# Maximum output voltage from photo sensor to detect high intensity pulse of light

I'm trying to work with the OPL550 photologic sensor http://optekinc.com/datasheets/opl560.pdf (Sense a pulse of light around 8kW/m^2 in intensity and around 100us in duration to trigger data acquisition.)

I believe I'm using the OPL550 totem-pole output. I'm reading all three transistors to be switches. If switch 1 is on, 2 will be on and 3 off. Thus Vout=Vcc. If switch 1 is off, 2 will be off and 3 on, Vout=GND.
It says that the maximum input voltage is 10V, but the maximum voltage output is 35V. Looking at the circuit, I think that max output is always limited to Vcc. Applying 3.3v and 1mA source causes a voltage drop of around 2V from my DC power supply.
Can someone point out if I'm wrong in my analysis, and perhaps what is going on? Thanks.

• As a point of interest are you fully certain of the energy contained in your light pulse? It's easily possible, but quite a lot. In case of a very well collimated LASER it'd still be quite strong to achieve that amount of power over the whole detector chip surface. Sep 23, 2015 at 14:53
• Also, why did you edit it back? the back-tick ` creates a code block between your "I'm"s that makes it very hard to read. -- Never mind, looked at the history, turns out we were simultaneously editing. Sep 23, 2015 at 14:54

Your analysis is basically correct.

The output will actively drive either voltage.

What is confusing you is that the 35V refers only to the types specified in that section, which are all -OC types, or, open-collector, which only actively pull towards ground.

So their output can be pulled all the way to 35V with a resistor and then the output transistor will pull that to ground when it is turned on, but nothing actually drives the output from the OP itself.

For your type, with totem pole, it actually doesn't get mentioned in that section that says 35V.

### EDIT

As well as that as Marko points out, your device has different maximum supply voltages, although all start at 4.5V, the OC types will go to 16V, and your totem pole 550 type will only go up to 5.5V, so it is really a pure TTL-level device.

35V is for models with open collector output, only. 3.3V is to little for supplying since in datasheet says 4.5 to 5.5V @ 8mA to 15mA power consuption.

• Yeah try a 5V supply, and if your numbers are right you are much above the maximum light level for max output. If it still doesn't work with higher supply voltage perhaps try attenuating the light a little. Sep 23, 2015 at 15:10

Let me try to do this systematically. The data sheet is somewhat confusing.

I'm reading all three transistors to be switches. If switch 1 is on, 2 will be on and 3 off. Thus Vout=Vcc. If switch 1 is off, 2 will be off and 3 on, Vout=GND.

Applying 3.3v and 1mA source causes a voltage drop of around 2V from my DC power supply.

Looking at the circuit, I think that max output is always limited to Vcc.

Close enough, but not quite. If Q1 is off, Q3 will be off, and Q2 will be on - sort of. Q2 will get its base drive from Vcc, but the transistor cannot turn on really hard. So you should figure on an output in the range of Vcc minus 1 to 2 volts, depending on the current being supplied. And this, I think, is consistent with your providing 3.3 Vcc and getting 2 volts out, although your description is not clear.

Note that the data sheet says that the 550 "will drive up to 8 TTL loads". TTL input levels are not the same as CMOS, and a low TTL input is 0.8 v, while a high input is 1.6 volts. So a driver intended for TTL does not worry a lot about the high output voltage.

It says that the maximum input voltage is 10V, but the maximum voltage output is 35V.

It sure does, but you need to learn how to read data sheets. These numbers come from page 4 of the data sheet, and this is labeled "Absolute Maximum Ratings". The numbers on this page should not (must not) be used for operation. They simply give limits that the manufacturer says the chip will survive. So, for instance, if you apply a Vcc of 10 volts to your 550, all this says is that you won't cause the chip to explode, and if you then turn the voltage down to something more reasonable, like the 4.5 to 5.5 listed on page 5, the chip should still work.

Likewise, the 35 volt output limit has a note attached - see the little (4)? Looking down at the bottom you'll see that (4) says that 35 volts can only be applied directly to open-collector types, and the 550 is not one of those. If the output might somehow be attached to a higher voltage, you need to put a resistor between the two to limit the current that the output has to handle.

The upshot of this is that you should provide 5 volts to the 550. At 3.3 it might work, and it might not, and it might sorta, kinda work. Page 5 says that the 550 should be operated in the range of 4.5 to 5.5 volts, and if you run it with ripple of less than 2 volts (that is, within the range of 4 to 6 volts) it won't false-trigger on the ripple. More than that, and it still might not false-trigger, but it might, and you've been warned. Running outside the 4.5 to 5.5 range is not guaranteed to cause problems, but the chip is not guaranteed to work to the specifications provided, either. You can do it, and it will probably work in the sense of detecting some light level shifts, but if it's slower than you expect, or less sensitive, you have no ground for complaints, so don't go whining to the manufacturer or bad-mouthing the part on internet forums.

Finally, I hope you're aware that this thing is grossly oversensitive for what you want. From page 5, the Positive-Going Threshold Irradiance (light trigger level, in plain English) is in the range of 0.25 to 1.4 mW/$cm^2$. Multiplying by 10,000 gives 2.5 to 14 watts per square meter, about 1000 times less than you've specified. This may or may not cause false triggers (I don't know enough about your system), but you might consider putting an optical attenuator on the sensor to reduce sensitivity.

• Thank you for the detailed answer Beast, it really helped me understand what I was getting. Yes, I assumed the Vout max was listed for every component, I'll keep in mind to read datasheets more carefully now on. On the bright side, the circuit is now working under overloaded irradiance just fine because I'm not concerned with sensitivity as just signalling a start.
– Sam
Sep 24, 2015 at 22:29
• @Sams - glad to help. My comments about the light level are just to suggest that you think about your dark level. Is it possible that stray light might get into the system and cause a false trigger? If so, you can easily put a x100 attenuation filter in front of the sensor and still reliably detect your ON condition. Sep 25, 2015 at 0:44