Hi All,
Haven't posted for a while, but I need some help with what could be a problem with photodiode use and a possible solution. I am using a photodiode in reverse-bias mode as a laser light detector (see attached schematic). The photodiode when supplied with +5V will put out 5.5V or more when the laser ( or other light source ) is brought right up close to the photodiode. As I understand this is normal behaviour for a photodiode. The overall circuit works great, but I have a concern that any voltage above 5V is going to cause damage to the LM311 comparator which has a 5V supply and 0V ground. As a possible solution I incorporated a 4.7V Zener diode around the sensitivy adjust pot ( no laughing in the back row, I'm only a novice! ). This seems to work after a fashion, but I didn't realise that these diodes draw a small amount of current. The result is that its use greatly decreases the sensitivity and upper voltage level that the photodiode can produce due to the fact that the Zener is drawing on what little current the photodiode can pass. I tried removing the Zener and swapping round the photodiode and 1M pot so that the photodiode can 'pull-down' the 5V supply but I found that this would result in a negative input to the comparator of less than -0.5V confused. Again, I am wondering if this can damage the LM311 which has it's ground at 0V. If these kinds of inputs are going to be a problem, any ideas how I can get round this?

Thanks,
Salvatore.

 

Your schematic seems to have the top and bottom of it snipped off.

Could you re-post it?

 

It would be helpful if you can provide us with the complete schematic.

hgmjr

 

LM311 can handle input voltage as high as (2*pos supply)Volts above the negative supply. Since your negative supply is 0V and pos supply is 5V, an input of less than 10 volts should do no harm.

 

Hi,
I have attached the whole circuit diagram. If as you say I can go up to 10V then I don't need to worry..! ;o)

Thanks,
Salvatore.

 

The most immediate issue I see with your circuit is that your POWER ON LED indicator, assuming that the LED is a standard one, will need a series current limiting resistor.

hgmjr

 

OK, try this:
1) Remove the Zener diode, pot, and photodiode from the circuit.
2) Connect one end of the 1 meg pot to +5v, and the other end to ground.
3) Connect the photodiode between the pot's wiper arm and ground.
4) Connect the pot's wiper arm to the input of the LM311.
5) For safety, connect a 10K Ohm resistor between +5V and the 1 meg pot; otherwise if the photodiode is conducting with the pot turned all the way up it will short the power supply.

Now the pot will be able to set the operating point of the photodiode at whatever voltage you wish.

And yes, like hgmjr said, you need a current limiting resistor for the power-on LED. If it's a standard red LED, use 220 Ohms.

Use 0.1uF bypass caps between Vcc and ground on each IC to minimize transients.

It would be a good idea to have separate regulators for your analog and digital devices.

 

Hi,
Both the LEDs have 5V integral current limiting resistor so no need for an external one.. ;o) I'll try the rearrangment of the photodiode and see what happens... Thanks for the input!

Salvatore.

 

From the components used in your frequency-to-voltage converter, it would appear that you are looking for a frequency at the F2V's input on the order several KHz. Also, I see that you have included a divider/counter to permit you to scale the incoming frequency to permit you to handle much higher pulse rates from your lazer light source.

If you run into difficulty with the frequency response of the photodiode, there are a couple of things you can do. If you happen to have response problems operating at the higher frequencies, let us know.

hgmjr

 

Hi hgmjr,
Yes, maximum frequency input to the LM2917 of 2500Hz. The circuit is for a tachometer for a turbocharger, maximum 150,000rpm. I've tested the original photodiode setup up to 30,000rpm (500Hz) using an electric router with no issues. For input up to 2500Hz, I wrote a bit of software to generate pulses through the parallel port to feed the comparator. Again, no problem, the LM2917 responds within the 0.3% design limit ( I believe it is possible to improve on this by adjusting the R1C1 combination to get the accuracy down to 0.1% ). I wanted to use a photodiode as opposed to a phototransistor for the faster response times. In a worst case scenario, with the laser reflecting off the face of a nut for example, the reflected beam will have a very short dwell time on the sensor (a microsecond or less @ 2500 revolutions per second), where as you know a phototransistor won't have time to respond. This is also where the divider comes in, which will divide the six ( or 2 or 8 ) short pulses per revolution into the one pulse per rev for the LM2917. So if the circuit responds ok to a direct input of 2500Hz from the parallel port and the photodiode has more than adequate response times, then in theory the whole circuit should work ok up to the 150,000rpm design limit...

 

Hi guys,
where can i find the NSC LM2917 or LM2907 models for Multisim 10 or Pspice???


Thanks
Simeo