Photodiode amplifier circuit help

I’m putting together a Morse code decoder circuit with Arduino (a photodiode sensor connected to an analogue pin, the Arduino will then decode the high and low light pulses the photodiode receives to be dots and dashes) and I’m currently working on the sensor portion of the circuit.

I have the following amplifier circuit to amplify the signal coming from the photodiode.

As well as this I also took the optical filter off of an old barcode reader and put it over the photodiode so that mainly the light from the red laser pen that I am shining at the detector should get through.

The speed at which I’m modulating the laser is proving to be the main problem; I’m modulating the laser by turning it on and off every 50 - 200 microseconds; if using a plain photodiode on it’s own the photodiode rather than reading this as highs and lows just waits till it has received enough pulses to saturate the sensor and then turns on, it’s acting more like a low pass filter.

So I chose to use this amplifier circuit instead however I’m struggling to get the correct resistor values to give me the sensitivity I need.

Can anyone recommend some resistor values for the above circuit or another amplifier circuit that might be better suited to the sensitivity and speed that I need?

For extra info, as well as the above I’m thinking about further reducing ambient light getting to the sensor by either pointing the sensor and the laser at a surface a few mm from the detector and measuring the reflections or by putting a thin piece of paper over the sensor that the laser would shine through and hit the sensor that way.

Any help much appreciated

Hi, What is your photodiode, type, part no, link to datasheet please.

http://www.electroschematics.com/tag/photodiode-circuits/

Tom.... :)

They didn't seem to come with any number or id, I bought them off of ebay, just 5mm photodiodes like these.

http://www.ebay.co.uk/itm/20PCS-5MM-Photodiode-/291378932949?hash=item43d789acd5

Oddwired: The speed at which I'm modulating the laser is proving to be the main problem; I'm modulating the laser by turning it on and off every 50 - 200 microseconds; if using a plain photodiode on it's own the photodiode

Any help much appreciated

Laser diodes have a startup time of milliseconds , its inherent in the design. You cannot digitally modulate them.

"this as highs and lows just waits till it has received enough pulses to saturate the sensor and then turns on, it's acting more like a low pass filter."

I would suggest its not turning on at all but your pulses may be getting added up by a capacitance until there is enough power to give an output from the laser.

try using an ir led instead of a laser

Ah, that will be an issue; for what it's worth at the moment I'm using an old laser galvanometer to move the laser back and forth over the sensor to simulate the effect but I was hoping to make a 555 timer circuit to switch the laser on and off once I've got the sensor working, if the laser won't do it I'll have to come up with something else later or stick with the current method :(

As for the capacitance suggestion that does sound likely now you mention it; either way the photodiode on its own wasn't working so I'm having a go with the op amp amplifier instead.

Using a tv remote or similar would be a simple way to test your reciever.

Oddwired: As for the capacitance suggestion that does sound likely now you mention it; either way the photodiode on its own wasn't working so I'm having a go with the op amp amplifier instead.

The amp is essential.

Just noticed the link.

They look like led to me not photodiodes.

Google

Using led as photodiode.

they can work but have drawbacks.

What value does Cf have? It will integrate the input signal and function as a low pass filter. You could lose a lot of performance there if it is too large.

Start from here:
http://www.linear.com/search/search.php?q=photodiode
http://www.linear.com/search/search.php?q=photodiode

I agree with suggestion, that what you have, may be not photodiodes, could be LED or phototransistor. Do you have a multimeter to check a current, for P-Diode dark current < 1 uA, P-transistors about 100x higher = 0.1 mA . Led easy to test with web-cam, you can see IR leds lights up if you apply a power and current 10mA ( use resistor of course).

Laser diodes have a startup time of milliseconds , its inherent in the design.
You cannot digitally modulate them.

No, it’s not correct. Lasers widely used in optic fiber communication links up to 100 Gbits. I have tested IR 300 mW laser 808 nm with 7 MHz 50% duty cycles, so above this it really work light low pass - modulation index drops significantly. But laser was cheap from BG. And as a sensor I use BPW34, which itself has pass band only 4-5 MHz

What is the purpose of this exercise. Morse code? What baud rate. Why not use a modulated IR diode. Modulated IR has less problems with ambient light. A narrow beam IR LED could bridge 50 meters. Is the (~1msec) switch time of a 3-pin IR receiver the problem. Leo..

Those look like IR diodes photodiodes are kind of grey looking not clear.

Magician: No, it's not correct. Lasers widely used in optic fiber communication links up to 100 Gbits. I have tested IR 300 mW laser 808 nm with 7 MHz 50% duty cycles, so above this it really work light low pass - modulation index drops significantly. But laser was cheap from BG. And as a sensor I use BPW34, which itself has pass band only 4-5 MHz

Sorry it is, and you have answered it yourself.

modulation !

Laser diodes can be modulated at high frequency between 20 to 100 % intensity.

If they are turned hard off they take miliseconds to start

I would add i used to test links you mention and eye height is around30%

As the op wants to use morse the off periods will be enough to kill the laser. He would need to use a carrier.

WAWA idea is the way i would go.

Recievers with built in amps can be had easily, unfortunatley they all have built in ir filters.

The op seems to want to use a visible laser though.

OP

I suggest 1 to 2 meg for your resistor and 200 to 300 pf for the cap as a starting point.

Once you have the reciever working you have a starting point.

A carrier is a great idea, with it you can vastly improve the S/N. Phase modulation would work well. I found a good tutorial on the amplifier circuit: http://ecee.colorado.edu/~ecen4827/hw/hw1/PhotodiodeAmplifers.pdf

Magician: Start from here: [http://www.linear.com/search/search.php?q No, it's not correct. Lasers widely used in optic fiber communication links up to 100 Gbits. I have tested IR 300 mW laser 808 nm with 7 MHz 50% duty cycles, so above this it really work light low pass - modulation index drops significantly. But laser was cheap from BG. And as a sensor I use BPW34, which itself has pass band only 4-5 MHz [/quote]

To be fair i dont know what you used.

Modules are available that can be fed with a digital signal.

Feeding 0 to x volts square wave into a "neat diode" can easily destroy it.

I would not have thought a laser pen would be equipped with this facility. It may have soft start protection though that will defeat any attempt at modulation.](http://www.linear.com/search/search.php?q=photodiode)

Laser diodes can be modulated at high frequency between 20 to 100 % intensity.

If they are turned hard off they take miliseconds to start

Here is a laser:

here is a circuits:

Tested at 7 MHz, start up time of laser <50 nanoseconds, so your statement “milliseconds” simply isn’t apply.
I know, that supply bias current may increase speed up, but it doesn’t change anything for 50 nsec pulses, so I can’t say if it makes difference on picoseconds scale.

Its nice to have a known working circuit.

All i can say is it goes against all my working knowledge.

Cw lasers can be driven in pulse mode but its very risky and only possible at low duty cycle/ average power.

I suspect you have succeeded by not driving to its full output , do you have any power comparisons ?

Most cheap cw modules have a considerable soft start period of 10 s of milliseconds upwards, although the diode ramp itself can be smaller with careful supply design.

I have used cw pulsed lasers in a lab but they were commercial devices and the driver box cost ££££.

Get the pulse or current setting wrong or the repetition rate and it was very very easy to blow the diode.

To clarify you are using a "neat " diode the op is using a laser pointer module.

All the commercial modules i have seen have an inherent startup delay to protect the diode.

The advantage of using commercial modules is that they are often classified by power output that can make their use legal in various places 2 to 3 mw normally.

Use of a neat diode like that can easily exceed legal levels, only proper testing will tell you that.

For the op the use of a 2mw pointer will easily give a long range if modulated correctly.

No one seems to have mentioned that the whole point in modulating the light is to make the receiver insensitive to unmodulated light, that is ambient light.
For that you need a tuned amplifier or band pass filter with gain. That original circuit was a low pass filter and so would amplify the ambient light as well as the signal.

wawa did mention it but we are trying to get his reciever working first and arguing while we waitt