Blinking LED circuit diagram based on LDR and resistor

This is perhaps the simplest LED flash, not relying on any semiconductors. This LED flash circuit utilizes common passive components such as a few resistors, a capacitor, and an LDR. This means that this LED flash does not rely on transistors or ICs to produce the flashing effect.

Circuit description

The LED-LDR combination is used to work like an amplifier. As the current through the LED increases, the resistance of the LDR decreases, causing the voltage on the right side of the circuit (the output) to increase.

As long as the voltage at the capacitor's "output" continues to rise, the LED will get positive feedback from the capacitor's charging current.

When the output voltage reaches a level almost equal to the supply voltage, charging of the capacitor is inhibited and the LED gradually decays.

As a result, the output voltage decreases and the LDR resistance increases. The LED eventually turns off completely by charging the capacitor.

The capacitor is discharged by the left resistor. This feeds some current into the LED, gradually causing the LED to become slightly forward biased again. As the process is repeated, the LED brightness increases.

The oscillator used in this LED/LDR flash circuit is similar to a relaxation oscillator.

Use high-intensity LEDs and adhere them face-to-face on the LDR within a distance of 5mm. This is necessary to provide enhanced gain to initiate the shimmer effect. The red LED selected in this setup is a standard high brightness type LED with a brightness specification of 3700 mcd @20mA.

As you can see in the GIF at the top, complete darkness is not necessary to initiate the flashing process. The flashing process continues to occur even in semi-dark environments.

However, if you increase the ambient light, the flickering will stop.

This is because, under bright ambient light conditions, the LDR resistance becomes very low, causing the oscillation to cease and the flickering effect to cease.

Parts List

Resistance is 1/4 Watt 5%

33 K - 1 No

15 K - 1 No

LDR - 1 No, any standard type shown in figure

LED - 1 No, any 20 mA bright LED

capacitor

22 μF/25 V Electrolytic - 1 Normally Open

power supply

The LED/LDR blinking circuit operates from a 3V supply, either from a small AC to DC 3 V adapter or from a few AAA 1.5 V batteries in series.