# Need Help With Motion Sensor Controlled Strobe LED. II

#### Rory Starkweather

Nov 13, 2014
77
This is about my SP-01-N4/CEG007100 project. Basically a motion detector controlling a strobe light.

I intend to power the control circuit with a 9 volt clock battery and the strobe circuit with a 9 volt lantern battery.

As someone has kindly pointed out, the CEG can produce about 3 volts. I would think that would be able to turn on a power transistor, if applied to the Base lead.

Suppose I use a generic power transistor as the switch and a current limiter to feed the LED strobe?

I'm trying to avoid resistors as much as possible because I have a lot more VRs and PTs than resistors.

I guess I have two questions.

1. Could it work?

2. Would it be efficient?

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,827
Yes, it would work. No, it would not be efficient. The cheapest current limiter is a resistor, but these unavoidably dissipate power. Try to choose a strobe circuit battery with terminal voltage only slightly greater than LED voltage, which is about 3 V. So a 4 V battery would be preferable to a 9 V lantern battery... if you can find one. I think three 1.5 V D-size cells connected in series to produce 4.5 V would be a good solution. You would need a 1.5 ohm, 2 watt, resistor in series with the power transistor, battery, and LED to drop 1.5 V at 1 A of LED current.

#### Rory Starkweather

Nov 13, 2014
77
As usual, you are very helpful. I will sketch it out in a minute.

#### Rory Starkweather

Nov 13, 2014
77
Whoa. Wait a minute. 1 Amp of LED Current? Does the Luxeon need that much?

I was considering two parallel sets of 'D'Cells. 4.5 Volts at max charge.

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#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,827
You don't HAVE to run it that hot (increase the value of the series dropping resistor), but the spec sheet does say 1000 mA maximum forward current, IIRC.

Wiring D-cells in parallel is never a good idea. The strongest will discharge into the weakest. If you feel you need more ampere-hours capacity, several series-connected cells driving a buck-converter will be a better option.

Or find cells with larger capacity. I remember using cells with binding posts sometime in the last century. They were quite expensive and not rechargable, about six or seven inches tall and a inch and a half in diameter IIRC. Originally used as ignition cells for Ford-type induction-coil ignitions and also as the "A" battery to light the filaments of early radio receivers. When the cell is used up, its carbon electrode can be removed for all sorts of nifty experiments.

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#### Rory Starkweather

Nov 13, 2014
77
TYVM, again. I like to look at even simple circuits and find as many options as possible before I actually start working on them.

I have a lot of voltage regulators and current limiters left over from stuff I used to do. I was been thinking about a small motorcycle battery powering both sides of the circuit, but that seems a little much.

Any advice on whether to use a relay between the two sides of the circuit, or an opto-isolator?

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,827
... Any advice on whether to use a relay between the two sides of the circuit, or an opto-isolator?

Hows about you sketch a schematic or wiring diagram of the circuit, snap a photo of the sketch, then upload the photo to this forum in your next post.

#### Chaos

Nov 27, 2014
1
Interested in what you come up with, it would be cool to make something like this up for a garage alarm.

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