Night LED Lamp with PIR Sensor - Guide to calculation parameters

Question

I would like to make some LED lamps for night use, therefore at low intensity, just enough to see where you are walking, but almost without waking up. ;-)
It should only turn on when it detects movement and turn off after a certain time.
It must work at low DC voltage (because it is common to have small unused power supplies or easy to build a battery pack).

The basic circuit:
Source: https://www.instructables.com/LED-Light-With-PIR-Motion-Sensor-Soft-Onoff-and-3y/

Among the many circuits available on the net I chose this one, for a few reasons.
Employs a common PIR sensor, requires few components and appears set to not consume more than necessary. Not least and unlike many other similar circuits, it includes an RC (Time Constant) filter for soft-on, soft-off functionality, which I'd like to keep.

Unfortunately, the author does not explain in detail all the calculations made.
Simply I'm not experienced enough to do without it.

So I would like a small guide that highlights the right procedure, formulas and calculations to define the value of the components, for a specific LED array. How to adapt the circuit to a given load avoiding waste of current, and perhaps how to verify that it works in the best conditions.

The first revision of the basic circuit, should have:

Basic parameters:

• HC-SR501 PIR sensor for motion detection.
• Use of standard 5 mm. LEDs (see the table), single or LED array (always of exactly the same type of LED)
• Regulated DC power supply between 5 and 12V (15V max.), chosen according to the number of LEDs used.
• Soft-on, soft-off function (RC Time Constant filter).

Notes and Questions:

• The Author used the BJTs he had (2N2222A). The NPN transistors I have are: BC547B/C, BC548B/C (all, max 100 mA), PN2222A (500 mA), MPS2222AG (600 mA) and SS8050 (1.5 A).
  Among them, which ones would be the more suited for this circuit?
• Since in a Darlington pair it's the second transistor (TR2) that does the hardest work, is it better to use different transistors or to keep an identical pair anyway, perhaps to simplify the calculations?
• My multimeter can measure the hfe of NPN and PNP transistors. I guess this can help.

Circuit Rev. 2

Thinking about a second revision, I would like to know how to add an LDR photoresistor to detect ambient light and activate the circuit only below a certain threshold. I've seen many examples, but I don't know how to integrate it into this specific circuit.

Thank you in advance.

Answer 1

I don't think battery power is very suitable for this circuit.

Just enough light to see- you may not need a Darlington at all, in fact you may be able to drive the LED directly from the module. There is apparently a 1K resistor in series (check that) and try just an LED to ground. A few mA into a modern bright LED produces quite a bit of light in a dark room. If you need more, a single transistor and a series resistor on the LED should be fine.

As far as using an LDR, if you refer to the datasheet for the Shenzhen Silvan BISS0001 chip used in these modules you will see the suggested circuit:

R2 is the LDR, R1 sets the threshold resistance. The light is disabled if the voltage at pin 9 is less than approximately 0.2Vcc. The modules appear to have pads for the LDR and a 1M resistor for R1, so you can locate the pads and add the LDR if it is not present. Check that the 1M resistor is present.

There are some English datasheets for the chip, but many of them seem to be incomplete.

^{simulate this circuit – Schematic created using CircuitLab}

Any of those transistors you mention will work in this circuit. I made the LED current around 13mA and the base current about 1/20 of that.