Is it possible to switch off a LED when current is applied to base of a transistor? I am using the circuit below but when current is applied to the base, the LED is ON. I need the LED to be off when SW1 is on. I am using 2N1613.
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\$\begingroup\$ Are you limited to this particular circuit or can you use other components? \$\endgroup\$– efox29Aug 22, 2015 at 5:48
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\$\begingroup\$ I can use any component. Transistors, gates just about anything. \$\endgroup\$– BasuAug 22, 2015 at 7:15
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\$\begingroup\$ Why is there the restriction that you must apply current to the base to switch the LED off? Couldn't this part of the circuit be changed? \$\endgroup\$– CL.Aug 22, 2015 at 7:20
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\$\begingroup\$ Actually yes. I am open to any solution. I need to turn off the LED when there is power. \$\endgroup\$– BasuAug 22, 2015 at 7:44
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3 Answers
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You could use the switch to shunt the base current away from the transistor:
If you really need to apply base current to switch the LED off, you could use the transistor to bypass the LED:
However, this wastes all of the current that normally goes through the LED. A more power-efficient circuit would use another transistor that gets its base current shunted away by the first transistor:
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\$\begingroup\$ Thank you! I think this will work for me. Can I use any NPN transistor? \$\endgroup\$– BasuAug 22, 2015 at 7:46
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\$\begingroup\$ Yes. (The 2N1613 and most other small-signal transistors have enough amplification to get away with less base current; you should be able to replace the 1K resistors with 10K.) \$\endgroup\$– CL.Aug 22, 2015 at 7:51
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\$\begingroup\$ I just tested it and it worked beautifully. Now suppose I use 2 of this circuit, the other one with the polarity reversed, how do I keep LED1 glowing? \$\endgroup\$– BasuAug 22, 2015 at 15:47
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\$\begingroup\$ So you want to control two LEDs with one switch? That would be a different question. \$\endgroup\$– CL.Aug 22, 2015 at 16:10
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\$\begingroup\$ Never mind, I got it working. Yes actually if I connect a LED between the R2 and the transistor connecting led- to ground, it works. \$\endgroup\$– BasuAug 22, 2015 at 16:13
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If you don't care about wasting power just put the transistor in parallel with the grounded LED. It will shunt the current away from the LED when the transistor is on, and turn it off.
answered Aug 22, 2015 at 5:34
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yes, it is. here is how you do it: break between led- and collector of the npn; have led- connected to ground; connect led+ to collector. done. here is how it work. when sw1 is off, npn is off, collector voltage will be 6v, and so led is on. when sw1 is on, npn is on, collector voltage will be 0, and so led is off.