Welcome! I can't help you with your circuit, but I can tell you how to send and receive private messages (PM's, not PMS).

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Generally speaking, there are quite a few methods to accomplish that, but you've left a lot to the imagination.

1. Define "loose connection". If it's literally acting like an loose/intermittent electrical contact, then max voltage will pass through occasionally. Do you want the LED to flash on these, or wait until the connection is made for a specific time period? (I'm thinking along the lines of switch debouncing.)
   
   
2. Should the LED remain on if the "solid" connection is lost?
   
   
3. Is 1.5 volts the total available for the whole device? What's the total available power? (Common battery capacities = http://en.wikipedia.org/wiki/List_of_battery_sizes.) Depending on the required complexity of the rest of the circuit, this can be a major factor in the design.
   
   
4. LEDs require more than 1.5 volts, but there are plenty of simple boost circuits to choose from. Again, the rest of the circuit will have an impact on this choice.
   
   
5. SSR = "Solid State Relay", or something else?

Pantez,

Thanks for your help so far. I was being vague about the circuit because I don't want to give away its intended use.

The "loose connection" is more like a gap. Once the gap is bridged, I want the "LED" to light up for 1 second every 30 seconds. If the gap is un-bridged, the LED should go off. The power to the LED is irrelevant because it isn't really an LED, it's just something that indicates the gap has been bridged. The circuit will be powered by a 1.5 volt battery. Let's just say that I want the LED to remain off if the power in the circuit is less than 0.5 volts, and I want the LED to flash whenever the power in the circuit is greater than 0.5 volts.

SSR = solid state relay.

Thanks, Erik

Well, 1.5 volts is not a digital ON or OFF so I don't suppose that will help you very much.

Generally a gap is used to stop all the voltage unless it is of such a high frequency that it can readily jump the gap. But the voltage will only divide if the resistance at the point of contact is somewhat close to the LED resistance.

So the question becomes one of how can you make sure the resistance is not a problem. An LED only requires about 10 to 15 mAmps to be quite visible. But it may take more than 1.5 volts to get it started. Perhaps you are really talking about a 3 volt system?

The voltage issues I was talking about are related to the overall circuit you want to create. If it's just a mechanical switch, LED, and battery, then things are simpler. If you want to illuminate the LED for a specific time period, or do other things, then the circuitry will require more voltage and more current.

The lowest Vf for any (visible wavelength) standard LED I've seen is 1.8 volts. I haven't experimented with powering LEDs below spec'd forward voltage, so I don't know what effect it will have.

I believe there are some specialty LEDs with built-in DC-DC converters that will permit operating at 1.5 volts, but I don't have the specs or availability handy.

Let's see if I can't simplify this... Forget about the gap. I have a battery. The pos and neg terminals are connected to an LED. Along the wire going from the neg terminal to the LED there is a component (A). If the electricity going to A is greater than X, then A allows the electricity to flow thru, thus illuminating the LED. If the electricity going to A is less than X, then no electricity is allowed to flow thru A. My question is: What kind of component is A?

"If the electricity going to A is less than X, then no electricity is allowed to flow thru A. My question is: What kind of component is A?"

A = Voltage controlled switch (It's a circuit, not really a single component.)

If the battery voltage was higher then I would guess that component "A" was a Zenier diode. You didn't mention a current limiting resistor to protect the LED but if your voltage is low enough then you can sometimes get away without one but you may have short life on the LED.

With a 1.5 V battery, your component "A" could also be a general purpose diode. A new device called a current limiting diode is on the market and those have an upper current range of about 20 mA. What ever the device is, there will be some voltage drop across it which reduces the voltage available to your LED.

You could have a problem finding a regular supply of such LEDs. I would do some more testing by building several and checking that they all work correctly.

The last two posts were extremely helpful. I googled "voltage controlled switch, Zenier diode and current limiting diode." I think the VCS is what I need, but it looks complicated and may increase the price of the device more than I wanted. The diodes mentioned above -- if I understand correctly -- are more of a "current control" ?? and will still allow current to pass thru the circuit even if the current is less than X? I can't have that. I need something where absolutely no current can reach the "LED" unless the current is greater than X.

I went to Radio Shack today and bought a few simple circuits to play around with, and a few books to refresh my memory of the basics. And -- bear with me -- I also bought a reed relay in hopes that I could hook that up to one of the circuits in order to only make it function (relay on) when the voltage in the circuit was greater than X. But after a bit of research, I think the reed relay has to come in contact with a magnetic field in order to activate. (??) And I was thinking... What if the current flowing thru the circuit went to a magnetic coil? And, once the coil had enough current, it triggered the reed relay? Would the coil drain the battery if the current was less than X?

I really appreciate your help and I promise to invite y'all to my beach house after I make my first million. :-)

Erik

Zener diode is about voltage control .

It seems that any time I had to use something mechanical, like a relay, it had more problems than it was worth. Here is a simple circuit that uses a few really cheap diodes and an NPN transistor. I assume the load is the LED and the supply voltage is at least a few volts?Your signal goes into R1 which is just enough to prevent it from frying the transistor input. Your signal voltage will turn on the transistor when you have exceeded the voltage required to forward bias the diodes. In this case 3*0.6 V plus 0.6V (for the Vbe junction) equals 2.4 volts give or take a few tenths of a volt depending on what kind of diodes you picked up. When voltage is applied to the input, a current will flow from the power source through R2 and it will then light up you LED. The voltage Vce is probably going to be 0.1 to 0.2 volts but it won't be much. The diodes in general act like a switch but it takes a little voltage to turn them on. The transistor helps make that threshold where the LED turns on a little more distinctive. You can add or subtract as many diodes as you need for your threshold level in increments of about 0.6 volts. If you need more accuracy than that, you can use a voltage divider in parallel with one of the diodes. I'll draw that if you have trouble with my description.

The current limiting diode was suggested if you couldn't come up with a good value for R2. Its purpose is to limit a current/thermal runaway effect in the LED.

When you make your millions remember you owe your thanks to CR4.

Solid state thermal sensor ? - if this could be of your use .