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Author Topic: Why LED works but BC547 is not Working ?  (Read 1418 times)
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Members,

Please help me out for my PIR Sensor example where i do make PIN 13 of Arduino Uno R3 HIGH when i get signal from PIR from Digital pin 2. LED works fine but when i attach BC547 with 4.7K registor to base pin of BC547, +V12 to Motor and Ground of Motor to Collector pin, and Emitter to Ground as per attached image and fritzing layout.

With same circuit, my LED get powered on but motor is not. I know i'm making something wrong but not able to find out where.

Please help me to make my DC motor run or energize my Relay ....


* PIR Motion Sensor - Schematic - 25-02-2013.png (70.56 KB, 732x326 - viewed 144 times.)
* PIR Motion Sensor - Schematic - 25-02-2013.fzz (7.29 KB - downloaded 15 times.)
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Why 4k7?  You probably need something smaller.

What's the measured voltage of the battery when the motor should be "0n"?
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And I think you might have the collector and emitter back to front


* cbe.jpg (18.74 KB, 301x262 - viewed 54 times.)
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when i connect power to pin 1, nothing to Base and ground of motor to pin 3 motor works does it mean that BC547 is shorted or not working ?
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Well, how did you select the base resistor?

Refer to the datasheet. Check your transistor type (A, B, C). Probably your using type B with 220 hfe approx.

Connect your motor to the 9V battery directly via multimeter and measure the current. BC547 can't handle currents greater than 100mA.

The below calculations are based on the assumption that your motor uses 70mA.
IC = 70mA
IB = IC/hfe = 70/220 = 318 uA.
VBE (sat) = 0.8V
Hence voltage across resistor is 5 - 0.8 = 4.2V
R = 4.2/318u = 13kohm
with 10% overdrive, R=R/1.1 = 12kOhm

You probably burnt your transistor. Design your amplifier before you actually try it.
« Last Edit: February 25, 2013, 11:09:30 am by xpleria » Logged

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xpleria : I think you pointed rightly, my DC 12V/200R motor is using more than 200mA as mine DMM cant measure more than 200mA so can't figureout actual current it is taking and i might have burnt my transistor as when i power CE motor works so i think it might be shorted... ( I measured it with 10A max setting and it is taking 1.75 A !!!)

Will try with another one and BC547 is B type... True... three lines written as BC, 547B, J5 E i do not understand what that lines was till you pointed out...

Thanks
« Last Edit: February 25, 2013, 11:36:31 am by vbnewcomer » Logged

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Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.
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BC547 is completely the wrong transistor for switching a motor - it's rated at 100mA maximum. For switching small motors, you can use BC337 or ZTX851, with a 100 to 220 ohm base resistor. For larger motors, use a logic level N-channel mosfet instead.
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Its not going to work the OP doesn't have the gnd tied on the arduino
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Its not going to work the OP doesn't have the gnd tied on the arduino

Good catch!
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Actually i was trying with relay but because it was not working and DC motor from friend came handy, i instantly try with that !!! i was trying to on relay of 6VDC (Sun HOLD - RAS0610) but somehow it was not working and hence try the DC motor and was totally un-aware about Amphere it was consuming as even my wall-wart DC converter is 500mA max output !!!

So my relay 6VDC/7A with coil resistance of 103 Ohm will work with the same circuit ...?
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