How to select a transistor for 1W LEDs on a 555 circuit

[eester B]

Hi, I have a 555 timer 'one-shot' circuit running off 12V, which lights up 2x 1W UV LEDs (350mA, 3.4V). My understanding is that the 555 only puts out 200mA, so I would need transistors to get full potential from the LEDs.
I'm looking at transistor calculators and reading up on it but the math/theory is beyond me at this point without some humanoid intervention.
How do I select a transistor / what transistor should I be using for this application?
Your input is greatly appreciated.
EB

 

[ronv]

A small switching transistor should do it. Look at a 2N2222 it can switch up to 800 ma. at 30 volts.
Since you have 12 volts to work with you can put the 2 LEDs in series for a total of 6.8 volts. You then need a resistor to drop the remaining 5.2 volts at .35 amps. Since 350 ma is probably the maximum spec. lets use 300 ma. so a resistor of 17 would be needed. Probably can't find a 17 ohm resistor, but 18 ohm is fairly common. The power would be I squared R or about 1.6 watts, so something like a 3 or 5 watt resistor.
So now we know the collector current (300 ma.) we can pick the base current that will saturate the transistor (30 ma) The base emitter drop is about .65 volts so the base resistor would be about 11.3/.030 or 376 ohms. use 390.

 

[eester B]

A small switching transistor should do it. Look at a 2N2222 it can switch up to 800 ma. at 30 volts.
Since you have 12 volts to work with you can put the 2 LEDs in series for a total of 6.8 volts. You then need a resistor to drop the remaining 5.2 volts at .35 amps. Since 350 ma is probably the maximum spec. lets use 300 ma. so a resistor of 17 would be needed. Probably can't find a 17 ohm resistor, but 18 ohm is fairly common. The power would be I squared R or about 1.6 watts, so something like a 3 or 5 watt resistor.
So now we know the collector current (300 ma.) we can pick the base current that will saturate the transistor (30 ma) The base emitter drop is about .65 volts so the base resistor would be about 11.3/.030 or 376 ohms. use 390.

That is just the explanation I needed (and to understand it myself for next time), thank you very much. Now, do I put the LED/LED resistor/(+) connection on the collector side, and the ground connection on the emitter side, or vice versa?

Thanks again
EB

 

[eester B]

I hooked up the circuit using 17.1 ohm (a 5W 12ohm and a 5W 5.1ohm resistor in series) between pin 3 and the transistor base, a 2W 390ohm resistor and the two LEDs between (+) and the collector. Emitter goes to ground. Hooked up like this, the LEDs are noticeably dimmer than when I had the LEDs hooked up in parallel to pin 3 using 1/2W 27ohm resistors. ((12-3.4V)/.35a = 25ohm). Is my transistor backwards or something? Results are opposite of what I was expecting.
Thanks again
EB

 

[alec_t]

Is my transistor backwards or something?

You've got the resistors swapped. The 390Ω should go between pin3 and base; the 17.1 should go between (+) and the LEDs.

 

[eester B]

Yes! Thank you!

 

[eester B]

Thanks for the help.
Now, I am unclear how to make my hall effect sensor trigger the 555 timer now.
Output of the hall sensor goes low when triggered, but the 555 trigger wants high voltage, so?
Do I connect the output of the sensor to the base of a PNP transistor (and transistor collector to (-) and emitter to 555)? Which transistor/how do I know which transistor?

I previously faked it somehow by connecting the sensor output to the 555 trigger pin, as well as to (+) through a 500K resistor. It worked for a bit but I think it fried either the sensor or the timer.

Thanks again! I'll upload videos of this when its done!
EB
Honeywell hall effect sensor:
785-SR13C-A1

 

[alec_t]

Output of the hall sensor goes low when triggered, but the 555 trigger wants high voltage, so?

The 555 trigger input needs to be taken low to start the timer.

 

[eester B]

The 555 trigger input needs to be taken low to start the timer.

Hmm, let me redefine the question since I don't think my definitions are correct (I don't really get what 'taken low' means).

The output pin of the hall effect timer connects to V+. The trigger pin of the 555 timer connects (through the trigger) to ground. Connecting the hall timer output pin to the trigger pin of the 555 timer does not trigger the timer circuit. How do I rectify this?

More simply, how do I make the hall effect sensor set off the trigger pin of the 555 timer?

Thank you
EB

 

[ronv]

It might be time to see a schematic.....

The 555 will trigger when the sensor goes from high to low. But we need to know what voltages everything is hooked to.

 

[eester B]

Schematic is attached
View attachment 67823

 

[alec_t]

The Hall device output pin should connect to pin 2 of the 555.
The 555's pin 2 should also have a resistor (10k, or thereabouts, will do) connecting it to the +12V supply.
Connect pins 6 and 7 of the 555 together (if pin 7 is left floating you won't be able to re-trigger the timer).

 

[eester B]

Again, thank you so much for the help! It worked like a charm, though I don't understand why the hall sensor needs to be tied in to the +12V like that.

Next question: I am doing a similar setup, but this time with five 3W LEDs (650ma max, 3.8V each). Using a 12V power supply, I am running two LEDs in series and three LEDs in series, parallel with each other (see attached diagram).
Since there are two parallel circuits, I need 650mA x 2 = 1.3 mA, so a base current of 130mA will saturate the transistor, so the base resistor is (11.3V - 0.13A)=87 ohm...
Is this accurate? I am not sure about splitting the line after the transistor (as shown) or if I need two transistors?
Thanks
EB

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