The circuit assembled on a breadboard for test

I made this circuit to support the backlit of my Servo Actuated Keycode Lock project. I wanted to add backlit to the touchpad, but the LEDs should turn on only if the ambient light is not sufficient. The project runs on batteries so the current consumption is critical. The door is located in front of an East window, so there is enough light during day. Only during night the backlit is necessary.

Here is the schematic circuit:

The circuit is based on a Schmitt Trigger. I have publish in the past many circuits based on this brilliant building block, like this Thermostat Circuit and this PC Fan Failure Alarm. If you do not know how the Schmitt Trigger works, i suggest you follow this link to read the theory.

The IC1 (741) along with the resistors R2, R3 and r4 performs the Schmitt Trigger. Instead of a waveform, the input to the 741 comes from a voltage divider, performed by a photocell (PH1) and a potentiometer R1. This potentiometer will eventually set the sensitivity of the circuit.

This is the photocell (LDR) that i use

The output of the Schmitt Trigger is driven through a resistor (R5) to the base of the transistor amplifier. The circuit as-is, may not be able to directly drive a relay with this transistor. If you plan to use this circuit to drive higher loads than LEDs, then you may consider use more amplification stages. I only plan to power some 3-4 LEDs, so the power is enough. Each LED will have its own limiting resistor.

The circuit has a hysteresis that can be adjusted by changing R4. This hysteresis works as follows: The LEDs turn on when a specific amount of light falls on the photocell. The LEDs will remain ON as long as the luminosity is bellow this amount. When the amount of light starts to increase again and goes above this specific point, the LEDs will NOT turn off. Instead, the amount of light must go a little bit higher for the LEDs to turn off. This difference is the hysteresis. The bigger the R4 - the smaller the hysteresis and vice versa.

Any resistor between 100 Ohms and 100 KOhms can be used. But keep in mind that above 40 KOhms, the hysteresis is very small, almost none, so practically there is no meaning to make such a circuit. The point of this circuit is to have this hysteresis. The reason is very simple. Suppose that you use this circuit for security light - the LEDs must turn on when the ambient light is not sufficient. Now suppose that this circuit is outside, and the sun begins to fall. There will be a time, that the amount of light is exactly at the point that the LEDs must turn on. In an ideal world, the LEDs would turn on and remain like this. But this will not happen. Instead, the LEDs will turn on and off every time that the light slightly changes, even for a tiny amount, causing the circuit to oscillate. This is where the hysteresis comes to save the day. When the LEDs turn on, the luminosity will have to change a lot until they turn off again. So, slight light changes will be filtered.

Light falls on the photocell, the LEDs are OFF	My hand drops shadow on the photocell and the LEDs are ON

Resistors
R1	Potentiometer 100 KOhm Linear Rotary 1/2W
R2	Resistor 47 KOhm 1/4 Watt 5% Carbon Film
R3	Resistor 47 KOhm 1/4 Watt 5% Carbon Film
R4	Resistor 22 KOhm 1/4 Watt 5% Carbon Film
R5	Resistor 1 KOhm 1/4 Watt 5% Carbon Film
R6	Resistor 10 Ohm 1/4 Watt 5% Carbon Film
R7	Resistor 10 Ohm 1/4 Watt 5% Carbon Film
R8	Resistor 10 Ohm 1/4 Watt 5% Carbon Film
PH1	LDR 5K-500K Photocell 5x4x2mm 150 mWatt
Transistors
Q1	BC548 Switching and Applications NPN Epitaxial Transistor
Semiconductors
IC1	LM741 Operational Amplifier
LED1-3	LED 3mm white high brightness

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