How do you attached a transistor to this digital output?

[davpankhur]

Hi,

Can someone please show me how to correctly attach a transistor or relay to this photosensitive sensor's digital output?
I would like the digital output to close a circuit for example, a LED light.

Specifications:
Working voltage 3.3V-5V
the output format: Digital switching output (0 and 1)
Size:32 x 13 x 7mm(L x W x H)
Color:Blue
Net weight:2g
Package weight:12g
http://www.ebay.com/itm/271509509755

Thanks in advance!

 

[Harald Kapp]

Do you have a circuit diagram to show us?

 

[davpankhur]

here is the circuit, just a quick drawing sorry.

 

[Harald Kapp]

Sorry I didn't make myself clear enough: We need the circuit diagram of the PCB to know what the available output looks like so we can recommend how to connect a transistor or solid state relay.

 

[davpankhur]

Sorry no i dont, i believe it just has a standard 3.3v digital output pin.

 

[Harald Kapp]

Here's general information on connecting a transistor as a switch.

 

[davpankhur]

is a mosfet transistor better?, i dont want to take away any current away from the device that is doing the triggering.
I think transistors need some electrical current to activate right?

 

[Harald Kapp]

A MOSFET that is turned on by 3.3V gate voltage is not so easy to come by.
A load of 100mA requires a base current of 1mA (assuming a Dc currrent gain of 100). Your output should be able to deliver that bit of current without problems.

 

[Ehsan]

If you could not turn on your device reduce the value of R1 to 100 Ohm or even 50 Ohm.

 

[davpankhur]

ok thanks Harald, ill stick with transistors. which resistor needs the least amount of current to trigger? sorry, ill read the transistor tutorial soon.

 

[davpankhur]

Hi Ehsan, thanks I'll give it a try, does the R1 resistor of 50 Ohm halves the current required?

 

[Ehsan]

does the R1 resistor of 50 Ohm halves the current required?

If you reduce the resistor value from 220 to 50 Ohm the current passing through becomes 4 times more.

Because we don't know the maximum output current of DO pin, then we have to do trial and error. That is why I started with 220 and told you to reduce it to let more amount of base current pass through.

2N3904 has maximum DC gain (h_FE=300) so if you get 1mA out of DO pin, then that circuit I posted can easily drive your 100mA load.
you can also use BC548C transistor with more DC current gain (h_FE=800).

 

[davpankhur]

Ok i get it now, the more base current, the more current can flow through the emitter. Thanks!

 

[BobK]

Ehsan's diagram will not work. The two grounds need to be connected together.

Bob

 

[Ehsan]

Ehsan's diagram will not work. The two grounds need to be connected together.

Bob

yes that's right.