# Need help designing a 'trip wire' type circuit.

1. Oct 12, 2012

### fabiobollinge

My first-year engineering group is designing an automatic shut off oven system. We will integrate smoke detector systems inside the oven. Details about that don't matter.

Smoke detectors work on a trip wire type alarm. Alpha particles are emitted from a small source of Americium, the alpha particles ionize the air and complete a circuit. When smoke enters the system, the ionization doesn't occur and the circuit is broken and the alarm is triggered.

We will be using just the ionization system, but not the built in circuit with the smoke detector, because we need to incorporate it into a larger system, and modifying the existing circuitry would be almost impossible.

Disregard the ionization system and replace it with a simple switch - on or off. And replace the entire alarm function with lighting a simple LED. We can modify the circuit from there, I just need a circuit, preferably a schematic too, that will light an LED once a connection is broken, or a switch is turned off.

Thank you for taking the time to read this!

2. Oct 13, 2012

### yungman

Look up an LED, adjust the R according to the equation given. When the switch open, LED will light up.

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3. Oct 13, 2012

### fabiobollinge

This works excellently, however the only way for it to work excellently is to allow a large current constantly through the trip wire. And for my project the current through the trip wire should be minimal, since it will be powered on for hours at a time, and it could also be battery powered!

If you have a solution, let me know!

4. Oct 13, 2012

### yungman

Next version will cost you a hamburger!!!

I don't know what current you require, if the current is high, you might want to consider a Darlington NPN to lower the current on the trip wire. The Base resistor can be half the $\beta$ times higher than the R.

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5. Oct 13, 2012

### fabiobollinge

Thanks! I used that idea, but I used two transistors! I got away with having a 1M resistor on the switch closed circuit, then that current is enough to just barely open the first transistor, but the small current from that is enough to nearly fully open the second transistor, lighting the LED.

6. Oct 19, 2012

### fabiobollinge

Hey again! So I tried working this into a larger circuit. But my team is going to try to use photoresistors, so when smoke blocks a laser beam, the resistance goes up, and that acts as opening the switch.

Here's the circuit I made using help from above (posts above this one ;) ). The idea is when left left most switch is opened, current is redirected the the first LED (LEDs are circles here), then the second switch opens (middle right) and turns on the second LED, and when the third switch opens (top right) the third LED goes on, and a transistor is opened, completely an exterior circuit (represented by the LED on the far right).

http://imageshack.us/a/img801/9838/oven20triggers.jpg [Broken]

FYI, the max current for the LEDs is 20mA, and current source is a 9V battery.

Now I would like to replace the switches with a system dependant on photoresistors, so when resistance is max, it acts an open switch, and vice versa.

Thank you

Last edited by a moderator: May 6, 2017