# Voltage controlled switch

#### noltex

Joined Apr 18, 2006
15
I need a simple circuit that will act like an SCR with the input of 1.5v, but without the self-latching problem that SCR's have. It needs to open when the input voltage is removed. The circuit is used to drive a 12v relay, but I only need the relay to be on momentarily - basically a one shot switch.

Joined Jan 10, 2006
614

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

To echo Gadget, we need more details about the circuit. How much current does it take to pull in the relay?

#### noltex

Joined Apr 18, 2006
15
The relay coil rating is 12vdc, 320 ohm and 37.5 mA. The input voltage to trigger the circuit is 1.5vdc. I need the circuit to turn on the relay when the input voltage goes high, then off when the input voltage is removed. Any help is appreciated.

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

The LM311 comparator can sink that much current on the output. Use a divider to put about 1 volt on the positive input pin (2) and the 1.5 volt control signal to pin 3. Put the 12 volts to the relay, and the LM311 will be the path to ground when pin 3 goes above 1 volt.

#### Søren

Joined Sep 2, 2006
472
Hi,

The LM311 comparator can sink that much current on the output. Use a divider to put about 1 volt on the positive input pin (2) and the 1.5 volt control signal to pin 3. Put the 12 volts to the relay, and the LM311 will be the path to ground when pin 3 goes above 1 volt.
Who's gonna suggest a microcontroller ?

This would IMO be the sensible way of doing it

Both the driving circuit and the 12V supply shares the ground connection of course.

#### beenthere

Joined Apr 20, 2004
15,819
HI,

Soren (sorry, no slashed o's) has the simplest solution. A 2N2222 would be a good sub for the transistor, by the way.

#### Søren

Joined Sep 2, 2006
472
Hi,

(sorry, no slashed o's)
Never mind, I know how hard it is to press ALT-0248 or write "oe" hehe

#### noltex

Joined Apr 18, 2006
15
Thanks Søren, that's exactly what I'm looking for. The application I'm using this for must be absolutely reliable, which typically means the simpler the better.

I'm a police detective and the circuit is part of a cell phone activated kill switch for a bait car, which is put out for the sole purpose of getting stolen. If the circuit should fail, it may be a long time before I get my car back. The rest of the circuit is finished, I was just stuck on needing an additional momentary relay activation, since the rest of the circuit is latched.

This has been a really fun project to work on, since I'm free to put just about any kind of gadget in it that I want, cameras, lights, door locks, whatever we want to come up with. I didn't want to mention the specific use earlier to keep from getting side-tracked. Thanks for the tremendous help.

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

Don't use a Ford Falcon for bait. A buddy tried to get rid of his by leaving it unlocked in a parking lot with the keys in it. Two weeks later, it was still there, but someone had left empty beer cans in it.

#### flx

Joined Nov 18, 2008
1
Mmmm, who doesn't just love the fresh smell of a 2-year old thread being dug up?

About that simple circuit that Søren posted "earlier"... I need to build something similar, but the input I have is barely line-level audio. So besides the fact that my signal is lower than 1.5V, it's AC also. Is there anything that can be added/modified/adjusted, and keeping it simple at the same time? Or are we talking about a whole new approach to the problem?

Basically, what I need to accomplish is to control a little DC motor (without a relay), which operates at 2.5 - 5V, 2-300 mA. My power source is 3.7V and the input signal is as described above. Whenever there's sound (above noise, of course) at the input, the motor has to spin. It doesn't matter how fast it spins, I just want to see it moving.

#### epcole

Joined Nov 18, 2009
8
I cant see the picture of the circuit soren posted I've hit refresh plenty of times and still doesn't show. can someone copy and re-post it please I'm working on a similar project with a 12v relay

#### SgtWookie

Joined Jul 17, 2007
22,230
Søren appears to be long gone, but here are a couple of ideas to try.

The transistor/resistor/load on the left is for when you have relatively high current available at 1.5v.

The Darlington transistor/resistor/load on the right is for when you have relatively high current available at 1.5v.

Note that the Darlington will have a much higher Vce (voltage between the collector and emitter) than the standard transistor will; the saturated transistor will have a Vce under 200mV, whereas the Darlington Vce may be as high as 1.1v.

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