LDR based Auto LED

[gauravkothari23]

Hi All,

I am trying to built a Night Lamp using LDR which auto lights up the White 2835 3V 4 in series 0.2W LED's when Dark.
The circuit i am using is capacitive power supply.

My problem is when in dark area the LED does not glow to full brightness. it lights up Dim. but when i remove the LDR from the circuit to check, LED glows to full brightness. but the same result or brightness i am not getting when connected with LDR

 

[aliarifat794]

When the LDR is connected, most probably it is not allowing enough current to pass through to the LEDs, resulting in them glowing dimly. Check the resistance values of your LDR in brightness and complete darkness.

 

[D.A.(Tony)Stewart]

what is your line voltage and Hz?

1. 230 Vrms 50 Hz
2. 120 Vrms 60 Hz?

1. or 2.

--- Updated Jul 15, 2024 ---

You can always add 2K2 or so in parallel with the 1k to improve the daytime cutoff. But you won't get 0.2W from this design.

Make sure the LDR does not pick up the LED illumination. That will kill the brightness. Use a collar around it.

 

[KlausST]

Hi,

my wild guess:
* LED light somehow gets to the LDR
* or GND bounce (bad PCB layout) leads to oscillations.

A scope or even a DVM gives the information what happens.

Klaus

 

[betwixt]

I'm surprised it works at all. You have Q1 wired as a shunt across the same supply that provides it's bias current.

I would disconnect the 'ground' end of R2 and move the chain of components R2 to D4 so they connect between the bridge output and the collector of Q1. Also swap R3 and the LDR into each others places, this also protects the transistor and LDR from excessive current when it is brightly illuminated.

Note that in darkness, the voltage across C2 and Q1 could rise to well over 100V, it would be wise to add a Zener diode rated at about 20V across C2 to limit the voltage to a safe level.

Do be careful with this kind of circuit, never touch any part of it and never attach a ground.

Brian.

 

[FvM]

Note that in darkness, the voltage across C2 and Q1 could rise to well over 100V, it would be wise to add a Zener diode rated at about 20V across C2 to limit the voltage to a safe level.

No. LEDs are already limiting the voltage.

The observed effect suggests that LDR has still too low resistance in dark state. Do you have a specification? Did you measure resistance with a multimeter? Possibly LDR has insufficient voltage rating.

 

[gauravkothari23]

I'm surprised it works at all. You have Q1 wired as a shunt across the same supply that provides it's bias current.

I would disconnect the 'ground' end of R2 and move the chain of components R2 to D4 so they connect between the bridge output and the collector of Q1. Also swap R3 and the LDR into each others places, this also protects the transistor and LDR from excessive current when it is brightly illuminated.

Note that in darkness, the voltage across C2 and Q1 could rise to well over 100V, it would be wise to add a Zener diode rated at about 20V across C2 to limit the voltage to a safe level.

Do be careful with this kind of circuit, never touch any part of it and never attach a ground.

Brian.

Yes,
I have even tried the circuit attached.
But the result is the same.
Resistance of LDR in Normal Light is approx 4K and in dark it goes around 500K
Voltage we have is 220V 50Hz
We have also covered the LDR with cover, so that the LED light does not illuminate on LDR