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I was working on a project and decided to use a BJT transistor as a switch. I haven't taken the electronic circuit courses which deal with transistors in my university yet so I wanted some advice if my idea is feasible or not.

Basically, I have an actual switch (Pololu Pushbutton Power Switch SV) that will control when my project will be turned on and off. But my device will be hooked onto a bike so I made it so that if an RFID tag is properly read (via an RFID reader), my BJT (which is in series with the battery source and the pololu switch; please see google doc for schematic) will be allowed to conduct current across it, hence powering the device. When the RFID tag is properly read, I will set a digital pin to HIGH (this pin is connected to the base of the BJT transistor) which will allow the current from the battery to flow across the BJT and power the the device. Will 40 mA delivered to the base be enough for current to flow across the BJT?

Here is my circuit diagram I have quickly drawn (it's located in a google doc folder): https://docs.google.com/document/d/1H8ES5edi69eQtyavTgFldQ_BlMkgLi44-MLGMWAWi6Y/edit?usp=sharing

Thank you for anyone who comments and please don't hesitate to ask me for more info! Since I have been researching BJT transistors on google, my knowledge on them is not concrete so please forgive me if I have some silly/stupid assumptions or I am totally doing this the wrong way.

As requested, here is the switch: http://www.pololu.com/product/750

Just some back ground for you guys on why I am doing this. This is so if someone tries to attempt to close the device when it should be operating, the device will still be operating since the digital pin is HIGH and current is still flowing accross the BJT transistor hence allowing the device to be still powered up. If the user wants to shut the BJT off, just swipe the correct RFID tag again and this will cause the digital pin to become LOW which will cause the BJT to stop the flow of current.

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  • \$\begingroup\$ As shown in your drawing, the Pololu switch (link to switch data?) will short the power supply - you definitely don't want to do that! What is the battery voltage? Is the transistor NPN or PNP? \$\endgroup\$ – Peter Bennett Jan 1 '15 at 2:44
  • \$\begingroup\$ Hello, thank you for the comment! Okay, the battery voltage will be 5.2V and the transistor I think will most likely be an NPN. I also posted a link to the pololu switch in the original post (located towards the end). \$\endgroup\$ – fghia002 Jan 1 '15 at 2:57
  • \$\begingroup\$ If you want to keep this potentially good question alive you MUST - Read what Peter said about shorting the battery with the switch. - Understand what he said. - Fix the diagram. As it is it makes no sense. The switch is connected across the battery. It will be a race to die if the switch is closed. \$\endgroup\$ – Russell McMahon Jan 1 '15 at 3:02
  • \$\begingroup\$ Hi Russel, sorry for the diagram and thanks for pointing my mistakes out. The switch is actually a pololu switch which is already built with internal components to stop it from shorting the battery. I have updated the google doc. with a picture I found on google. \$\endgroup\$ – fghia002 Jan 1 '15 at 3:07
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Your circuit should look something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

A high output from the Arduino will turn on Q2 which will pull Q1's base low, turning it on as well. Q1 should be selected to handle the current required by your device. You should also have a resistor between Q1's base and emitter to be sure it turns off when Q2 turns off.

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  • \$\begingroup\$ Thank you! I'll be sure to test this out. Just wondering, why wouldn't just one BJT transistor with its' base wired to a digital pin not work? \$\endgroup\$ – fghia002 Jan 1 '15 at 6:06
  • \$\begingroup\$ The base of Q1 in the diagram must go to about 9V to fully turn off. This is why you cannot drive it directly from a digital pin. \$\endgroup\$ – caveman Jan 1 '15 at 8:20
  • \$\begingroup\$ And if anyone can comment, would the PNP transistor MPS2907AG work for my case? My device will be drawing varying amounts of currents (ranging from ~80 mA to ~280 mA) so I chose the PNP transistor with a max collector current of 600 mA. Here is the data sheet: verical.com/pd/on-semiconductor-gp-bjt-mps2907ag-147937 \$\endgroup\$ – fghia002 Jan 1 '15 at 20:56

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