LDR working the opposite way

Thread Starter

zazas321

Joined Nov 29, 2015
432
So im just testing LDR. I came up with the simplest circuit i could find. I tryed to make it but LED seems to be working the opposite way.. It lights when there are some light in my room, but when i turn off the light, the led also goes off
 

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dl324

Joined Mar 30, 2015
9,332
Swap the position of the LDR and pot.

Don't bother posting pictures of your breadboard; they're essentially useless because there isn't enough detail. Assuming someone would be inclined to trace it out.

Post your schematic right side up to make it easier to read, and draw the transistor symbol correctly.

Don't bother posting videos of how LDRs work or how to use them. I don't watch those videos because most are a complete waste of time.
 

Thread Starter

zazas321

Joined Nov 29, 2015
432
Yooo.. i have built my dark sensor but strange thing happens when i wire some leds in parallel. One LED worked fine to dark sensor but when i add few more LEDs to the circuit some of them lights up , some of them wont light up at all and others light less brighter than other ones.. Why such a mess happens? ;/
 

dl324

Joined Mar 30, 2015
9,332
strange thing happens when i wire some leds in parallel. One LED worked fine to dark sensor but when i add few more LEDs to the circuit some of them lights up , some of them wont light up at all and others light less brighter than other ones.. Why such a mess happens? ;/
Post a schematic.

You shouldn't operate LEDs in parallel unless each has it's own current limiting resistor. If you actually placed several LEDs in parallel using the same current limit resistor, then some are hogging current.
 

Thread Starter

zazas321

Joined Nov 29, 2015
432
Hm... I have just tryed to put for example 5 same green LEDS in parallel and they light up all nicely, but when i change one green light to a red one and i have 1 red and 4 green, the one red glows brightly and the other 4 green LEDs glows very very bad:eek: Is this because their voltage differs?
 

dl324

Joined Mar 30, 2015
9,332
Hm... I have just tryed to put for example 5 same green LEDS in parallel and they light up all nicely, but when i change one green light to a red one and i have 1 red and 4 green, the one red glows brightly and the other 4 green LEDs glows very very bad:eek: Is this because their voltage differs?
Even with the same color LED, you can get different brightnesses due to current hogging. LEDs of different colors will likely have different forward voltages, which will also result in current hogging.

The correct way to do this is to have each LED, or chain of LEDs, have a current limiting resistor that can also be used to match brightness.
 

AnalogKid

Joined Aug 1, 2013
8,229
Hm... I have just tryed to put for example 5 same green LEDS in parallel and they light up all nicely, but when i change one green light to a red one and i have 1 red and 4 green, the one red glows brightly and the other 4 green LEDs glows very very bad:eek: Is this because their voltage differs?
Yes. In very round numbers, small red LEDs have a forward voltage (Vf) of around 1.8 V, while similar green LEDs come in at around 2.1 V. When connected directly in parallel, the RED LED comes on and holds the voltage across the green LEDs at a level that is too low for full brightness.

ak
 

Thread Starter

zazas321

Joined Nov 29, 2015
432
Okayy! sooo to sum up everything! If i understood good , for example if i add 50 LEDs to the parralel they will all have 4,5V and to light up brightly i will need to calculate the right current for them, in this case 20mA * 50 will be 1A. So i have AC to DC adapter 4,5V which can give 1A so thats exactly what i need? :) And if i want to add different colours LEDs in parralel circuit i will need to add resistors to all of them? :eek:
 

Thread Starter

zazas321

Joined Nov 29, 2015
432
Yes. In very round numbers, small red LEDs have a forward voltage (Vf) of around 1.8 V, while similar green LEDs come in at around 2.1 V. When connected directly in parallel, the RED LED comes on and holds the voltage across the green LEDs at a level that is too low for full brightness.

ak
Hmmm i thought in this case the current matters:eek: I mean if i power my circuit with 4,5V battery so every LED in parallel will get 4,5Volts and with this voltage is more than enough to light any colour LED brightly.
 

dl324

Joined Mar 30, 2015
9,332
LEDs are typically operated at 20mA or less unless they're multiplexed.

If you assume a 2V drop for the LED, 680 ohms and 4.5V would give you about 4mA (3.7mA from I=2.5V/680 ohms). To get the actual current, measure the voltage and use Ohm's Law.
 

crutschow

Joined Mar 14, 2008
23,802
The chart shows the voltage versus current for that particular LED.
Normally you pick a current and then use the LED voltage at that current to calculate the proper series resistor needed to get a resistor voltage drop for that current with the supply voltage you have.

You apparently do not fully understand series and parallel circuits.

Two diodes in parallel will share the same voltage, so the LED with the smaller voltage will not allow the LED with the higher voltage to glow brightly since it has only the smaller voltage across it.

Two LEDs in series will share the same current, but will have a voltage drop across each of them, as shown from their data sheets at that current (which will be different for different LEDs).
The total voltage drop is then the sum of those two LED voltages.
 
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