Making a light detector using a phototransistor and an open drain comparator

Question

I want to make a light detector using a phototransistor. To do that I connected a phototransistor and apotentiometer to the open drain comparator as in the diagram below.

When the light is dim, everything is OK, but when light is bright (normally or even slightly bright) then the LED lights even when voltage of the potentiometer is more than 700mV higher than the voltage of the phototransistor.

Specifically, when I turn the potentiometer knob, the voltage between the voltage source and the comparator output rises gradually until it hits maximum around the point where comparator inputs values are equal and then it remains constant.

So my problem is that comparator doesn't work like a step function, but more like a gradually increasing function.

What am I doing wrong?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

In all practical cases you will want to include some amount of hysteresis. Otherwise, you will experience jitter in the output. And allowing that is always a bad thing. So you must design in some hysteresis behavior. For this, you'll need positive feedback.

Also, the TEPT5700 has only two pins: collector and emitter. The NPN base is floating and responds to photons impinging so as to generate recombination current to allow current from collector to emitter.

Finally, the TEPT5700 specifications focus on \$V_\text{CE}=5\:\text{V}\$. You are using \$V_\text{CE}\le 3.3\:\text{V}\$.

You can achieve this with a simple circuit using two BJTs:

^{simulate this circuit – Schematic created using CircuitLab}

Left side is the circuit with hysteresis. Right side is the circuit that activates an LED on the basis of light input.

You can make some adjustments, if needed, for light level. For example, for a wider hysteresis band you might try setting \$R_2\$ and \$R_3\$ to \$47\:\text{k}\Omega\$ and \$R_1\$ to \$8.1\:\text{k}\Omega\$. Etc. Basically, play around with \$R_2\$ and \$R_3\$ and also, separately, \$R_1\$.

I've intentionally left off a potentiometer for the above circuit. But you can sink unwanted phototransistor current by reducing \$R_1\$, for example. So that's a likely place for a potentiometer. But I'll leave that detail to you.

Answer 2

So my problem is that comparator doesn't work like a step function, but more like gradually increasing function

This sounds like the op-amp gives constantly on/off signal to the led, as the voltage of the phototransistor and the Voltage of the potentiometer are getting really close + due to the parasitic/stray capacitances the comparison of the two is fluctuating.