Is there a circuit or device that can be used to switch on a low power circuit without itself using power? I am trying to take the output (min. 30-50mV) from a photodiode and use it to turn on a circuit that is connected to a +5V power source (4-AAA batteries). However, the switch itself can not draw any power.

I have tried using an NPN BJT. But it took 735mV to turn on the circuit and I can only use about 30-50mV. Thank you for any help.

Dave

 

Is there a circuit or device that can be used to switch on a low power circuit without itself using power? I am trying to take the output (min. 30-50mV) from a photodiode and use it to turn on a circuit that is connected to a +5V power source (4-AAA batteries). However, the switch itself can not draw any power.

I have tried using an NPN BJT. But it took 735mV to turn on the circuit and I can only use about 30-50mV. Thank you for any help.

Dave

No you have to take a small amount of power from the 5 volt source to make it work otherwise you have to use another voltage source to power the control circuit.

 

You could use some FET and have the diode as a part of a resistor divider, but that would still consume some current all the time.

Maybe you could use a phototransistor?

 

How about a small solar cell? Shining light on it will produce a voltage and current.

 

kubeek, thanks for the phototransistor idea, I will look into that. I'm not sure if a JFET would work. From what I understand it controls current and I need a way to control voltage, however I could be wrong. I guess another possibility is trying a MOSFET like the IRL510, but I have yet to figure that out.

I actually did try a solar cell, one from a calculator, and it did work. But what I forgot to mention is that the reaction time needs to be in the microseconds range and unfortunately the solar cell was too slow. I also looked into photoresistors but they were too slow as well.

Is there a way to possibly store voltage in a capacitor and then "knock it out" using a photodiode?

 

I think it would help if you say exactly what the application is.