I'm pretty new to electronics and need help on a project I'm designing.

I have a varying DC input voltage and I want to design a circuit that switches on/off according to some threshold voltage. For example, if the DC input reaches 1V, the switch throws on and allows current through, otherwise below this threshold voltage the switch is off.

I've been thinking I should use either a BJT or MOSFET but I'm not sure how I would implement it. Any help is greatly appreciated!

 

You could use a comparator, such as an LM339 or LM393.
You could also use a TL431 circuit as a comparator with a built-in voltage reference.

Need more info about the voltages and currents involved.

 

Voltage source is shown in the graph attached, I need the threshold voltage at which the switch turns on to be at 0.9V. The current is going to be around 265mA

 

And what is the voltage source for that current?

 

Not really sure, all I really know about the input source is that the voltage varies from 0.3V to 2.0V, and that the maximum current is about 254mA.

 

Not really sure, all I really know about the input source is that the voltage varies from 0.3V to 2.0V, and that the maximum current is about 254mA.

So what power supply is available to power the circuit?
Can't really design a circuit without that info.

 

Oh sorry for being unclear,

There is a 5V line to power everything, but ideally the switching circuit would be completely passive. I'm trying to avoid using any ICs to control the switching and would rather have it use transistors only or something similar

 

Transistors are active circuit elements, the same as IC's.
You could build the circuit with transistors but it wouldn't be as accurate or temperature stable as using ICs.

 

The only way to get a precise switching threshold without using an IC is to pretty much replicate the internal IC circuit with discrete parts. This can be very instructive, but the net result is less reliable and way more expensive in both dollars and time.

Back to the switch, a saturated PNP transistor can switch +5 V pretty well, but there will be a 0.1 V - 0.2 V drop across the transistor. An oversized P-channel MOSFET will have less drop, and the contacts of a small relay will have none.

ak