# Help with specialty alarm circuit

Thread Starter

#### oceanrowsolo

Joined Aug 20, 2009
2
Hi, I'm using a special device, a radar target enhancer, and need a circuit design that will do the following:

1. Runs on 12vDC, when a signal is detected from a radar source, the circuit should:
A. Trigger a LED lamp
B. Trigger an audible alarm that can be switched off

When a radar signal is detected, a line with 5vDC goes to 0 vDC, this is the trigger/alarm line provided by the radar target enhancer already in place.
Since the unit will be 'hit' by radar every second or seconds, it would be good to have a timer circuit, perhaps adjustable, that would 'ignore' the multiple hits for X period of time (something like 1 minute).

This is a marine application for use on a transoceanic row boat. Any help or suggestions is greatly appreciated. I can solder and follow a circuit design, but don't have the skills to create the design. Please help if you can....

Thanks kindly!!

W. Vidmar

#### beenthere

Joined Apr 20, 2004
15,808
A "target enhancer" sounds is if it should be something to give a larger return to the radar, like a corner reflector, also called a Mongolian ghost trap.

You seem to be interested in a radar detector that indicates when you are receiving radar energy. Radars use a wide range of frequencies. Would you happen to know the frequency and repetition rate of the radar you wish to detect?

#### jj_alukkas

Joined Jan 8, 2009
753
You need a low logic trigger.

In the following circuit, replace the relay with a 12V low power buzzer with switch along with an LED indicator in parallel. If your buzzer or siren is a heavy one, connect it after the relay.

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#### KMoffett

Joined Dec 19, 2007
2,765
jj,

With 5v logic input and a +12V supply the PNP transistor and relay will be ON whether the input is +5v or 0v. If there is a 5v supply for the logic, maybe he could use a 5V relay. That could switch the 12V LED and buzzer.

Ken

#### jj_alukkas

Joined Jan 8, 2009
753
Oh ya, thats right. Forgot it. Thanks for pointing it out.

#### KMoffett

Joined Dec 19, 2007
2,765
Cascade two NPN transistors, two resistors, and the relay from the +12V to the second collector would allow logic level switching of the 12V relay.

ken

#### Bernard

Joined Aug 7, 2008
5,227
Just throwing this out as a straw man- to throw darts at: Assuming +5V is not available, using optical-coupler with +5V on signal line there should be no current flow thru emitter; when sig. is lo , 20mA will flow, grounding R2, triggering C555. Select R3-C for desired blockout time[ 3MΩ, 47μF etc ]. C555 has poor hi capability, so a FET is used to drive load.R5 to fit LED's used. "S" is sonalert or other sounding device. Have a good-safe trip.

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Thread Starter

#### oceanrowsolo

Joined Aug 20, 2009
2
Hi,

Thank you for your suggestions and schematics.

The device I'm using, an active radar target enhancer, already exists and has three wires coming from it; +12VDC, Ground, and a +5VDC signal that goes to 0 VDC when hit by marine ship radar.

With the above in mind, which of the proposed solutions would be best?

Beenthere; the device already works (commerically made) and is in use on ships and unmanned buoys around the globe - hence no need to worry about freqs and such. And yes, the device gives out a signal to make me look at though I'm a 150' blob on the other ships radar, rather than the 24' row boat I will be on....

Thanks kindly,

W. Vidmar

#### jj_alukkas

Joined Jan 8, 2009
753
Cascade two NPN transistors, two resistors, and the relay from the +12V to the second collector would allow logic level switching of the 12V relay.
Thats the solution with minimum components.

#### Bernard

Joined Aug 7, 2008
5,227
JJ's two npn's ,good idea with relay or inhibit timer using 555 IC as one-shot. Blockout time about 1 sec per micro farad.

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#### Bernard

Joined Aug 7, 2008
5,227
Sorry Ken mis read post.

#### jj_alukkas

Joined Jan 8, 2009
753
I think 2 transistors make it even easier ..

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