LED to power a DC motor

Phototransistor + relay driver + relay?

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

Hi,

Its basically an output from the smoke detector IC which is required to drive a DC motor.

Regards, Krishnamurthy.S

You can get to the pin of the IC? Battery voltage? Is there a current limiting resistor on the led? Does the IC pull to ground (battery minus?) to light the led or does it go high? Will the motor work from a separate power source? How much current will the motor draw (and what kind of motor - little toy motor or earth mover?)

That is the basic information one would want.

Yes, I can get to the pin of the IC and the voltage is as per the requirements of the smoke detector chip, would be around 9V. I am in the phase of designing the circuit. The motor works from a separate source. I should drive a blower, and hence is a big one.

A good solution would be to add an optoisolator in series with the existing LED. You might have to modify the existing dropping resistor due to the increased v drop of two devices.

The output of the optoisolator can be used to drive a small load, or a large one via a relay.

One thing to consider: The existing LED might be used for more functions than just the alarm output. My smoke detector's LED flashed briefly every few seconds. It also flashes for a low battery condition. You may need some logic on the opto's output to condition the signal.