hobbyking FET switch

Using Arduino General Electronics
1

Hi guys
I have one of these:
http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8833

or actually 15 which I have bought for a project based on the text they sell it with which states that it is a simple on/off switch,
That doesnt appear to be the case,
This is the "manual" it comes with :

I have connected it as per the diagram and sure enough motors and relays etc work , but only because it seems to be feeding the 5v out to the motor upon "desactivation" I say this because the gate of the FET has 1.5v on it until the switch activates then it loses that and the motor starts,

this is the datasheet for the FET where the +5v is shown going straight to the battery in the diagram :

I havent looked yet to see if the other is the same or not,

Can anyone help me out ? I need to use it to switch a wire to ground, I am planing to use it to "PUSH" a switch that simply connects a pin to ground,

Any ideas?

2

Looking at that "schematic" it looks like a high side switch, not a low side switch. Another clue is in the fact that it uses a P channel FET.

That won't switch a wire to ground, it switches a positive supply on and off.

What you need is a simple N-channel FET (logic level) or an NPN BJT.

3

Hi ,
Do you think i could change the FET and use the same board, ?
On the current board there is a 1.5v switching voltage to the FET which is HIGH in the inactive state and LOW when activated,
the drain and source are not really connected to anything else,

The second FET may be different, I have to find a board that hasnt had the surface scratched off and will elt me read it ,

Thanks,

4

No, you can't just change the FET. The whole way the circuit operates is completely different.

All you need is a single small signal N-channel logic level FET, or a small signal NPN BJT, such as the good ol' BC337, and two resistors.

switchbypass.png

5

do you think i could use the input to the current FETs to trigger the one you mention ?

6

spruce_m00se:
do you think i could use the input to the current FETs to trigger the one you mention ?

You provide a voltage on the input to that circuit and it "presses" the button - how you provide that signal is up to you. Normally you'd connect it direct to an Arduino IO port.

7

in which case I will have a go at using the output on the existing board,
the price of the existing board and the size were an obvious advantage over using arduino in this project,
However I want to learn how to make the output stage of the circuit so that I can use arduino in the future for projects with severl outputs,

thanks,

8

spruce_m00se:
Hi guys
I have one of these:
http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=8833

or actually 15 which I have bought for a project based on the text they sell it with which states that it is a simple on/off switch,
That doesnt appear to be the case,

It is the case, just not the way you wanted.

spruce_m00se:
TPC8107 pdf, TPC8107 Description, TPC8107 Datasheet, TPC8107 view ::: ALLDATASHEET :::

Any ideas?

You have a P-channel FET, you needed an N-channel. There's no easy way to make it work.

9

as far as i know, a switch doesnt involve needing a positive or negative votlage to work, it just connects two points as does a relay,

For now I have a subminiture relay on the output of the original module in order to do what I want,

10

http://www.ebay.co.uk/itm/NPN-Transistor-BC337-Pack-of-15-/190712040857?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item2c6752cd99

just checking that this is what is referred to

11

Yup. A handy common NPN to have in your kit. I have a big pile of them here :wink:

12

and for that price its probly not worth digging around old boards looking for one,
:slight_smile:

13

spruce_m00se:
and for that price its probly not worth digging around old boards looking for one,
:slight_smile:

I consider that expensive, but I'd be buying from a proper supplier with other items as well, so no delivery charges. For a single component or very small quantities most suppliers hammer you on delivery...

14

True,
but considering the project its being used on, its negligable, still I know what you mean, shipping kills most deals on the internet,

Especially if you get something from china and customs catch it :frowning: