rookie guy here.

I am experimenting with a control circuit for making an LED cube, but discovered some wierd abnormality in the transistors i was using.

I wanted to test the circuit by replacing an arduino with R2, and simulate ON when the resistor is closed to 5v, and OFF when the R2 part of the circuit was open. This did NOT work on my planned transistor, D882-Y. I thought i'd try with another transistor, and opted therefore for another transistor i had laying around, BC547B. This worked.

To sum it up, D882-Y had a closed circuit between Collector and Emitter by default, while BC547B needed voltage on the base to open up, as expected.

I tried the D882-Y both ways without any success.

Am i doing something wrong, or is my transistor defect?


D882-Y BC547B


enter image description here

  • \$\begingroup\$ What kind of LED are we talking about here? measure the voltage of your circuit directly, the D882 my have a higher drop. \$\endgroup\$ – PlasmaHH Mar 4 '15 at 22:45

When R2 is connected to the + of the battery the transistor will be turned on.

When R2 is disconnected the transistor won't have a clue what to do since you aren't telling it what to do.

An output of LOW on the Arduino doesn't mean "disconnected", it means "connected to ground".

To turn OFF the transistor your should connect the base to the - of the battery rather than leave it flapping in the breeze.

Better still, provide a pull down resistor between the base and - that is at least 10x bigger than the resistor R2 - say 10KΩ would be good. That way when R2 is disconnected the 10KΩ resistor turns the transistor off.

Also you should move the LED to the collector side of the transistor:


simulate this circuit – Schematic created using CircuitLab

While you can have the LED on the emitter it takes special consideration with regards to your resistor values, and it's simpler to just move it to the collector.

| improve this answer | |
  • \$\begingroup\$ I changed my circuit to look like this, as you told me to, but it is still not working: puu.sh/gnZFs/3817bfe642.png Note the 33koHm resistor because that was the lowest i had that was atleast 10 times R1 \$\endgroup\$ – Liam S. Crouch Mar 5 '15 at 22:07

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