Switch Relay from just 2v ?

Thread Starter


Joined Aug 15, 2006
Hi all,

I have a small problem - I am working on a little project that requires me to switch a 12v supply on using a 2v signal.
I have been searching Farnell and Maplin (UK) but can't find any relays that will activate on my 1.8 ~ 2v supply
Any ideas how I can do this or if such relays even exist ?


Thread Starter


Joined Aug 15, 2006


Joined Jan 10, 2006
Well, the transistor will turn on at approx 0.7 volts. A resistor is series from the switch signal to the base to limit the base current should be all thats required. Try something like 1 or 2 k ohms. Fairly non critical.


Joined Aug 8, 2005
Note that the relay you suggest has rated contacts of 2 Amps Max, therefore your supply load and the current going through those contacts should always be less than that 2 Amps.

The relay coil for the 12V version is rated at 960 ohms (see spec sheet), therefore:

0.0125A = 12V / 960 ohms

This means the xstr you choose should have an Ic of at least twice that or 25mA (this math indicates that the Ic should not be much of an issue in your application).

Now, technically, what you would do is divide 12.5mA by the xstr's hFE to find the maximum base resistor value (e.g., 12.5mA / 100 = 125uA, then 1.8V - 0.7V / 125uA = 8.8K).

The minimum base resistor value is mainly determined by the amount of loading the input signal can tolerate, or the extent to which you want to load it. Figure that 2.0V - 0.7V = 1.3V and 1.3V / 2.2K = 590uA, which indicates that a 2.2K resistor shouldn't be a problem (assuming your input signal can drive at least 1mA).

The upshot is, just about any NPN xstr with more than about twice the operating voltage (2 x 12V = 24V CEO) will work with the circuit you describe, and a 2.2K base resistor will be more than adequate. Therefore, the listed 2N3904 is quite common and would be a good choice.