Electrical – On/Off Transistor Switch by pulses of Voltage

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So I have a remote switch to control a light bulb. The transmitter circuit basically consists of:

-transmitter IC ( RF 433M Hz)

-DC power supply

-Switch ( turn on/off by applying voltage, transistor maybe, or thyristor? )

-Mechanical Touch sensor that generates a pulse of 1.5-2.5 V within 0.5 s per press. The current generated here is almost negligible, around 20 nA.

(See the schematic)

All I need to do is to turn on the switch (transistor) when I press the mechanical sensor that is connected to the gate of the transistor to allow the DC power supply feed the "Data pin" and then transmit it through antenna.

The problem is, I don't know what type of transistor ( or any other electronic device) that can be used to detect such a small voltage/current pulse and turn ON.

Any suggestion? If you have totally different ideas to tranmist the data with that sensitive mechanical touch sensor you're most welcome to share with me and I'll appreciate it 🙂

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Best Answer

Ok, we have the MOSFET choice solved: BSS138, for example, can easily conduct with a gate voltage as low as you request. Now, your circuit is missing a few things. First, you should have a pull-down at the data input pin, because when the FET isn't conducting, there is no voltage applied at all, and this isn't good. Here is what it would give:

schematic

But: A mosfet actually turns on when its gate voltage is above Vgs(th) with respect to the voltage at its source terminal. Not necessarily ground. So, look what happens when the gate is, for example, at GND+2.5V: The mosfet starts conducting because its source is at ground (due to the pullup). Fine. But when the output (source voltage) starts raising, the Vgs voltage actually reduces. And when it reaches 2.5V-Vgs(th), the mosfet can't conduct more. So you'll never reach the required 5V at the output. The highest you'll get is about 1.5V (2.5V - Vgsth).

In short: The above circuit doesn't work.

Here is what works:

schematic

simulate this circuit – Schematic created using CircuitLab

When the input is at 2.5V, M1 start conducting, which lowers the voltage at the gate of M2 (BSS84, it is a P-channel FET), and delivers the full 5V at the data input PIN. Otherwise, the pin is pulled down by R2.

This is a basic level shifter, actually.

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