Introduction: How to Convert a Negative Pulse Into a Positive Using a Transistor

hello all, this is my first attempt at an instructable so I hope you can understand it.  I ran into a problem when building a transistor scr to turn on leds in my car with my keyless entry remote. The scr (latching transistor) turns on and off with +12v positive pulses and my keyless sends negative (ground) pulses to lock/unlock the doors. I wanted to use this to turn on flashing leds (like a car alarm) when I locked the doors and off when I unlocked it. So I decided that I would build this to convert the negative pulse to positive. after looking for days I couldn't find anything that suited me so I finally figured this out on my own. I figured I would make an instructable here to help somebody else who couldn't find it anywhere else. So here we go........ 

Step 1: The Schematic

here is the schematic for this project. it is a pretty basic circuit, But its function will come in handy for a lot of different projects you will build.

Step 2: Parts List

here is the parts list we will need.                                                                                                                                                                                                                  1 pnp transistor I used a 2n3906                                                                                                                                                                      330 ohm resistor                                                                                                                                                                                               10k ohm resistor                                                                                                                                                                                               1 led

Step 3: Attach the Led and Transistor to the Board

first we need to attach the led and transistor to the breadboard. you can put these anywhere on your board. I placed these ones so I could make all my connections without crossing jumpers. the led is a normal 5v led. you can use any color you like. the led connections become the output leads if you want to connect this to something else like I have. The transistor is a pnp 2n3906. any pnp resistor will work. An npn resistor will not work for this project.

Step 4: Make Connections to the Led

Next we make the connections to the led. Put a small jumper from the negative (cathode) side of the led to the ground rail on the board. An easy way to tell the negative side of the led is a flat spot on the bottom ring of the led. this is the negative leg of it. then we put the 330 ohm resistor from the positive (anode) side of the led to the collector leg of the transistor. some transistors have different pins so always look at the datasheet for the one you are using. I used a pnp (a npn transistor will not work for this) 2n3906. the collector is the third pin on this transistor.

Step 5: Finish the Connections at the Transistor

Now we will finish our connections at the transistor. First we will put in the last resistor. Take your 10k resistor and put one end (resistors are bipolar so it doesn't matter which side you use) on the base of the transistor. On the transistor we are using it is the middle pin (again some transistors are different so make sure you check the datasheet for the one you are using). Put the other end of the resistor in an open hole on your project board. Now put a jumper in a hole in the same row as the resistor. this will become your - input. next connect the emitter leg of the transistor directly to the positive rail on your project board. All the connections at the transistor is complete.

Step 6: Connect Your Power Supply and Test Your Circuit

That's all there is to it. Connect your power supply and test your circuit. take the input wire from your 10k resistor and touch it to the ground rail on your board. the led should light up. If it doesn't, The first thing to check is your led. If it is backwards, It will not light up. if that's ok then check all of your connections. this is a fairly basic circuit so you should be able to troubleshoot it easily. Well this is the end of my first instructable. I hope its able to help some of you.

Comments

author
rspellicer (author)2013-10-10

Just a bit of a detail that you may want to correct. The transistor you are using is a PNP, however, the schematic has the symbol of an NPN. Otherwise, neat instructable :)

author
xobmo (author)2013-10-06

Great explanation! Just to let you know there are no 5 volt LEDs. I am creating circuits too!

author
aclark32 (author)xobmo2013-10-07

most leds are 3.8v, but to walk into radio shack or other cheap suppliers like ESCO the bags are indeed labeled 5v. I just figured by saying 5v. that I would avoid a lot of questions about whether 3.8v ones would work..lol

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