# Need help with relay circuit.

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#### jjcf89

##### New Member
Hi. I am trying to figure out how to get a relay or if necessary a series of relays to trigger from a LED that has a voltage of 2V and power a household 120V AC. The plan is to get a horn or alarm to trigger when the LED lights. The LED is soldered onto the circuit board so the new circuit would have to be connected in parallel with it.

Thanks for any help.

#### crutschow

##### Well-Known Member
You could use a transistor to turn on a relay but you will need a low voltage DC source. Do you have any available, from the board perhaps? Or would you have to generate it from the 120VAC?

#### MikeMl

##### Well-Known Member
Hi. I am trying to figure out how to get a relay or if necessary a series of relays to trigger from a LED that has a voltage of 2V and power a household 120V AC. The plan is to get a horn or alarm to trigger when the LED lights. The LED is soldered onto the circuit board so the new circuit would have to be connected in parallel with it.

Thanks for any help.
If you are getting intimate enough with the appliance to solder wires onto the LED, why not go a little further and find the switch that lights the LED in the first place, or chances are that you will find a much higher voltage level than the forward drop across the LED.

There is likely a current limiting resistor in series with the LED. If you simply took the output from the other end of the current limiting resistor, you might be able to drive a small DC relay directly. The contact rating would have switch the AC to your final load.

#### jjcf89

##### New Member
You could use a transistor to turn on a relay but you will need a low voltage DC source. Do you have any available, from the board perhaps? Or would you have to generate it from the 120VAC?
There are several different a/c voltages available on the circuit board. 6.3V, 28v, 18v. But I haven't found a DC source yet.

#### jjcf89

##### New Member
If you are getting intimate enough with the appliance to solder wires onto the LED, why not go a little further and find the switch that lights the LED in the first place, or chances are that you will find a much higher voltage level than the forward drop across the LED.

There is likely a current limiting resistor in series with the LED. If you simply took the output from the other end of the current limiting resistor, you might be able to drive a small DC relay directly. The contact rating would have switch the AC to your final load.
Well unfortunately the goal of the project is to clip onto the LED so that the new circuit can be removed easily allowing us to rma the unit in case of problems not caused by us.

Finding the switch might still be a viable solution if I can manage to trace the traces and still clip onto the switch. But if we can keep it simple to just clipping onto the LED that would be good.

#### MikeMl

##### Well-Known Member
How about just coupling a photo-transistor to the LED optically?

#### jjcf89

##### New Member
How about just coupling a photo-transistor to the LED optically?
Would it be reliable? And how would I get it to power 120V. Would I still use a relay?

#### MikeMl

##### Well-Known Member
Yes, but it would take a small external AC power supply, like a wall-wart. Unless you can steal the relay coil power from the existing circuit, you will need to use the wall-wart.

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#### jjcf89

##### New Member
So there are phototransistors that deal with AC?

#### MikeMl

##### Well-Known Member
So there are phototransistors that deal with AC?
Is the LED driven by AC or DC? You can tell if you watch it out of the corner of your eye while moving your eyes. If it flickers, it is driven by AC; if it is steady, it is driven by DC.

A Phototransistor just responds to the light that it sees. It will put out a current proportional to the instantaneous illumination. If the illumination has an 120Hz modulation riding on it, then the phototransistor output will have have the same variation...

#### jjcf89

##### New Member
Is the LED driven by AC or DC? You can tell if you watch it out of the corner of your eye while moving your eyes. If it flickers, it is driven by AC; if it is steady, it is driven by DC.

A Phototransistor just responds to the light that it sees. It will put out a current proportional to the instantaneous illumination. If the illumination has an 120Hz modulation riding on it, then the phototransistor output will have have the same variation...
The LED is definitely driven by 2VDC. Well what I want is when the LED comes on a 120VAC be switched on. How would the photo transistor tie into it? Would it drive the relay?

#### MikeMl

##### Well-Known Member
Here is a typical phototransistor that has good sensitivity to Red light.
It costs about $1 in single qty. It puts out about ~0.05mA with low illumination. If you had a relay with a 100mA DC coil, it would take a current gain of 50mA/0.2mA = 1000 to turn on the relay, so two NPN darlington-connected transistors would do it. The Pot adjusts the sensitivity. #### Attachments • 138.6 KB Views: 92 Last edited: #### jjcf89 ##### New Member Here is a typical phototransistor that has good sensitivity to Red light. It costs about$1 in single qty.

It puts out about ~0.05mA with low illumination. If you had a relay with a 100mA DC coil, it would take a current gain of 50mA/0.2mA = 1000 to turn on the relay, so two NPN darlington-connected transistors would do it. The Pot adjusts the sensitivity.
What would be the benifits of using the phototransistor instead of just clipping onto the LED?

#### MikeMl

##### Well-Known Member
No clips. Non invasive. You dont even have to open the appliance.

#### jjcf89

##### New Member
The LED is on the inside of the appliance anyways. I was thinking the clips would be a bit more secure.

#### MikeMl

##### Well-Known Member
The LED is on the inside of the appliance anyways. I was thinking the clips would be a bit more secure.
The clip version:

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#### jjcf89

##### New Member
When you say wall wart, do you mean a transformer like for example the kind that you use to power a set of computer speakers.

#### MikeMl

##### Well-Known Member
When you say wall wart, do you mean a transformer like for example the kind that you use to power a set of computer speakers.
Yep. It should have a full-wave rectifier in it, as well as a filter capacitor. If you can find one that doesn't have an internal capacitor, you can put one in the circuit. It should say something like "12VDC - 250mA" on On the Wall-Wart. If you want to use a relay with a different coil voltage, the Wall-Wart voltage nominally should match the relay coil voltage.

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#### MikeMl

##### Well-Known Member
...

But, I've always wondered about ground plane. Your diagram has 2 separate grounds.

Is there a difference in the 2 grounds Appliance vs Wall wart or Are they just both at 0vdc and not to worry ?
Actually, I was trying to show that the relay circuit is not grounded until the bottom clip lead is attached to the appliance. Suppose that for some reason, the cathode of the LED was not connected to "ground", but say the series resistor and the LED were reversed. By having the relay circuit "float", it would still work... Note the current through the relay coil, which is all that matters...

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#### killivolt

##### Well-Known Member
Actually, I was trying to show that the relay circuit is not grounded until the bottom clip lead is attached to the appliance. Suppose that for some reason, the cathode of the LED was not connected to "ground", but say the series resistor and the LED were reversed. By having the relay circuit "float", it would still work... Note the current through the relay coil, which is all that matters...
Oops I deleted my question brings clarity to my limited knowledge of ground planes 2 separate grounds has always been a problem in my mind.

Thanks.

kv

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