Small relay and transistor circuit trigger at <1v

[Jaw174]

Hello,
I am building a simple latching relay circuit. what transistor out ther will allow me to switch a 5v relay from a 1v, very low current signal? i am guessing that i need a transistor with a very high gain...

the relay will be passing 5volts at 2 amps, and activated at 5 volts.




The outut is from a cellphone, spliced off the vibrate motor.


i am getting a peak of .03 amps - at .02amps continious.

basic circuit:

**broken link removed**


i would be modifying this so that the relay would latch and stay "on" using a line from 5v to the signal-in wire when switched to the Normaly open side. of course if i do that, the max voltage on the transistor base has to be at least capable of 5

Ideas on what transistor to use?

 

[audioguru]

If you don't use a resistor in series with the base of the transistor to limit the current then the transistor might blow up.

If you don't use a resistor in series with the LED to limit its current then it will probably blow up.

If you don't use a reverse diode across the relay coil to limit the voltage spike it produces when it turns off then most parts will blow up.

The transistor number and its current gain depends on the relay's coil current and how much input current from the cell phone is available.

 

[b_reagle]

You could do it this way, but there is no RESET.

 

[b_reagle]

Sorry. I meant this way..

 

[Jaw174]

Ok, i know all that- from a high level those are the least of my worries.

also- the signal is definatly going to be very low current... which realy is the question...

in my testing i used a 2222 and a 2n3904 - 2 basic PNP transistors. i am using a bigger relay for testing that triggers somewhere around 7.5 volts.

my problem is that i need the same voltage into the transistor to get it to do anything... that does NOT work... especialy when i have that low of a voltage in the transistor.

 

[b_reagle]

Can you use a MOSFET?

 

[b_reagle]

I think 30mA input into a transistor is enough to drive your relay. The 2222, is it a 2N2222? That is a NPN, same as your picture. If you use a MOSFET then just voltage being present at the input will drive the relay.

 

[Jaw174]

a mofset sounds like a better idea.

any suggestions?

i was using an NPN version 2222- not exatly that diagram- i just threw that up for example.

 

[b_reagle]

IRF3711ZL instead of NPN.

 

[b_reagle]

It need 1mA input.

 

[Jaw174]

great, thanks.. ill pick one up and give it a try.

 

[Roff]

IRF3711ZL instead of NPN.

He said he has a 1V signal. That won't turn on IRF3711ZL, or most any other MOSFET.

It need 1mA input.

It doesn't require any steady-state gate current - but like I said, it won't work.

If you really have a volt open circuit, and 20mA short circuit, then an NPN should work. I would put 47 ohms in series with the base for insurance.

 

[Jaw174]

ok - anny suggestions on what to try next then?

it seems the output of the NPN's i have tried - a 2222 and 3904 - matches the input voltage

 

[Roff]

ok - anny suggestions on what to try next then?

it seems the output of the NPN's i have tried - a 2222 and 3904 - matches the input voltage

I don't understand what this means. Post a schematic of what you have tried, and tell us how it falls short of your requirements.

 

[Jaw174]

ok -soooo what can i use.

**broken link removed**

 

[rezer]

ok -soooo what can i use.

**broken link removed**

If your issue is getting the relay to work off a 1V signal, what Roff suggested should work. A Mosfet won't in this application.
My questions are this:
How are you latching the relay? I read that in one of the posts. Is this something you are trying to do?
What are the symptoms of your circuit? What is it doing/not doing?

 

[audioguru]

Your base resistor is 1k so the base current is only 0.3mA which might be too low for the transistor to turn on the relay. Use 47 ohms then the base current will be 6mA.

EDIT:
You have the relay contact connected to the collector of the transistor so the transistor will blow up.

 

[Edwin2]

The relaycontact may also not be connected to the leftside of the baseresistor !
You might blow up the cellphone !!

Edwin

 

[Roff]

The relaycontact may also not be connected to the leftside of the baseresistor !
You might blow up the cellphone !!

Edwin

No one has suggested that he connect the relay contact to the left side of the base resistor.

 

[Willbe]

You might find a transistor that can stand 30 mA base current.
Or find a germanium transistor.

Another way may be to shunt the silicon b-e junction with a silicon diode and a small series resistor.
This diode will steal some current from the base at Vbe > 0.5v and you can adjust the diode series resistor to have the base current close to the abs. max. rating.