Controlling +V line with a Picaxe?

[Haku]

I have a Microvision laser pico projector which runs off 5v 1a and I'd like to be able to have an auto-off timer for it controlled by a Picaxe.

My initial thought would be to use an n-channel MOSFET to connect/disconnect the ground line of the power, which would be a quick & easy solution, but this could cause problems because the projector also has a ground line from the VGA connection.

How can I easily switch the +5v line on/off from a Picaxe?

 

[jedynakiewicz]

Put a PNP transistor into the 5v supply; control the base from the PICAXE through an appropriate resistor, probably around 1K. Take the pin low to turn on; high to turn off. A TIP32A will switch 3A 60V but with a gain of 25 you would probably need to drive the base through another transistor.

 

[Haku]

I have some PNP & NPN power transistors I bought for making H-bridges, D882P NPN & B772P PNP. Could I use those to control the +V line?

I only managed to make the H-bridges work with those transistors because I copied an existing circuit which works well, if I don't have a schematic or circuitboard to copy I can struggle with using them.

 

[AllyCat]

an appropriate resistor, probably around 1K. Take the pin low to turn on; high to turn off.

Hi,

That's rather optimistic for 1 Amp, but you might just be able to drive the B772P directly from a PICaxe pin with a resistor of about 220 ohms (giving a base current of 20mA). The minimum gain is given as 60, but with a VCE of 2 volts, so for a VCE < 0.5 volts you might need nearer to 50mA base current. It's worth a try, but otherwise use an NPN driver/inverter with say 2k2 from PICaxe pin to its base and 100 ohms from the collector to the PNP base.

Cheers, Alan.

 

[Billo]

A P-Channel MOSFET would simplify things somewhat. You would just need the MOSFET , no additional transistor or even a resistor is required. Anything rated at 3 amps or more would not even need a heat sink. Something like the Fairchild FQU11P06TU would be ideal. Cheap too at under $1 at Mouser in North America.

 

[AllyCat]

Something like the Fairchild FQU11P06TU would be ideal.

Hi,

I'm not sure if that's really a "logic level" FET, with a maximum gate cut-off at 4 volts (< 250uA). In practice it might well work, but as you already have some apparently suitable bipolars, I'd give them a try first.

Cheers, Alan.

 

[Haku]

I had a dig through my salvaged parts and found a AO4409 SMD p-channel MOSFET.

Time to fire up the soldering iron & do some testing

 

[Billo]

Hi,

I'm not sure if that's really a "logic level" FET, with a maximum gate cut-off at 4 volts (< 250uA). In practice it might well work, but as you already have some apparently suitable bipolars, I'd give them a try first.

Cheers, Alan.

Have used it. Works.

 

[Haku]

Thanks for the help guys, the AO4409 works a treat to turn on/off the +V line to the pico projector, and it's great that it's a SMD the same size as a SMD 08m2, meaning the possibility of a very small 'blob' on the power cable to give it timer control.

Now the hard part of working with the limited IO lines of an 08m2 for control & visual feedback (LED) of the timer setting...

 

[bluejets]

Why not just use a simple relay.??

 

[Haku]

Why not just use a simple relay.??

I thought of using a relay but the list of points against includes:
- Too bulky
- Makes noise when switching
- Uses more power
- Most importantly; no 5v relays to hand

 

[Haku]

I got round to transferring the circuit from breadboard to stripboard a couple of days ago, not my most elegant soldering (it's always hard to make stripboard soldering look good) but when powered up it worked first time without a problem

 

[boriz]

Sweet. A work of art!