIC18F45K22 microcontroller with red laser and BPW34 as photodiode. I tried with CTMU unit in pic with receiver circuit being switched ON using transistor BC547. Readings I got thorough this is not satisfactory as readings keep on changing depending on intensity of light falling on photodiode. I'm stuck in this one and can't able to move further. Is there any alternative solutions to this or how can I implement this overall ?? And is it possible to use counters in pic ???
 

Thread Starter

Lokheshkumar A

Joined Oct 16, 2016
13
The rise time of the BPW34 is 100nS. In that time light has travelled 100 feet or 30 metres.
Look for PIN photodiodes.
Readings are changing significantly when intensity of light falling on photodiode changes. Is there any relation between intensity and its response time ? What circuit can I use for detecting even small portion of light using this photodiode ?
 

AlbertHall

Joined Jun 4, 2014
12,345
Yes, the intensity of the light will change how long the delay is until the output reaches your threshold. You cannot fix that without using a much faster photodiode.
 
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