# Using a transistor correctly to "connect" Maximite computer and circuit

My question is mostly if I am doing it right.

I have a snap circuit kit (kinda like a breadboard for kids). I also have a Maximite computer (kinda like a Arduino running basic with monitor out). The Maximite has some I/O pins that according to the technical details are high at 2.6V to 3.3V. Also, the maximum current that a pin can deliver is 25mA. The snap circuit kit uses between 3V and 6V depending on the circuit. What I would like to do is to use the Maximite to turn things on and off on in the snap circuit. For example, I might want to turn a 6V lamp or motor on and off.

I am pretty sure that I would want a transistor for this. In particular I think a n channel MOSFET is what I need. I looked up the datasheet for an IRF530 and see that the "Gate Threshold Voltage" is between 2.0V and 4.0V. If I understand things, that means that this MOSFET will "turn on" when the gate is given between 2.0V and 4.0M. Is that right?

Concretely, I am thinking of doing the following:

My question is really just if I am doing this the right way and if there are any other things that I should keep in mind or think of. (I would also welcome other suggestions on how I might do what I want to do.) Specifically I am wondering if it is correct to connect the two grounds/negative terminals.

simulate this circuit – Schematic created using CircuitLab

• In short, no. Look at the Vgs they used for the Rds(on) measurement in the datasheet, and aim to drive the gate with that. In more detail, see Andy's answer.
– user16324
Commented Feb 22, 2018 at 17:33

I looked up the datasheet for an IRF530 and see that the "Gate Threshold Voltage" is between 2.0V and 4.0V. If I understand things, that means that this MOSFET will "turn on" when the gate is given between 2.0V and 4.0M. Is that right?

No, it will begin to turn on at a voltage that may be as high as 4 volts. By turn on, it guarantees that with a gate voltage that might be as high as 4 volts, the drain current is 0.25 mA.

This characterisitic is better suited for understanding how it might turn on properly: -

If the gate source voltage is (say) 5 volts then typically you could expect a drain current of 1 amp would drop about 0.2 volts across the device.

Regarding the circuit in your question, you have drain and source the wrong way round and you will need a pull-down resistor (say 10 kohm) between gate and source. Here's a picture from the internet that gives you the general idea: -

• Thanks for the answer. To reversing the source and drain and adding the resistor, is the specific MOSFET then a good choice for what I am trying to do within the voltages (6 V and 3.3 V) or what should I be looking for? Commented Feb 22, 2018 at 17:31
• A 6 volt lamp of 100 ohms will take 60 mA and this is massively higher than the guaranteed 0.25 mA so you should design the circuit to deliver possibly 5 volts to the gate or choose a MOSFET with a lower threshold voltage and check the graph of Id against VDS in the data sheet (as per my picture) to see that at 3.3 volts you can pull at least 10 x 60 mA. Alpha and Omega semiconductors have some very low gate voltage mosfets: aosmd.com/products/mosfets/n-channel Commented Feb 22, 2018 at 17:37
• The AO3400 is a good one to look at. Commented Feb 22, 2018 at 17:39