Battery Saving LED Circuit

[dmoore210]

I am attempting using a Light Dependent Resistor in a circuit with a battery. Something very simple where in daylight the current will go through the resistor and bypass the LED - LED OFF. And in darkness the current will go through the LED - LED ON.

My question is, during daytime that will be a decent amount of current going through the battery and killing it quickly. Is there a way to construct this circuit and only have current going through this circuit during darkness and preserving the battery as long as possible?

 

[Diver300]

You can use a transistor. You have a resistor that turns on the transistor. The current in the resistor can be a lot less than the current in the LED. In daylight, the Light Dependent Resistor (LDR) bypasses the transistor, so the resistor current goes through the LDR instead of the transistor. That causes the transistor, and therefore the LED to turn off.

View attachment 61533

 

[agusirawan]

I ever tried the circuit like above with no luck, except
the vr change to potensio. and change the resistor value.
if not, the led continues on, or continues off.

 

[Blueteeth]

You could use a comparator which will give a pretty sharp 'on/off' threshold. With hysteresis to stop is oscillating states at the threshold. The LM393 as far as I recall, uses very little power and depending on the resistor values and battery voltage - its quiescent current draw should be quite low.

A possible alternative to an LDR would be a photodiode, or perhaps even an LED as a light sensor. Both can produce small votlages based on the incident light, at very low currents - enough to trip a comparator, but low enough to use uA when in the 'off' state.

 

[agusirawan]

I will try to expriment again. But I have only lm741. hope not many different with 393.
My goal actually, long life battery when light about 3 months (when the led off). So the led still
can on when dark about 1 hour. So we can switch to another standard emergency lamp.

I dont satisfied with standard emergency lamp that must be on/off manually, or
must trickle charge battery and auto on when no light. Fast broken.

Thanks for ur suggestions.

 

[PCBWING]

Maybe you could use the solar battery to replace the Light Dependent Resistor and then select the appropriate charge and discharge module.

 

[alec_t]

What is the dark resistance of your LDR? If very high then a CMOS Schmitt gate / MOSFET circuit might be useful to conserve power.

 

[agusirawan]

I try to answer from the last exp,

Basic circuit like above
R10 change to 1K
VR change to VR1K As potensio, the 3rd leg to ground
LDR when light on in house - 10K
LDR when black shade by finger close - 90K
Trans BC547.
Battery 5.5v 2000mah rechargable in full.

The circuit run perfectly with adjusted pot,
but the battery drain < 24hour.

-------- another try ----
When use R10K fixed the BC547 break out in 10seconds.

--- another try ---
When use vr as vr on two legs. The lamp continue on. cannot off.

 

[agusirawan]

Forget...
--- another try ---
change r10K to 30 K (base on 10rmin*90Rmax*^1/2 = 30K)
BC547 break out too.

 

[alec_t]

You could try this.
Day-time current drawn is only ~ 36μA (probably less than the battery self-discharge current), so the circuit doesn't need an 'off' switch. Night -time current is ~16mA for the component values shown.

 

[agusirawan]

alec_t thanks for your circuit.

will try in next holidays. maybe i need to change u2, because
not popular in my country. search dsheet u2 first.

i like your circuit with 4.8-5.5v.
really need this because my area often lights off.

 

[alec_t]

If it helps, U2 can be any IC having a Schmitt-trigger input and inverting function, providing it can run from the supply voltage you are using.