Will this transistor/ relay set-up work?

Projects General Guidance
1

I'm using an Arduino Pro in my car.

I'm using this transistor:

& this relay

Will this work or do I need a smaller Relay?

2

what are you trying to do?

3

You need a 125R resistor between the pin 5 and the gate of the FET.
You need a reverse bias diode across the relay.
You need to connect the -ve of the battery to the arduino ground.

4

Grumpy_Mike:
You need a 125R resistor between the pin 5 and the gate of the FET.
You need a reverse bias diode across the relay.
You need to connect the -ve of the battery to the arduino ground.

  1. How did you calculate the size of the resistor and why is it needed exactly.
  2. Do I need a reverse bias diode because of how the transistor is wired to the relay?
  3. I did the sketch pretty fast, I will ground the Arduino to the battery and make a 5v regulator for the (+)

I'm just starting to get into this stuff so go slow :smiley:

Thanks so much,

Andrew

tigerbomb8:
what are you trying to do?

Use an RFID reader to act as my ignition switch (or kill switch), since I have no key.
I guess I could use a smaller Relay, I just have this one laying around.

5
  1. How did you calculate the size of the resistor and why is it needed exactly.

The FET gate looks like a discharged capacitor. A resistor of 125R keeps the peak current below the 40mA that it is "safe" to draw from an arduino.

  1. Do I need a reverse bias diode because of how the transistor is wired to the relay?

No you need it because of the back EMF generated by the coil.

and make a 5v regulator for the (+)

You need 12V for the relay, so what is the regulator for?

6

Grumpy_Mike:

  1. How did you calculate the size of the resistor and why is it needed exactly.

The FET gate looks like a discharged capacitor. A resistor of 125R keeps the peak current below the 40mA that it is "safe" to draw from an arduino.

  1. Do I need a reverse bias diode because of how the transistor is wired to the relay?

No you need it because of the back EMF generated by the coil.

and make a 5v regulator for the (+)

You need 12V for the relay, so what is the regulator for?

  1. Ohhh, So no matter what size transistor I have I should always have a 125 Ohm resistor? What Watt?
  2. Never though about that. Would this work? http://www.radioshack.com/product/index.jsp?productId=2036270
  3. I meant to say a 5v regulator for the Arduino since it will be in my car.

Thanks!

7

pwnm30rdi3:
Use an RFID reader to act as my ignition switch (or kill switch), since I have no key.
I guess I could use a smaller Relay, I just have this one laying around.

cool you should make this into and enter it into the instructables shopbot contest

i was going to make something like this. but to start the car i would need to put my wallet in a specific spot.

8

tigerbomb8:

pwnm30rdi3:
Use an RFID reader to act as my ignition switch (or kill switch), since I have no key.
I guess I could use a smaller Relay, I just have this one laying around.

cool you should make this into and enter it into the instructables shopbot contest

i was going to make something like this. but to start the car i would need to put my wallet in a specific spot.

I should enter!
I will have to swipe a RFID key tag near the reader then I will be able to start the car, right now you could press 2 buttons and drive off with my car.
If the car was fuel injected I would program it to turn over the starter too and start the entire car when the key was swiped.

9

should always have a 125 Ohm resistor?

With an FET then yes, with a bipolar transistor no.

What Watt?

It doesn't mater, so little power is being dissipated here.

Would this work? http://www.radioshack.com/product/index.jsp?productId=2036270

Yes

10

should always have a 125 Ohm resistor?

I could only find a 120 Ohm 1/4 watt resistor. Hopefully it works?

Would this work? http://www.radioshack.com/product/index.jsp?productId=2036270

I also could only find a,
Prv - 400v Max
Vf- 1.2v @ 3A
Industrial Rectifier, will this work?

Thanks so so so much!!

11

Hopefully it works?

Yes it will work but will not provide enough protection. The absolute maximum output current for a pin is 40mA, even that will produce damage if sustained. Your resistor will limit the current to 5 / 120 = 41.6 mA, you are better getting a resistor greater that the 125R you need.

The diode is fine.

12

Grumpy_Mike:

Hopefully it works?

Yes it will work but will not provide enough protection. The absolute maximum output current for a pin is 40mA, even that will produce damage if sustained. Your resistor will limit the current to 5 / 120 = 41.6 mA, you are better getting a resistor greater that the 125R you need.

The diode is fine.

I found some 130 Ohm 1/4 watt resistors today.
Thanks for all your help. I would have probably messed something up terribly without your help!
You are the best!!!