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can i use LED on collector?

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Hesam Kamalan

New Member
Hi,
i want to build a dancing light with micro, LED and transistor. is this schematic correct?


i want to increase the LEDs. is this schematic correct, too?
 

bananasiong

New Member
Hi,
Both the schematics are correct. The first one turns the LEDs on when the base is driven low while the second one turns the LEDs on when the base is driven high, i.e. when the transistor is turned on. You can increase the number of LED, provided that the voltage drop of the LEDs are not higher or equal than the supply voltage. The supply voltage must be higher than the total series voltage drop of the LED.
What color of LED are you using? Different color LED has different voltage drop.
You can put them in parallel too. Something like, a few parallel of series pairs.

*EDIT: The load is on the collector usually, so the answer is yes ;)
 
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Hesam Kamalan

New Member
thank you bananasiong,
i tested that schematics. but when i used two LEDs (serial or parallel), the intensity of LEDs was low. so i can't use some LEDs in serial or parallel.

another question:
in my project, i have used PIC16F84 and 12 LEDs. colors of LEDs are various such as blue, green or red (for example 6 blue, 4 red, 2 green). for estability of voltag i've used regulator 7805.
my question is, what do you prefer about connecting LEDs to forward of regulator (12LED * 30mA = 360mA) or backward of regulator (that i have 14V)?
 

bananasiong

New Member
Hi,
For series connection, assume the voltage drop for each LED is 1.8 V. So 3.6 V for 2 LEDs. If the supply voltage is 5 V, the voltage across the resistor is 1.4 V. Ohms law, 1.4 V/330:eek:hm: = 4.24 mA. The LEDs are not expected to be very bright with this current. Try reducing the resistance, but do calculation for not higher than 30 mA. Also, remember to include the base resistor while driving the transistor via the PIC.

I don't get these:
another question:
in my project, i have used PIC16F84 and 12 LEDs. colors of LEDs are various such as blue, green or red (for example 6 blue, 4 red, 2 green). for estability of voltag i've used regulator 7805.
my question is, what do you prefer about connecting LEDs to forward of regulator (12LED * 30mA = 360mA) or backward of regulator (that i have 14V)?
 

Hesam Kamalan

New Member
schematic of my project is:



i have used 12 LEDs that connects to each pin of PIC seperatly.
you think that wich of thees is better?
1) as you see in the schematic picture i connect anode of LEDs to VCC (5 volt) or node2.

2) or i connect anode of LEDs to 12 volt (node1)
if i connect anode of each LEDs to 12 volt and voltag drop of 1.8v, the voltage across the resistor is 10.2v. so 10.2/330=30mA

i think if i connect each colector to 12v the intensity and brightnes are higer. what you think so?
 

bananasiong

New Member
Hi,
For driving only the LED, transistor is not needed. The PIC is capable of sourcing 25 mA on each output pin. Just connect the LED directly via a resistor.
 

matk95

Member
No Becuse the transistors are still coneted to the regulator output and this will use the same if not more curent than directly powering them from the pic. and the regulator will not get hot
 
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audioguru

Well-Known Member
Most Helpful Member
The PIC will burn out in your original circuit because the max allowed current from a PIC output is 25mA but without current-limiting resistors the current into the base of each transistor will try to be 60mA and the PIC will overheat.

With an output current into an LED of 25mA an output from a PIC will have about a 1V loss.
 
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bananasiong

New Member
Hi,
The maximum allowed current for each pin of the PIC is 25 mA, but according to the datasheet, if the whole PORT is used for sourcing or sinking, 200 mA is shared by 2 PORTs, so average each pin sources or sinks around 12.5 mA. That is still enough for the LED.
 

Mikebits

Well-Known Member
I thought AudioGuru suggested you put current limit resistors on the bases of your transistors. Maybe I was wrong.
 

audioguru

Well-Known Member
Most Helpful Member
If you don't use current-limiting resistors in series with the base of each transistor (the transistors are not needed) then the micro-controller will burn up and die.
 

Mikebits

Well-Known Member
It's not a 'suggestion' it's a necessity :D

My point was, if the OP is going to ask for suggestions, then he should take them. I think it has been mentioned more than once that the transistor is not even needed, yet the transistor is still there and without the suggested base I limit.
 

Sceadwian

Banned
The transistor is needed if enough LEDS are used, his schematic shows 11 LEDS, unless they're multiplexed the pass transistors are required, with the HFE of most common transistors the current hardly needs to be even 1ma to saturate.
 

Mikebits

Well-Known Member
The PIC I/O pins are capable of delivering 25ma per pin. Since this is a dancing LED circuit, it is unlikely all led's will be lit at the same time for very long if ever. So it seems to me the transistors are not needed.
 
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