How to get better phototransistor sensitivity from laser light?


First time poster here, please be indulgent.
I’m trying to replicate an analog object detection sensor by using a line laser (650nm) and analog readings from an array of phototransistors (using a MCP3008 SPI ADC connected to an esp32). I chose this particular phototransistor since its peak wavelength sensitivity curve matched approximatively my laser wavelength ([])

The phototransistor is used in this configuration

I found that its responsiveness is better with the lowest pull-down resistor possible(0 ohms) while having a vcc of 5V and the line laser directly shining at it (half an inch away). The max analog value (on a 10bit scale) I got is only 2.1 V, while the lowest is only 1.8V ish. Ideally, i would like to use the full range of 5V (direct shine) to 0V (full object blocking).

I’ve looked at this previous thread, where @APDahlen used another model with 2 transistors acting as a darlington pair. Is this to get a gain on the signal ? If so, how can I get the same results on a tight budget and with a analog values. My array consists of 8 phototransistors and this would mean 16 transistors per module, and that would be pricey considering the sensor is only a subsystem.

I’ve looked at photoresistors, but I would prefer phototransistors which are RoHS compliant.

Any recommendations or insights would be helpful, since I do have a strong mechanical background, but I’m still learning electronics along the way.

Thank you,


Unfortunately I can’t seem to find the documentation from an analog light level based measuring system I designed for a patented infinite resolution non-contact rotational sensor back in the early 90s (it became obsolete around 2002).

I do seem to recall that photodiode short circuit current measurements where vastly superior for this and where used by the aerospace and defense industries for laser guided tracking systems. It did require some very high gain amplifiers to make small photodiode current changes usable, but it had the best range, linearity, lowest errors due to ambient temperature, and lowest manufacturing cost.

Most of my education on photodiodes for light level applications came from the custom photodiode manufacturer, and other manufacturers, white papers and application notes.

Sorry i can’t remember more, or find any useful old documentation (it’s probably all paper in storage boxes in the back of the warehouse or got shredded).

If that’s not a typo, it’s a solid indication that something’s either broken or incorrectly configured; a resistance of zero in the circuit above should yield a zero voltage output regardless of incident light.

Given a high enough load resistance and suitable inputs, that circuit should have no trouble generating outputs spanning most of a 5v supply range. Chances are fair that you’ll see something like that result once you sort out why on earth you appear to be seeing ~2v across a short circuit.

Hi @rick_1976,

My apologies, I meant that I simply removed the pull-down resistance altogether, leaving the phototransistor directly connected to the MCP3008 adc channel input like pictured below :

Ok, in that case the question kinda does a 180 and one might wonder why they’re only reading half-scale when dumping current onto a high-impedance ADC pin with no obvious place for it to go…

1000 lux is an illuminance comparable to that found at a decent workbench, and likely far less than what’s imposed by a laser pointer point-blank. Per the datasheet, your transistor oughta be conducting upwards of a mA under those conditions, which should be more than enough to draw an unburdened ADC pin to within saturation of the 5V supply… Have you measured the analog signal directly?


I just did a quick analog bench test with a multimeter, and in fact it appears that I need to increase the pull-down resistor quite a bit before getting substantial voltage delta values.

Here is my janky setup :

The fiber optic cable reduce quite a bit the perceived brightness and thus it looks like I need to increase my pull down resistor value. Right now sitting at 3MΩ, I can see a more substantial drop. If my understanding is correct the phototransistor and the pull-down act as a voltage divider ?

From my previous mistake, I noticed a bigger delta when removing the pull-down, is this logical ?
Does removing the pull-down make it of “infinitely” large value ?

If someone could correct me or confirm my intuition that would be great. Otherwise I’ll keep trying higher values for now, it look like it could potentially work OK for my application.
Also, does increasing the resistance value of the pull-down have an effect on the phototransistor reaction time ?

All ideas welcome,


It may be useful to conceptualize the transistor as a current source instead, but the end result looks a lot like a divider… Six of one, half dozen of the other…

An empty space where a resistor isn’t may indeed be thought an “infinite” resistance for many practical purposes. If, as indicated previously, you’re only able to achieve a 0.3v change across that, it’s an indication that your current source (transistor) isn’t doing much at all.

A simple reading of the datasheet indicates that phototransistor should pass 0.1uA (at most) in the dark and at least 100uA under modest light, with a minimum 0.4v drop across the device to be expected.

Stated differently, you shouldn’t be having much trouble getting a signal that varies between 0 and 4.5v or so across resistors as low as 50K.

Have you double-checked the polarity of the transistor in the circuit?

1 Like

Well, a quick look into the datasheet and it gives me this :

Is it odd that I tought Collector is commonly the longer pin ?
Anyways, in the datasheet it says that collector is the shorter pin (which I wrongly assumed at this point).

Results speak for themselves with a 47k resistor now.

That will teach me a valuable lesson haha.

Thank you for the valuable information and your time!