# pulsed time of flight receiver circuit for laser range finding....

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##### Member level 1
can somebody help me in building receiver circuit with pin photodiode (centered at 960nm) for calculating the time and converting it into voltage that is readable for ADC of pic16f877?

#### keith1200rs

##### Super Moderator
Staff member
That is not a trivial design. What is the specification? Laser power, optics diameter, range, accuracy etc?

Keith

##### Member level 1
thanks keith for the response....
specification are:
pulse width= 30ns
pulse repetition frequency(PRF)= 1kHz
laser power (max)=25W
wavelength= 905nm
range= 5 to 100m (or more if possible)
accuracy= as much as possible

regards:

#### MahmoudHassan

##### Full Member level 6
mmmm i have sugestions not sure for it's correct or not

simply photo diode will receive a pulse laser it will contact if we treat it as a switch connecting it's collector to input port in PIC
when pulse comes from laser collector is connected to ground and so pic counts pulse
and when no pulse the port is open high so not count pulse
it will do so for specific time and then you can use it to display counts or whatever !!
iam not sure it correct i hope this help

#### keith1200rs

##### Super Moderator
Staff member
If you do the calculations for the speed of light you will see that a PIC timer is way too slow.

I assume you have a pre-built laser pulser? Have you also got your detector, amplifier & TDC chosen? If not, you need to sort out how you are going to convert time into a signal suitable for the PIC and how to handle the wide dynamic range of the signals.

Anyway, to design the optics you need to take in to account the laser size & detector size as well as eye safety. Typical optics might be 30mm diameter. I assume you will have separate transmit & receive optics side by side. You need to do some calculations to make sure that you will still get some signal at the shortest distance when the optics are focused and pointing at the farthest distance. Larger photodetectors can help there, or receive optics with a field of view wider than the laser stripe.

You have to put a figure on accuracy to do any design. Precision of a few mm can be achieved but that would be the state of the art for pulsed time of flight and I don't think you will easily achieve that. 100m is quite a short distance which makes some things easier but the shortest distance can be a problem due to the effect of the high current laser pulse. Are you planning a calibration pulse or optical delay?

Keith

tony_lth

### tony_lth

Points: 2

##### Member level 1
Re: pulsed time of flight circuit for laser range finding....

thank you MR Mahmoudhassan and Mr Keith. i realy appreciate your concern.
actually i'm working with the amplifier and TDC circuitery but i couldn't sort out any. i have the detector i.e PIN photodiode (S1223). but i couldn't find any circuitery.

i'll be very thankful if you can help me with the circuitery.

#### keith1200rs

##### Super Moderator
Staff member
That photodiode is rather large and slow for a laser rangefinder. 1m represents around 6.7ns return path. Your 30ns laser pulse will be 5m long. Normally I would be looking at photodiodes <1mm diameter with maybe 2pF of capacitance. Also, I don't know you laser stripe width but if it is 300um for example, there is no point having a detector which is 10 times wider unless your optics is equally asymmetrical.

You need to look at transimpedance amplifiers for amplifying the photodiode signal. The simplest TDC would have a fixed threshold and a comparator. I once made a very crude indicator by integrating a fixed voltage from the start pulse to the received pulse. The voltage is then proportional to distance. For any sort of accuracy you need something smarter so you can find the pulse centre.

Keith

### erikl

Points: 2

Points: 2

#### keith1200rs

##### Super Moderator
Staff member
Yes, the Acam TDCs are good. There is another make but I cannot remember who it is but can check when I am back at work if you want to know. There is a lot of analog design work required before you get a decent start/stop signal for the TDC though.

Keith

Points: 2

##### Member level 1
thank you so much Mr. keith, i realy appreciate your help.
Actually i have designed a receiver circuit in which i'm using a pin photodiode, preamplifier, comparator, positive edge D-flipflop, transister and capacitor. i have applied "current mode time to amplitude converter" method.
now my problem is how to charge the capacitor to get 5V across it in 80ns? and hold that value until read by ADC of microcontroller.
i mean what value of capacitor and the constant current should i use that will charge the capacitor?
and secondly can i generate 80ns pulse at 1kHz PRF with pic 16F877?

#### keith1200rs

##### Super Moderator
Staff member
I am not sure where 80ns comes from. Your maximum distance is 100m which corresponds to 666ns return path.

Anyway, assuming the rest of your circuitry works the conversion to voltage is straightforward. Use a current source to charge a capacitor through a fast analog switch which is turned on from the start pulse to the stop pulse. Use the stop pulse to interrupt the micro, read the ADC and clear the capacitor with a second analog switch.

If you use a 1mA current source then 100pF would charge to 1V in 100ns. So, that would reach 5V in 500ns. You need a fast, low resistance switch with low capacitance. If the switch capacitance is high then you need a higher charge capacitor so it isn't affected too much. Switch capacitance should be a relatively stable effect which could possibly be calibrated out as it would probably produce a fixed offset in the result.

You could use a single MOSFET instead of an analog switch because you aren't dealing with both charge & discharge in the same device. The same applies to the discharge switch.

I would be interested to know how well it works. As I said, the photodiode seems a bit large and slow and 5m will be a challenge unless you couple the laser through some fibre.

Keith

Points: 2

##### Member level 1
thank you so much Mr keith, it was realy informative.
i would love to share my exprience. but uptill now i am realy depressed because things are getting worse day by day. its becoming more complicated and i hve little with me.

#### keith1200rs

##### Super Moderator
Staff member
You can always send me a private message if that will help. Laser rangefinders are not trivial circuits but I have been designing them for 25 years so I may be able to help.

Keith

greyghost

Points: 2