Light sensor detecting when the lights turns on AND when the light turns off

Question

I would like to create a circuit using photodiode that detects when the lights turn on AND when the lights turn off. The output should look something like that:

I want to create the simplest possible circuit. The precision is not important, only reliability. The minimum distance between voltage spikes will be in order of seconds. The duration of spikes should be about 100ms.

I would be grateful for at least some keywords to google the solution. So far I found only circuits detecting when the light is on.

Thank you

Answer 1

As Chris Stratton said in his comment:

By far the simplest circuit in terms of what you visibly construct would be a 6 or 8 pin MCU

And I fully concur. (The number of unused pins doesn't really matter for complexity, so use whatever MCU you have the easiest access to).

Typical Photodiodes (like this cheap INL-5APD80) have "day" photocurrents in the order of 10 to 100 µA.

^{simulate this circuit – Schematic created using CircuitLab}

Your freedom lies in picking R1; the higher R1, the less light is enough to make V_meas high.

Note that your MCU allows you to be flexible there, if you configure it to attach its ADC to the GPIO1 pin. Then, you do things like "if it's been really dark for a long time, we increase our sensitivity", because you're no longer triggering based on a fixed voltage level as if you were using GPIO1 as digital input.

The upside of using GPIO1 as digital input might be (depending on the MCU you pick) that you can send the MCU into the deepest of sleeps, and use GPIO1 changing state to trigger a wakeup, making this a relatively power-efficient circuit.

However, small problem here: Microcontroller (like this cheap STM32F0) have inputs that don't have infinite input impedance. (no problem here, the leakage current on datasheet page 72 is specified to be below 0.2 µA, so if your photocurrent is, say, 40 µA, you're pretty safe.)
If you need to sense things quickly on an ADC input, this can become a problem, and you'd need to add a voltage buffer (typically, made out of an opamp) to ensure things keep working; for really fast sensing, there's little way around more advanced amplifier designs (TIAs), but really, this is not your use case. You don't want to receive megabits per second with your photodiode.

---

Should you really want to go with Fredled's discrete logic approach, you can do it with a single XOR gate, and a delay.

^{simulate this circuit}

XOR's output is high exactly iff one input is high and one is low.

You can adjust how long that is by twiddling the time constant of your low-pass filter (in green box).

Answer 2

Something I did a few years ago.

Everything to the left of U1D is deleted, replaced by the output of the light sensor circuit connecting to pins 11 and 12. The nice thing about the Schmitt input is that the input signal can change fairly slowly, and the circuit still will make a clean output signal as long as the input is monotonic. There is no need for diodes to help the timing capacitors recover because the output period is so much shorter than the input period.

For 100 ms (-ish) pulses:

C1 = C2 - 0.1 uF

R1 = R2 = 1.2 M

Answer 3

I designed what you need 2 weeks ago so you are lucky. Because i don't have currently a computer you need a current change detection circuit which can be done really easily with a current source(variable) 2 resistors, 1 opamp and 1 capacitor. You connect 1 resistor in series with the capacitor and you put the entire thing parallel to the 2nd resistor. Let's call this part of the circuit A. You connect A in series with the current source and create a closed loop putting somewhere a GND as well. The 2 ends of the resistor which is in series with the capacitor you connect them to the inverting and non inverting input of the opamp. And here you go!

Answer 4

A photodiode will transmit a signal when there is light, stop transmitting when there is none. To convert this on/off signal, you can try the following circuit:

SHT1 is a schmitt trigger used to create a signal equivalent to the inverter schmitt trigger INV1. (It can be omitted if the Photo diode PHOTD1 already generate a signal equivalent). When PHOTD1 output is high, SHT1 turns high and INV1 turns low. C1 let a positive signal pass to the input of SHT2 for a short duration. Inversely, when PHOTD1 is low, INV1 turns high and SHT1 turns low and C2 let a short signal go to SHT2.

D1 and D2 prevent negative current to appear at the output of SHT1 and INV1 (this can damage the ic's). D3 and D4 prevent opposing signals to arrive at SHT2. R1 pulls the input of SHT2 low when no current go through neither D3 nor D4.

^{simulate this circuit – Schematic created using CircuitLab}

Another circuit proposed by Olin Lathrop here I would just add a schmitt trigger between the RC circuit (R1 + C1) and the input pin 2 of the XOR gate (IC1A).

Chose which one is the best.