Need help in change 6 volt to 12 volt circuit

[mrel]

Hello
Using this circuit see attach picture of circuit I use to light a 3 volts led the led turn on when no light hit cadmium sulfide photocell.
What need to do to change this 6 volts circuit to 12 circuit using 12 volt led as mention in attach file
mrel

 

[audioguru]

A 2N2907 is a PNP transistor. Your circuit will not work because its LED and battery polarities show that it needs an NPN transistor.
With an NPN transistor, in the dark the photocell is a high resistance so the transistor and LED will be turned off. When there is light then the photocell is a low resistance that turns on the transistor and LED.

To make it work from 12V then re-calculate the resistor values.

To make the LED light when there is no light and turn off when there is light then have the LED powered from its resistor connected to the positive supply and use the transistor to short the LED when there is light (but then the circuit draws current when there is light).

 

[AnalogKid]

Also, a 2N2907 can not handle 1.68 A. For that you will need a medium power transistor. Because the transistor is acting as an amplifier rather than a switch, you also will need a heatsink.

ak

 

[audioguru]

I wondered why the circuit used a 330 ohm resistor to limit the LED current to only 13mA but listed a very high power LED.

 

[mrel]

A 2N2907 is a PNP transistor. Your circuit will not work because its LED and battery polarities show that it needs an NPN transistor.
With an NPN transistor, in the dark the photocell is a high resistance so the transistor and LED will be turned off. When there is light then the photocell is a low resistance that turns on the transistor and LED.

To make it work from 12V then re-calculate the resistor values.

To make the LED light when there is no light and turn off when there is light then have the LED powered from its resistor connected to the positive supply and use the transistor to short the LED when there is light (but then the circuit draws current when there is light).

 

[mrel]

audioguru
The circuit i show in this post i found this circuit on a website the circuit original the circuit had BC547 npn transistor when i build the circuit the sensor will not turn on the led when there was no light on the sensor ,So I change transistor to PNP 2n2907 now the led light up when there is no light on the sensor.
The main purpose i post this circuit look for help to find what i need to change this circuit to make this circiuit run 12 volt instead of 6 volts using the led picture in this post.
mrel

 

[mrel]

Also, a 2N2907 can not handle 1.68 A. For that you will need a medium power transistor. Because the transistor is acting as an amplifier rather than a switch, you also will need a heatsink.

ak

 

[mrel]

AnalogKid
Can you tell me what kind transistor to replace the 2n2907 to make this circuit run the high power led i post picture led with description,to run on 12 volts.
Also you mention i need heat sink, i do have heatsink for led ,do i need any other heatsink?
mrel

 

[mrel]

I wondered why the circuit used a 330 ohm resistor to limit the LED current to only 13mA but listed a very high power LED.

 

[mrel]

audioguru
I found circuit on the internet so i don't know why there 330 resistor.
Only i thing i did to this circuit is change the BC547 to a 2n2907 transistor
mrel

 

[audioguru]

The original circuit used an NPN BC547 little low power transistor to feed about 12mA to the ordinary little red LED. But the circuit does not do what you want.

 

[Willen]

AnalogKid
Can you tell me what kind transistor to replace the 2n2907 to make this circuit run the high power led i post picture led with description,to run on 12 volts.
Also you mention i need heat sink, i do have heatsink for led ,do i need any other heatsink?
mrel

I am not answering, I am suggesting you how to use 'QUOTE' button. Just click on QUOTE button and see the text box (where you write reply) below where you can see some text already written. Go there and see the last word there, you can see a [/QUOTE] there. Just start to write your reply below the word and click reply.

 

[audioguru]

Can you tell me what kind transistor to replace the 2n2907 to make this circuit run the high power led i post picture led with description,to run on 12 volts.

The circuit is designed to turn on an ordinary low power little LED when there is light and turn it off when there is no light. You want the opposite.
The circuit needs to be re-designed to light an LED when there is no light and re-designed to light a high power LED. It probably needs a driver transistor and a power transistor.

 

[Colin]

You cannot use a single transistor and an LBR and a high powered LED and expect the circuit to work.

 

[Les Jones]

mrel,
You need to define EXACTLY what you want. The circuit you posted does not make any sense for the following reasons. 1 The 12 volt high power LED will not work from the 6 volt supply in the circuit. 2 Even if the supply was 12 volts the 330 ohm resistor would only allow a few mA to flow through the LED so it would be very dim. 3 Changing the NPN transistor to a PNP transistor only works by chance. The emitter and collector of the PNP transistor are the wrong way round for the supply polarity. If connected the right way round it would work as an emitter follower so there would be no voltage gain. This would result in very poor switching of LED. In fact the brightness of the LED would vary with the light level. The reason it works at all (Which can only be with a single low power LED. NOT with the LED array you show with you circuit as this will consist of a number of strings of 3 LEDs in series.) is that the transistor is working to some extent even though the emitter and collector are reversed due to the low supply voltage. (Probably less than the reverse vbe rating.)
If as I suspect you want to drive the high power LED you need some means of limiting the current. A resistor is not very good as you only have about 1.5 volts more than the LED array needs so it would not give good current regulation. For the switching I would suggest using a comparator chip (Such as an LM393) driving a power MOSFET.

Les.

 

[mrel]

Les Jones
You ask me i should sayYou need to define EXACTLY what you want.
I am looking for simple circuit to build using 12 volts high power led in circuit using Cadmium Sulfide photocell to turn the Led on when it is dark,the circuit need to use 12 volts to power the Led.
The attach file I post in the original post is for a 6 volts circuit ,I need 12 volts circuit also in the original post I provided specification for the led.
mrel

 

[Les Jones]

The confusion is because it appeared that the circuit and the LED next to it appeared to come from the same article. The circuit was wrong and did not match the type of LED.

Here is a circuit that I think will do what you require.

The value of the fixed resistor and potentiometer will have to be calculated using the data sheet for the LDR. The potentiometer 's value should be about the same as the dark resistance of the LDR. The fixed resistor should limit the current to less than the LDR's current rating. You need to calculate the value of "R" to match the current rating of the LED and it's wattage to match the power dissipated in the resistor. You may have to adjust the value of the resistor slightly from the calculated value to get exactly the current you want through the LED. The left hand side of the circuit does the switching. It shuts down the current regulator when the LDR is illuminated. The right hand side is a current crude regulator to control the LED current. I have not built this but it should work.

Les.

 

[alec_t]

Here's a variation on Les' circuit, using the other half of the 393 (dual comparator IC) instead of the BC337 to provide current regulation, and with hysteresis added to the first comparator so that passing clouds etc don't cause false switching of the light. Trimmers allow adjustment of hysteresis, dark/light threshold and LED current.

The blue graph simulates LDR resistance. For 'kV' read 'kΩ'.
Note that, as with Les's circuit, a small heatsink will probably be needed for the FET (as well as a large heatsink for the LED). A 100uF cap across the supply, as Les shows, would be advisable.

 

[Les Jones]

Hi Alec,
Your circuit is better than mine for a number of reasons. 1 You have added hysteresis. 2 Your constant current circuit uses a more stable reference voltage the the vbe of a transistor that I used. 3 It uses a lower reference voltage which means there is less voltage drop across the sense resistor. This is important in this application as we have very little voltage to spare for the constant current regulator. (12 volt supply and I assume 3 white LED forward voltage drops only leaves about 1.5 volts.) Thanks for mentioning adding a heatsink to the power MOSFET. I forgot that the OP has no electrical knowledge.

Les.

 

[mrel]

The confusion is because it appeared that the circuit and the LED next to it appeared to come from the same article. The circuit was wrong and did not match the type of LED.

Here is a circuit that I think will do what you require.
View attachment 94415
The value of the fixed resistor and potentiometer will have to be calculated using the data sheet for the LDR. The potentiometer 's value should be about the same as the dark resistance of the LDR. The fixed resistor should limit the current to less than the LDR's current rating. You need to calculate the value of "R" to match the current rating of the LED and it's wattage to match the power dissipated in the resistor. You may have to adjust the value of the resistor slightly from the calculated value to get exactly the current you want through the LED. The left hand side of the circuit does the switching. It shuts down the current regulator when the LDR is illuminated. The right hand side is a current crude regulator to control the LED current. I have not built this but it should work.

Les.

Les
The value of the fixed resistor and potentiometer will have to be calculated using the data sheet for the LDR.
(1) As for the Cadmium sulfide photocell using as (LDR) purchase at radio shack the package don't have specification,would not know value of the fixed resistor and potentiometer will have to be calculated.
(2) As for 12 volt led calculated I calculated resistor 10 ohm ,is this right if i built the circuit you post.
(3) I will try attach led specification see the post.
mrel