[SOLVED] Can I use a FET transistor to switch 1.5 volts?

[JohnJohn20]

I am designing a project for my electronics class that switches an LED on when it gets dark (and off when it gets light). Pretty basic except I want it to run off a 1.5 volt battery.

So I have a Joule Thief voltage booster to power the LED. That works OK.

But when I use a BC547 to switch the power to the Joule Thief circuit there is a voltage drop of about 0.6 volts across the BC547 and the LED is not as bright.

So, I believe FETs have less of a voltage drop across them but I am not sure if 1.5V is enough to switch the FET on. can someone a suitable FET to do this?

Thanks.

 

[crutschow]

There are MOSFETs rated for 1.5V operation such as these.

 

[BradtheRad]

Joule thief operation requires that there be a certain amount of current flowing in both windings of the transformer. Bias current on one side. Power current on the other. You only get this by using a transistor.

Try reducing the bias resistor value. This should result in raising current through all components. The led should be brighter.

 

[JohnJohn20]

Cheers Brad. No I want the FET to switch the 1.5V supply to the Joule Thief (which uses a BC547) on and off.

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Hi crutschow.

Thank you. That answers my question.

Could you perhaps suggest a FET that can conduct 50mA with Vgs voltage of 1 volt?

Thanks.

 

[FvM]

You didn't mention at all how you are controlling the FET respectively other kind of electronic switch.

In case of a FET, it's no problem to switch low voltage with effectively no voltage drop. But you need a certain amount of gate voltage.

 

[Audioguru]

50mA is close to the 100mA maximum allowed current for the BC547. If the base current is 1/10th the collector current then its typical saturation voltage loss is less than 0.2V.
A BC337 transistor is much more powerful so with its base current 1/10th the 50mA collector current its typical saturation voltage loss is only 0.03V.

Are you using the transistor as an emitter-follower instead of as a switch? Or is the base current much too low?

 

[JohnJohn20]

Thanks FvM. I only have the 1.5 V supply and a LDR (which I want to turn on when the resistance is more than 50Kohm). As FETs are voltage driven I think this is not possible. Right?

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Brad. After reading your post I tried putting the LDR between 0V and the base of the Joule Thief transistor and increased the bias resistor to 10K. Sort of works OK but turns on before it gets truly dark.

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Thanks Audioguru, I am using the first transistor as a switch and your suggestion works very well. I learn something everyday.